Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 Zoological Journal of the Linnean Society, 2019, XX, 1–51. With 32 figures. Reappraisal of the morphology and phylogenetic relationships of the alligatoroid crocodylian Diplocynodon hantoniensis from the late Eocene of the United Kingdom JONATHAN P. RIO1*, PHILIP D. MANNION2,, EMANUEL TSCHOPP3,, JEREMY E. MARTIN4 and MASSIMO DELFINO5,6 1 Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK 2 Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK 3 Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, 10024–5192, USA 4 Université Lyon, ENS de Lyon, Université Lyon 1, CNRS, UMR 5276 Laboratoire de Géologie de Lyon: Terre, Planète, Environnement, Lyon, France 5 Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, 10125 Torino, Italy 6 Institut Català de Paleontologia Miquel Crusafont, Universitat Autònoma de Barcelona, Edifici ICTAICP, Carrer de les Columnes s/n, Campus de la UAB, 08193 Cerdanyola del Vallès, Barcelona, Spain Received 30 September 2018; revised 14 March 2019; accepted for publication 28 March 2019 Diplocynodon is a genus of basal alligatoroid comprising nine species, which spanned the late Palaeocene to middle Miocene of Europe. Despite recent revisions of most Diplocynodon species, one of the earliest named and most complete, Diplocynodon hantoniensis, has not been re-described for over 150 years. This species is known from the remains of numerous individuals from the Priabonian (late Eocene) Headon Hill Formation, which crops out at Hordle (Hordwell) Cliff in Hampshire, United Kingdom. Here we re-describe and diagnose Diplocynodon hantoniensis, providing the first detailed description of postcranial anatomy in Diplocynodon, and indeed any basal alligatoroid. Diplocynodon hantoniensis is diagnosed by four autapomorphies, including retention of the ectopterygoid–pterygoid flexure through ontogeny and a unique anterior process of the ectopterygoid adjacent to the posteriormost maxillary alveoli. A critical review of previously referred remains from elsewhere in Europe and the USA restricts Diplocynodon hantoniensis to the late Eocene of the UK. Through comparisons with extant crocodylians, the well-preserved postcranial skeleton enables the interpretation of numerous muscle attachments in the forelimbs and hindlimbs, providing a potentially rich source of character data for future phylogenetic analyses. Based on a comparison of humeral morphology between a large sample of crocodylian species, we outline two new morphological characters in the humerus. We include D. hantoniensis in a phylogenetic analysis, including all putative Diplocynodon species (103 taxa scored for 187 characters). We use four different character-weighting schemes: equal weighting, implied weighting (k value = 8) and extended implied weighting with k-values of 4 and 8. In general, these weighted analyses produce congruent results with the equal-weights analysis, and increase the resolution within Diplocynodon. We recover a monophyletic Diplocynodon in three of the four analyses. However, the fourth analysis, with the strongest downweighting of homoplastic characters and missing data (extended implied weighting with k = 4), recovers the Palaeocene Diplocynodon remensis outside Diplocynodon. Our comprehensive revision of one of the most completely known Diplocynodon species facilitates comparisons in the genus, as well as between other basal alligatoroids, and forms the basis for comparing postcranial anatomy in other fossil crocodylians. KEYWORDS: Crocodyliformes – Crocodylomorpha – Diplocynodon – Eocene – implied weights – phylogenetic analysis – postcrania. *Corresponding author. E-mail: jonathan.rio12@imperial.ac.uk © Zoological Journal of the Linnean Society, 2019, XX, 1–51 1 This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 2 J. P. RIO ET AL. INTRODUCTION been recovered from the beds at Hordle Cliff (Benton & Spencer 1995), including mammals (e.g. Owen, 1848; Alligatoroids such as Deinosuchus and Leidyosuchus Cray, 1973; Hooker, 2014), turtles (e.g. Seeley, 1876; first appear in the fossil record in Campanian (latest Hooley, 1905), frogs (Holman & Harrison 2003) and Cretaceous) deposits in North America (Brochu, 1999). squamates (e.g. Hoffstetter, 1942; Holman et al., 2006). After diverging into alligatorines and caimanines The first report of crocodylian remains from the beds approximately 66 Mya (Brochu, 1999; Roos et al., 2007; at Hordle Cliff was presented by Edward Charlesworth and possibly earlier: see Bona et al., 2018), alligatoroids on behalf of Searles Wood in the Annals and Magazine of dispersed in the Palaeocene, with caimanines present in Natural History in 1844. This short account reported the South America (e.g. Brochu, 2011), alligatorines living discovery of: ‘A great portion of the head of an alligator, alongside basal alligatoroids in North America (e.g. having nearly all the upper range of teeth (42 in number) Brochu, 1997) and basal alligatoroids in both Asia (Wang remaining, along with the humerus, dermal scutæ and et al., 2016) and Europe (Martin et al., 2014; Puértolas- other parts of the skeleton’ and that ‘Mr. Wood proposes to Pascual et al., 2016). call the Hordwell alligator A. Hantoniensis’ (Wood, 1844: Diplocynodon is a genus of basal alligatoroid p. 351). A second account of Hordle Cliff was published by (Brochu, 1999) whose fossil record extends from the Wood (1846: p. 6), in which he reported, under the genus late Palaeocene to the middle Miocene of western Alligator, that: ‘A considerable portion of the skeleton of Europe (Martin, 2010; Martin & Gross, 2011; Delfino a crocodile, resembling the New World type, for which & Smith, 2012; Martin et al., 2014; Díaz Aráez et al., I propose the specific name Hantoniensis’. Wood (1846: 2017). Nine species are currently considered valid, pls 1, 6, 7) also figured some cranial remains, a femur comprising: (1) Diplocynodon remensis Martin et al., and a cervical vertebra, under the name ‘Alligator 2014 from the late Palaeocene of France, (2) the middle Hantoniensis’. Because the 1844 publication lacks an Eocene species D. darwini Ludwig, 1877 (Hastings & accession number or figures to identify the material, Hellmund, 2015) and D. (Baryphracta) deponiae Frey Wood (1846) is the accepted authority for the name et al., 1987 (Delfino & Smith, 2012) from Germany Alligator hantoniensis. and D. tormis Buscalioni et al., 1992 from Spain Both the Marchioness of Hastings and Owen (3) D. elavericus Martin, 2010 and D. hantoniensis presented new crocodylian remains from Hordle Cliff (Wood, 1846) from the late Eocene of France and the at the Seventeenth Meeting of the British Association UK, respectively, (4) D. (Hispanochampsa) muelleri for the Advancement of Science held at Oxford in June Kälin, 1936 from the early Oligocene of Spain (Piras & 1847, which were published as reports the following Buscalioni, 2006), (5) the type species D. ratelii Pomel, year (Hastings, 1848; Owen, 1848). According to these 1847 from the early Miocene of France (Vaillant, 1872) reports, Lady Hastings exhibited two crocodylian skulls and Spain (Díaz Aráez et al., 2017) and possibly from from Hordle Cliff, that Owen (1848: p. 66) argued more the early Oligocene of France (Brinkmann & Rauhe, closely resembled modern crocodiles than alligators or 1998) and Italy (Kotsakis et al., 2004) too and (6) D. gavials, based on ‘the festooned contour of the alveolar ungeri Prangner, 1845 from the middle Miocene of border of the upper jaw’ and that ‘the inferior canines Austria (Martin & Gross, 2011) and France (Ginsburg rest in grooves or notches upon the outer margin of the & Bulot, 1997). Whereas remains of nearly all of these upper jaw when the mouth is closed, and their entire species of Diplocynodon have been described or revised crowns are exposed’. Consequently, ‘Owen proposed recently, one of the earliest named, Diplocynodon to call the extinct species from Hordle Crocodilus hantoniensis, has not been revised for over 150 years. Hastingsiae, in honour of the accomplished lady by The Headon Hill Formation is a late Eocene whom the singularly perfect examples of the species had (Priabonian) freshwater unit in the Hampshire Basin been recovered and restored’ (Owen, 1848: p. 66). This that crops out in southern England and the Isle of report also noted that Mantell, who was in the audience, Wight (Edwards & Daley, 1997; Daley, 1999). One mentioned the presence of ‘a fossil species of Alligator mainland locality, Hordle (=Hordwell) Cliff, near the (Alligator Hantoniensis) described by Mr. Searles Wood, town of Lymington (Hampshire) has yielded abundant from a specimen of the lower jaw and teeth found in fossil remains from several beds of the lowermost the freshwater beds at Hordwell Cliff ’, but that Owen unit of the Headon Hill Formation: the Totland Bay ‘had not seen this specimen’ (Owen, 1848: p. 66). The Member (formerly the ‘Lower Headon Beds’; Edwards year associated with the first appearance of ‘Crocodilus & Daley, 1997). These remains were first reported Hastingsiae’ in the literature is, therefore, 1848 (see in a series of papers by Wood (1844, 1846), Owen also: Gramann, 1958), not 1847 as is often reported in (1848), theMarchioness of Hastings (1848, 1853) and the literature (e.g. Woodward, 1885). Wright (1851). In addition to numerous invertebrate Owen (1850) subsequently described and figured this and plant fossils (e.g. Wood, 1846; Hastings, 1853; material as Crocodilus Hastingsiae. He also commented Chandler, 1961), a diversity of vertebrates has also © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 3 upon a suite of similarities with the type specimen of Switzerland (Pictet et al., 1857), Germany (Gramann, Alligator hantoniensis, noting that the skulls of these two 1958), France (Vignaud et al., 1996) and the USA taxa were almost indistinguishable. However, the one (Weems, 1999). Finally, we outline several anatomical notable difference pertained to the nature of the occlusion features of the usually neglected postcranial skeleton, of the jaws. Whereas the fourth dentary tooth in the including numerous muscle scars that could form the skulls of Crocodilus hastingsiae occludes into a notch (i.e. basis for future phylogenetic characters. it remains visible with the jaws closed), that of Alligator hantoniensis occludes into a pit (i.e. it is obscured with the jaws closed). Although Owen (1850) suggested that Institutional abbreviations this difference might merely reflect individual variation, AMNH, American Museum of Natural History, he was also wary that this was a feature consistently New York, USA; CAMSM, Sedgwick Museum, used to distinguish modern crocodiles from alligators. Cambridge, United Kingdom; FMNH, Field Museum As such, he maintained that Crocodilus hastingsiae of Natural History, Chicago, USA; HLMD, Hessisches represents a crocodile, rather than an alligator, and Landesmuseum, Darmstadt, Germany; IRSNB, tentatively retained Alligator hantoniensis as a distinct Institut Royal des Sciences Naturelles de Belgique, taxon. The crocodylian-bearing bed was subsequently Brussels, Belgium; MACN, Museo Argentino de informally named the ‘Crocodile Bed’ by Wright (1851) Ciencias Nauturales ‘Bernardino Rivadavia’, and this fine white sandstone unit is one of the most Buenos Aires, Argentina; MNHN, Muséum national fossiliferous layers at Hordle Cliff (Wright, 1851). d’Histoire Naturelle, Paris, France; NHMUK, Natural Pomel (1853: p. 124) referred Alligator hantoniensis to History Museum, London, United Kingdom; QM, the Diplocynodon on the basis of shared features with the Queensland Museum, Brisbane, Australia; SMF, French species Diplocynodon ratelii, which he had named Senckenberg Naturmuseum, Frankfurt, Germany. several years earlier (Pomel, 1847). However, this new combination, Diplocynodon hantoniensis, seems to have gone unnoticed for the next three decades. Meyer (1857: p. 538) also noticed close affinities between the French and Systematic Palaeontology UK remains, but did not make any taxonomic referral. Eusuchia Huxley, 1875 Subsequently, Huxley (1859) described the dermal armour Crocodylia Gmelin, 1789 of the Hordle Cliff crocodylian. He contended that only one Alligatoroidea Gray, 1844 crocodylian was present at this locality and that this taxon should be referred to as Crocodilus hastingsiae. Woodward Diplocynodon Pomel, 1847 (1885) also only recognized one taxon and argued that the Diplocynodon Hantoniensis Wood, 1846 differences in occlusion were likely individual variation. Although Woodward (1885: p. 509) commented that the Alligator hantoniensis Wood, 1846 Hordle Cliff crocodylian ‘combines the characters both Crocodilus hastingsiae Owen, 1848 of Crocodiles and Alligators to a remarkable extent; and Crocodilus hastingsiae Owen, 1850 it also possesses some features that would quite entitle it to rank as a distinct genus’, he also made no mention Diplocynodon hantoniensis Pomel, 1853 of Pomel’s (1853) referral to Diplocynodon, continuing to Crocodilus hastingsiae Huxley, 1859 refer to the material as Crocodilus hastingsiae. Finally, Crocodilus hastingsiae Woodward, 1885 following Pomel (1853), Lydekker (1887) argued that this material should be referred to Diplocynodon hantoniensis Diplocynodon hantoniensis Lydekker, 1887 (see also: Lydekker, 1888). The latter assignment has since Diplocynodon hantoniensis Buscalioni et al., 1992 been followed in the literature and was supported in later Diplocynodon hantoniensis Brochu, 1999 phylogenetic analyses (e.g. Buscalioni et al., 1992; Brochu, 1999), with the differences in occlusal morphology likely to reflect ontogenetic variation (Brochu, 1999). Holotype: NHMUK OR 25166 (holotype of ‘Alligator Here, we provide the first revision of Diplocynodon hantoniensis’), anterior portion of a skull (Fig. 1). Type hantoniensis since Owen (1850), presenting a locality: Hordwell Cliff, near Lymington, Hampshire, comprehensive description of all cranial and postcranial United Kingdom. Type horizon: Totland Bay Member, remains from Hordle Cliff accessioned in NHMUK Headon Hill Formation; Priabonian (late Eocene) and CAMSM. We provide a new diagnosis for the (Edwards & Daley 1997; Daley, 1999). species and incorporate our anatomical observations into a recent phylogenetic analysis. We also briefly Synonym: Crocodilus hastingsiae Owen, 1848. comment on referrals to Diplocynodon hantoniensis Crocodilus hastingsiae (Owen, 1850; Huxley, 1859; from approximately contemporaneous deposits in Woodward, 1885). © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 4 J. P. RIO ET AL. Figure 1. The skull of Diplocynodon hantoniensis, holotype of ‘Alligator hantoniensis’ (NHMUK OR 25166) in dorsal (A, B) and ventral (C, D) views. Abbreviations: en, external naris; ch, choana; fr, frontal; if, incisive foramen; lac, lacrimal; mx, maxilla; na, nasal; occ, occlusal pit; pa, palatine; pmx, premaxilla; pmx 1, premaxillary tooth 1; pt, pterygoid; sof, sub-orbital fenestra. Scale bar = 10 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 5 Holotype: NHMUK OR 30393, from the same locality OR 30234, OR 30235, OR 30241); metapodials (R 1053 and horizon. partim, R 5267 partim, OR 25244 partim, OR 25254, OR 30240, OR 30242 partim, OR 30243, OR 30246, OR 30247 Referred material (NHMUK): Skulls (R 1041, R 1042, partim, OR 30336); phalanges (R 1053 partim, R.1054 OR 25167, OR 25170.a, OR 29694, OR 30287 partim, OR partim, R 5267 partim, OR 25243). CAMSM: skulls (TN 30392, OR 30414 partim); rostrum fragment (OR 30381 907 partim, TN 917, TN 918 partim); lower jaws (TN 904; partim); premaxillae (OR 30289 partim, OR 30369, OR TN 907 partim); dentaries (C31041, C31042, C31043, 30370, OR 30414 partim); maxilla (R 5010 partim, OR plus 8 unnumbered specimens); angulars (C31044, 25168, OR 25169, OR 25170, OR 30289 partim); nasal 31045); cervical vertebrae (C31026–40); dorsal vertebrae (OR 30289 partim); jugals [OR 25188 (with partial (C31012–25); caudal vertebrae (C30999–31011); quadratojugal), OR 25219, OR 30289 partim]; quadrates osteoderms (C31046–95; C31352–87). (OR 25187, OR 25193. OR 30372, OR 30382, OR 30414 partim); pterygoid [OR 30251 (with fragmentary Revised diagnosis: Diplocynodon hantoniensis can be ectopterygoids and palatine)]; ectopterygoids [OR 25216 diagnosed by the following unique combination of characters (with pterygoid fragment), OR 25217]; basioccipital (OR (autapomorphies marked by an asterisk): (1) premaxillary 36812); lower jaws [R 1043, R 1045 partim, R 5267 partim, surface posterolateral to naris with a deep notch; (2) short OR 25178, OR 30274, OR 30381 partim, OR 30394, OR anterior ectopterygoid process, which forms the medial 30396 (two rami in connection), OR 30397]; dentaries (R wall of the posteriormost maxillary alveolus, before veering 1044, R 5010 partim, R 5121, R 5212, R 5216, R 5217, OR medially to the suborbital fenestra*; (3) ectopterygoid– 30318 partim, OR 30321, OR 30287 partim, OR 30289 pterygoid flexure retained throughout ontogeny*; (4) partim, OR 30338, OR 30365, OR 30378, OR 30383); surangular–angular suture meets the articular dorsal angulars (R 5217 partim, OR 30285, OR 30289 partim, to its ventral tip in the mandibular adductor chamber*; OR 30307, OR 30308, OR 30318 partim, OR 30414 (5) broad preorbital ridge on the lacrimal; (6) prominent partim); articulars [OR 25192, OR 30282, OR 30283, OR laminae and sulci either side of the choana. 30284, OR 30388 partim, OR 30289 partim, OR 30363, OR 30363.a, OR 30388 partim (joined with 30289), OR 30414 partim]; surangular (OR 30414 partim); tooth- Description: skull bearing bone (OR 25260); isolated teeth (R 1043, R 5009, General proportions, preservation and ornamentation: R.6323, R 6852, OR 25165, OR 25598, OR 30317 partim); Several remains represent portions of the skull, proatlas (OR 30289 partim); odontoid process (OR 25186); including three nearly complete skulls: NHMUK OR axes (two unnumbered specimens); vertebrae (R 1041, R 30392, NHMUK OR 30393 (Figs 2–4) and CAMSM 1045 partim, R 1046, R 1047, R 1048, R 1049, R 1050, TN 907. All three of these skulls are large: NHMUK R 1051, R 1052, R.1068 partim, R 5213, R 5267 partim, OR 30392 is 375 mm long from the anterior tip of the OR 25175, OR 25177, OR 25176, OR 25179, OR 25180, premaxillae to the posterior margin of the skull table OR 25181, OR 25182, OR 25195, OR 25199, OR 25201, (measured along the midline on the skull surface), with OR 25207, OR 25208, OR 25209, OR 25210, OR 25213, NHMUK OR 30393 and CAMSM TN 907 comparable OR 25214, OR 25215, OR 25222, OR 25223, OR 25224, in size (Table 1). The preserved portion of NHMUK OR OR 30289 partim, OR 30402 partim, plus unnumbered 25166 also indicates a similarly sized individual. At specimens); ribs (R.1068 partim, OR 25191, OR 25248, the other end of the spectrum, NHMUK OR 25170.a is OR 30402 partim, OR 30414 partim); osteoderms (R 1043, a partial juvenile skull that is about 60 mm long from R 1045 partim, R 1068 partim, R 5214, R 5267 partim, the anterior tip of the maxillae (the premaxillae are OR 25174, OR 30318 partim, OR 30366, OR 30367, OR not preserved) to the posterior edge of the skull table. 30368, OR 30388 partim, OR 30414 partim, plus one Preservation varies significantly from specimen to unnumbered specimen); scapulae (OR 30247 partim, OR specimen, with different degrees of completeness, but 30414 partim); coracoids (OR 25245, OR 30359); humeri deformation is generally minor. For example, the nasals of (OR 25237, OR 30206, OR 30206.a, OR 30219, OR 30247 NHMUK OR 30393 are slightly lowered in respect to the partim); radii (OR 25244 partim, OR 30401); ulnae (OR maxillae. Some partial skulls, including NHMUK R 1042, 25239, OR 30236, OR 30237, OR 30247 partim, OR OR 25170a and OR 29694, are still partly embedded in 30389); radiale (R.1054); ilia (OR 25252, OR 25253, OR matrix, whereas others have been internally reinforced 30362, OR 30414 partim); ischia (OR 30354 partim); with plaster or a cement-like material, including NHMUK femora (R 5215, R 5267 partim, OR 25231, OR 25232, OR 30392. The surfaces of the two best-preserved skulls OR 25238, OR 25244 partim, OR 30210, OR 30211, OR have been partly covered by plaster in order to provide 30213, OR 30214, OR 30222, OR 30223, OR 30228, OR the impression of completeness. 30247 partim, OR 30399, OR 30414 partim); tibiae (R The overall appearance of the skull is that of a 5267 partim, OR 25236, OR 30215, OR 30216, OR 30217, generalized crocodylian, with the rostrum (the region OR 30242, partim, OR 30414 partim); fibulae (OR 30233, anterior to the orbits) representing about 65% of the © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 6 J. P. RIO ET AL. Figure 2. The skull of Diplocynodon hantoniensis (NHMUK OR 30392) in dorsal (A, B) and (C, D) ventral view. Abbreviations: amp, scar for musculus adductor mandibulae posterior; bo, basioccipital; ch, choana; en, external naris; fr, frontal; itf, infratemporal fenestra; jg, jugal; lac, lacrimal; mef, median eustacian foramen; mx, maxilla; na, nasal; ob, orbit; oc, occlusal pit; pmx, premaxilla; pf, prefrontal; po, postorbital; qd, quadrate; qj, quadratojugal; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra. Scale bar = 10 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 7 Figure 3. The skull of Diplocynodon hantoniensis (NHMUK OR 30392) in lateral (A, B) and occipital (C, D) views. Abbreviations: 5, 6, maxillary tooth numbers; bo, basioccipital; bs, basisphenoid; eam, external auditory meatus; ect, ectopterygoid; eo, exoccipitals; fm, foramen magnum; fr, frontal; itf, infratemporal fenestra; jg, jugal; lac, lacrimal; leu, lateral eustachian foramen lsg, lateral squamosal groove; meu, median eustachian foramen; mx, maxilla; pf, prefrontal; pmx, premaxilla; po, postorbital; pt, pterygoid; qd, quadrate; qj, quadratojugal; so, supraoccipital; sq, squamosal; vf, vagus foramen; XII, foramen for cranial nerve 12. Scale bar = 5cm. entire length of the skull. There is no medial dorsal boss Cranial fenestrae and openings: The dorsally facing on the rostrum. Canthi rostrali are clearly absent, but naris is entirely hosted within the premaxillae (Figs preorbital ridges are prominent in adult individuals 1, 2). It is approximately circular and has a rounded, (see description of the lacrimal). In posterior view, convex anterior rim, nearly straight lateral rims the skull table is approximately planar, except for a (especially in NHMUK OR 25166) and a concave modest depression in mature specimens (e.g. NHMUK posterior rim (as a result of an anterior projection of OR 30392 and 30393) (Fig. 3D), at the dorsal exposure the medial edge of each premaxilla). The rim of the of the supraoccipital. The lateral sides of the skull external naris is flush with the dorsal surface of the table are convex in juvenile specimens (NHMUK OR surrounding premaxillae, which is posterolaterally 25167 and OR 25170.a), and the paroccipital processes marked by a depression that is nearly as developed have short squamosal rami (Figs 5, 6). However, the as the one that characterizes Alligator (Figs 1B, 2B). lateral sides are straight or slightly concave in adult Whereas the lateral walls of the naris are nearly specimens (NHMUK OR 30392 and 30393), in which vertical, the anterior walls are very shallowly inclined the squamosal rami are long. posteroventrally. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 8 J. P. RIO ET AL. Figure 4. Articulated skull and mandibles of Diplocynodon hantoniensis (NHMUK OR 30393) in dorsal (A, B) and lateral (C, D) view. Abbreviations: art, articular; dt, dentary; emf, external mandibular fenestra; en, external naris; fr, frontal; itf, infratemporal fenestra; jg, jugal; lac, lacrimal; mx, maxilla; na, nasal; ob, orbit; pmx, premaxilla; pf, prefrontal; pof, preotic foramen; po, postorbital; qd, quadrate; qj, quadratojugal; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra; su, surangular. Scale bar = 5 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 9 Table 1. Skull measurements of Diplocynodon hantoniensis (mm) Feature NHMUK OR 30393 NHMUK OR 25166 NHMUK OR 30392 CAMSM TN 907 Dorsal skull length 375 NA 375 344 Narial length 37 40 33 41 Narial width 29 40 35 42 Snout length along medial axis 255 240 240 213 Rostrum width at notch/pit 97 102 84 82 for fourth dentary tooth Orbit length 60 NA 59 58 Interorbital width 25 NA 23 25 Supratemporal fenestra length 46 NA 44 41 Supratemporal fenestra width 25 NA 26 28 Skull table anterior width 86 NA 79 77 Maximum skull width 227 NA 220 215 across quadrates The dorsal margin of each orbit is composed of the The infratemporal fenestrae are triangular in shape frontal, the posterior margin by the postorbital, the and are approximately as long as the orbits, although posterolateral and lateral margins by the jugal, and their surface area is smaller overall than that of the the anterior margin is formed by the lacrimal and orbits. The most completely preserved infratemporal prefrontal. The orbits are deeply concave medially, but fenestra is the right opening of NHMUK OR 30392, straight or slightly convex laterally, and are truncated which is missing only a very small portion of the by the postorbital bars posteriorly. The orbital rim is quadratojugal, close to the postorbital (Fig. 3B). The flush with the jugal surface (a hint of a crest is present jugal forms the ventral margin and anterior angle of the in NHMUK OR 30392), but raised into a crest along the infratemporal fenestra, and the dorsal angle appears prefrontal edge. In the juvenile partial skull NHMUK to be formed by the quadratojugal (CAMSM TN 918). OR 25170a, the posterolateral edge of the prefrontal From the juvenile specimen NHMUK OR 25167 (Fig. is extremely high and forms a vertical laminar ridge 6), it is clear that the quadratojugal formed the entire delimiting the anteromedial surface of the orbit (Fig. posterior corner of the infratemporal fenestra. 5). It is not clear if palpebral bones were originally The external auditory meatus is clearly visible absent or are simply not preserved in any available in NHMUK OR 30392 and 30393 (Figs 3B, 4D) and specimen. has a cloverleaf shape. Whereas the dorsal margin The supratemporal fenestrae are oval-shaped, with of the external auditory meatus is composed of the their long axes oriented anteroposteriorly. They are squamosal, the quadrate forms the bowed posterior much smaller than either the orbits or infratemporal rim, as well as the ventral and anterior margins. The fenestrae in adult specimens, but comparatively quadrate–squamosal suture extends dorsally along the large in the juvenile individual NHMUK OR 25170.a, posterior margin of this opening. The quadrate forms which has a very broad supratemporal fossa. The a narrow ridge on the floor of the external auditory medial rim of each supratemporal fenestra is slightly meatus, extending medially into the ear canal. A large raised into a ridge in the largest specimens, with a preotic foramen pierces the quadrate anterior to the depressed interfenestral bar (Figs 3B, 4B). However, external auditory meatus (Fig. 4D). in smaller specimens (e.g. NHMUK OR 25167 and The elliptical foramen magnum, delimited by the 25170a) the rim of the supratemporal fenestra is exoccipitals and the basioccipital, is equal in width to nearly flush with the parietal surface (Figs 5, 6). the occipital condyle (Fig. 3D). Post-temporal fenestrae Whereas the posterior and medial fenestral margins are rather small: they are delimited by the parietal are curved, the anterolateral margins are straight and squamosal dorsally, and by the supraoccipital and the fenestrae taper in width anteriorly to form an and exoccipital ventrally. The lateral carotid foramen acute corner (Fig. 2B). The rim of each supratemporal opens lateral to the basisphenoid, and the small fenestra does not significantly overhang the lateral Eustachian canals open dorsally to the large, supratemporal fossa. A shallow fossa occurs at the medial canal. anterior corner of the supratemporal fenestra. The The suborbital fenestrae are fully visible in NHMUK medial parietal wall of the supratemporal fenestra OR 30392 (Fig. 2D) and CAMSM TN 907. Anteriorly, is imperforate. they reach the level of the ninth maxillary alveolus, © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 10 J. P. RIO ET AL. but reach the tenth alveolus in NHMUK OR 25166 (Fig. 1D). The lateral edge of the suborbital fenestra is straight, not bowed medially at the ectopterygoid– maxilla suture. Each suborbital fenestra has a posterior notch (best observed on the right opening of NHMUK OR 30392). The posterior corner of the suborbital fenestra is formed by the pterygoid, with the pterygoid–ectopterygoid suture situated on the posterolateral margin of the fenestra. Anteriorly, the suborbital fenestra tapers to an acute tip. The anterolateral margin of the fenestra is longer than the posterolateral margin. The incisive foramen is relatively large and is located far from the tooth row, with the anterior rim corresponding to the third alveolus, and the posterior margin corresponding to the fourth or fifth alveolus. Whereas this foramen is circular in NHMUK OR 30392 and CAMSM TN 907, it is teardrop-shaped in NHMUK OR 25166, with an acute anterior margin, resulting from the intrusion of the occlusal pits for the first dentary teeth. The internal choana is best preserved in CAMSM TN 907 and the juvenile specimen NHMUK OR 25167 (Fig. 6D). The posteroventrally oriented choana is heart-shaped and situated entirely within the posterior region of the pterygoids, with its posterior rim close to the posterior edge of the pterygoids. The anterior rim of the choana is flush with the pterygoid surface, whereas the lateral rim develops a large lamina that separates the choana from a slightly depressed area. In addition, the midline of the anterior margin of the choana is interrupted by a posteriorly directed process of the pterygoid. This is observed in all specimens where preserved. In NHMUK OR 30392, this process extends far posteriorly into the choana, although it does not fully bisect it. The posterior rim of the choana, preserved in NHMUK OR 25167 and CAMSM TN 907, is smooth, lacking the sharp notch that characterizes Caiman, but there is a small concavity on the posterior edge. Although slightly incomplete in its median sector, these specimens show the presence of a sagittal bony septum recessed within the choana. Premaxilla: The premaxillae completely encircle the external naris. They are characterized by five alveoli, even in adult specimens. Palatal laminae are flat, with no obvious ornamentation, except for a slightly rugose texture leading up to the alveolar necks (e.g. NHMUK OR 25166). Numerous small foramina pierce the premaxillary palate, adjacent to the alveoli. The dorsal Figure 5. Juvenile skull of Diplocynodon hantoniensis process of the premaxilla is relatively short and wide, (NHMUK OR 25170a) in dorsal view. Abbreviations: fr, posteriorly reaching the level of the second or third frontal; jg, jugal; lac, lacrimal; mx, maxilla; na, nasal; maxillary alveolus. The premaxilla–maxilla suture ob, orbit; pmx, premaxilla; pf, prefrontal; po, postorbital; is slightly irregular, rather than straight. One large qd, quadrate; so, supraoccipital; sq, squamosal; stf, reception pit for the first dentary tooth occurs lingually, supratemporal fenestra. Scale bar = 1 cm. in between the first and second premaxillary alveoli. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 11 Figure 6. Juvenile skull of Diplocynodon hantoniensis (NHMUK OR 25167) in dorsal view (A, C). B, ventral view; D, enlargement of the choana in B. Abbreviations: bo, basioccipital; ect, ectopterygoid; fr, frontal; itf, infratemporal fenestra; jg.s, jugal suture; pa, parietal; po, postorbital; qd, quadrate; qtj, quadratojugal; sg, squamosal groove; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra. Scale bar = 5 cm in A, B and C, but 3 cm in D. Posteriorly, the premaxillae are separated along their septum. Confluent alveoli, in which the septum is not midline by the anterior process of the nasal. complete, having a V-shaped fenestration, is observed Maxilla: The well-preserved maxillary tooth rows of in one small, isolated maxilla NHMUK OR 25168–9. NHMUK OR 30392 host 17 alveoli on each side (Fig. In dorsal view, at the level of the fifth maxillary 2C, D). In ventral view, the tooth row has a slight alveolus, and slightly medially inset, there are two lateral concavity at the level of the sixth and seventh protuberances. On the ventral surface of the skull, the alveoli, but it is nearly straight posteriorly. The fourth maxilla terminates a significant distance anterior to and fifth alveoli are about the same size. They are the postorbital bar, forming a short edentulous process not fully confluent in any of the three best-preserved that is as long as the distance between the last two skulls and are separated by a thin (but complete) bony maxillary alveoli (Fig. 2D). The penultimate alveolus © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 12 J. P. RIO ET AL. is slightly less than twice the diameter of the last ridge is present in the smaller specimen NHMUK OR alveolus. The medial walls of the last two maxillary 30381 and a clear boss is present in the juvenile skull alveoli are formed by the ectopterygoid. NHMUK OR 25170.a, immediately anterior to the Nasal: An acute process of the nasal protrudes anterior orbital corner (Fig. 5A). anteriorly between the premaxillae, but does not reach Jugal: The jugal forms the ventral margin of the the external naris. In NHMUK OR 30392 (Fig. 2), this orbit and extends posteriorly past the level of the nasal process terminates about 20 mm posterior to the infratemporal fenestra (Fig. 4D). The lateral and external naris. The nasals broaden posteriorly along ventral margins of the jugal are heavily ornamented the maxilla–nasal suture, reaching a maximum width with pits, whereas the medial side is smooth. A very at their midlength. Whereas the nasal–prefrontal large medial jugal foramen is visible in the isolated suture is concave, the nasal–lacrimal suture is linear. juvenile jugal NHMUK 20219, the juvenile partial Lacrimal: The lacrimal forms the anterolateral rim skull NHMUK R 1041 and in NHMUK OR 30393. of the orbit. It is longer than the prefrontal (Fig. 7A). Three of the four isolated jugals catalogued as NHMUK OR 30381 preserves the right lacrimal duct, NHMUK OR 30289 have large medial foramina, but which is large and opens immediately lateral to the the fourth one shows a foramen of moderate size. Two lacrimal–prefrontal suture. The lacrimal and the nasal medial jugal foramina are visible in the isolated jugal appear to be in broad contact, lacking any posterior arch NHMUK OR 25188 (Fig. 7B) and in the left jugal process from the maxilla (Fig. 7A), although the of NHMUK OR 30392, although there is only one poor demarcation of sutures makes this morphology foramen on the right jugal. The ectopterygoid is sutured difficult to establish conclusively in the four largest onto the anteromedial side of the jugal, ventral to the skulls. Preorbital ridges are present in all three of the postorbital bar. The ventral portion of the postorbital adult skulls preserving the lacrimals: they are more bar that is formed by the jugal is inset from the dorsal prominently developed in NHMUK OR 30393 and edge of the jugal, with a moderately developed sulcus. CAMSM TN 907 than in NHMUK OR 30392. A low At the level of the postorbital bar there is a small step Figure 7. Selected skull details of Diplocynodon hantoniensis. A, NHMUK OR 30381, fragment of the rostrum showing the anterior extent of the prefrontal, frontal and lacrimal; B, NHMUK OR 25188, isolated left jugal in medial view; C, NHMUK OR 30251 isolated pterygoids and ectopterygoids in left lateral view. Abbreviations: fr, frontal; lc, lacrimal; pf. prefrontal. Scale bar = 4cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 13 in the dorsal outline of the jugal, reminiscent of the NHMUK OR 25170.a, the usual condition is observed. condition in Alligator and Caiman, and differing from In NHMUK OR 30381 and 29694, the elements may the straight dorsal outline of Crocodylus. Posteriorly, have been pulled apart at their sutures and filled with the jugal terminates in an acute process that is matrix. Consequently, this unusual morphology might sutured ventrolaterally to the quadratojugal. This merely be an artefact of preservation. process extends slightly further posteriorly than the Postorbital: The postorbital bar is incomplete in most level of the basioccipital tubera. specimens and often reconstructed with plaster. Prefrontal: The prefrontals are separated from one However, it appears strongly inclined laterally and another along the midline by the frontal process and inset from the lateral surface of the skull based on by the posterior tip of the nasals (Fig. 4B). Around NHMUK OR 30392 and CAMSM TN 918. The bar is the orbit, the prefrontal surface is raised into a ridge slender and circular in cross-section, with a short and (Figs 4, 5). The prefrontal pillars are poorly preserved low process. A deep fossa excavates the lateral surface and/or partially obscured in all specimens, limiting of the dorsal portion of the postorbital bar, pierced by anatomical observations. Ventrally, they form a suture a large foramen. The postorbital is in contact with a with the dorsolateral margin of the palatines, forming thin dorsal projection from the quadratojugal, but not two walls. with the quadrate (see NHMUK OR 30392, CAMSM Frontal: There is no distinct step at the base of the TN 918) (Fig. 3B). frontal process in two of the largest skulls, NHMUK Squamosal: The squamosals form the posterolateral OR 30392 and NHMUK OR 30393, and is very weakly corners of the cranial table and the posterolateral developed in CAMSM TN 907. However, a slight step margins of the supratemporal fenestrae. Their dorsal is present in both the juvenile skull NHMUK OR surfaces are flat in both juvenile (NHMUK OR 25167 25167 (Fig. 6) and the much larger, though not fully and 25170.a) and fully grown individuals (NHMUK grown, individual represented by NHMUK OR 30381 OR 30392, 30393, CAMSM TN 907). A prominent (Fig. 7A). A very steep slope is visible in the very small squamosal groove is delimited by subparallel rims. individual represented by NHMUK OR 25170.a (Fig. A linear series of foramina pierce the groove, as well as 5). The step in both NHMUK OR 25167 and 30381 the dorsal margin of the groove. Dorsal to the external is highlighted by the ornamentation of the frontal: it auditory meatus, there is a slight ventral deflection is pitted posterior to the step (similar to that of the along the ventral margin of the squamosal. The skull table), but smooth along the frontal process. The squamosal–quadrate suture is visible on the right side dorsal surface of the frontal is gently concave and of NHMUK OR 30393 (Fig. 4D), extending along the its dorsolateral (orbital) margin is slightly upturned. posterior margin of the external auditory meatus. In The anterior process of the frontal extends beyond lateral view, the posterodorsal edge of the squamosal the orbital margin, to the level of the anterior tip of is curved gently ventrally. The squamosal rami do not the prefrontal and the midlength of the lacrimal. It is reach the tip of the paroccipital processes. difficult to discern the anterior extent of the frontal Parietal: The parietal forms the medial margin of the relative to the jugal in NHMUK OR 30392 and 30393 supratemporal fossa and fenestra, a large portion of as a result of poorly defined sutures, as well as the the post-temporal fossa and the roof of the braincase. plaster that covers the dorsal surface. However, based Posterior to the supratemporal fenestra, the dorsal on NHMUK R 1042 and OR 29694, the anterodorsal tip surface of the parietal is concave and the interfenestral of the jugal lies just posterior to the anterior extent of bar has a sagittal sulcus (Fig. 4). The medial wall of the frontal process. In the juvenile specimen NHMUK the supratemporal fossa is steeply inclined, facing OR 25170.a, the anterior extent of the frontal and laterally and slightly dorsally, and the parietal rim of jugal appear to be approximately equal; however, the the supratemporal fenestra only slightly overhangs frontal process is not well preserved in this specimen. the supratemporal fossa. On the medial wall of On the skull table, the linear frontoparietal suture the supratemporal fenestra, the laterosphenoid– enters so deeply into the supratemporal fenestrae that parietal suture is horizontal. The post-temporal the parietal and postorbital are not in contact. fossa is prominent, forming a shelf that is principally Often in crocodylians, the anterior frontal process composed of the parietal, anterior to the post-temporal bisects the nasals for a short distance. The morphology fenestra. Laterally, the parietal approaches, but does in this area is extremely difficult to discern in the not contact, the squamosal on the posterior wall of the largest specimens of Diplocynodon hantoniensis (i.e. supratemporal fenestra. The parietal–quadrate suture NHMUK OR 30392, 30393 and 25166). In NHMUK undulates strongly. OR 30381 (Fig. 7A) (and possibly also NHMUK OR Quadrate: The quadrate condyle is characterized by a 29694), the frontal appears to invade the nasals small, ventrally deflected medial hemicondyle, and by anteriorly via two separate processes, between which a dorsal notch (open medially) that hosts the foramen the nares project posteriorly. However, in the juvenile aëreum (Fig. 3D). The latter opening is relatively small © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 14 J. P. RIO ET AL. and placed on the dorsal surface of the quadrate, far exoccipitals faces posterodorsally. The paroccipital from the mediodorsal edge. Whereas the articular process extends posterolaterally, sitting above the surface of the medial hemicondyle faces posteriorly posterior quadrate rami. The ventral margin of the and ventrolaterally, the lateral hemicondyle faces paroccipital process above the cranioquadrate canal is posteroventrally. On the midline of the ventral surface marked by a ventrally directed lamina. This lamina of the quadrate ramus, there is a well-developed terminates immediately medial to the cranioquadrate attachment scar for M. adductor mandibulae posterior canal, continuing towards the foramen magnum as (Holliday et al., 2013) [scar ‘B’ of Iorndansky (1973)] a faint horizontal ridge. Neurovascular foramina (Fig. 2D). This scar develops into a large sharp lamina, pierce the exoccipitals and are well preserved in projecting approximately 10 mm from the ventral NHMUK OR 30392, 30393 and 25166. Four foramina surface of the quadrate in NHMUK OR 30392 and are present on the right exoccipital of NHMUK OR NHMUK OR 30393. The scar continues ventrally 30392 (Fig. 3D). Two foramina occur close together at along the posterolateral braincase wall, although it the level of the foramen magnum for the hypoglossal becomes less pronounced. (XII) nerve. Slightly ventral and lateral to these Quadratojugal: The quadratojugal is well preserved on foramina is the vagus (X) foramen. Further ventrally, the right side of NHMUK OR 30392 and CAMSM TN close to the lateral margin the descending process of 918, in which it is represented by a small step situated the exoccipital, approximately at the dorsoventral far from the posterior corner of the infratemporal midpoint of the occipital condyle, is the much larger fenestra, immediately dorsal to the midpoint of the lateral carotid foramen. This foramen sits within a posterior fenestral margin (Fig. 2B). As can be observed fossa and appears to be orientated posteroventrally on NHMUK OR 25188 (Fig. 7B), a moderately developed and slightly laterally. process extends along the medial surface of the lower Basisphenoid and basioccipital: The basisphenoid temporal bar. The quadratojugal has a long, apically forms the anterior margin of the median eustachian very thin process that reaches the dorsal corner of the foramen and extends ventrally on to the posterior infratemporal fenestra. Posteriorly, the quadratojugal pterygoid processes. Ventral to the basioccipital, extends to the level of the quadratic condyles, forming the basisphenoid is anteroposteriorly very thin, but the lateral margin of the lateral hemicondyle. The dorsoventrally tall. The median eustachian foramen ventral suture with the jugal is approximately parallel opens between the basioccipital plate and the with the quadrate–quadratojugal suture. basisphenoid. The lateral eustachian foramina are Supraoccipital: The supraoccipital is well exposed poorly preserved; however, in NHMUK OR 30392, the on the posterior surface of the skull and delimits left foramen is visible, albeit filled with matrix (Fig. 3D). the ventromedial rim of the post-temporal fenestra. It occurs roughly in line with the median eustachian The dorsal exposure of the supraoccipital is clearly foramen, but slightly posterior to it. The external visible in the juvenile specimen NHMUK OR 25170.a surface of the basioccipital faces posteriorly, ventral (Fig. 5), in which it forms a broad and dorsoventrally to the spherical occipital condyle. A small vertical, short triangle. This triangle becomes taller in sagittal crest extends along the basioccipital plate. mature specimens, in which the sutures delimiting Braincase and orbitotemporal region: One unnumbered the supraoccipital are not easily discernible. The juvenile specimen in the CAMSM preserves some of posterior surface of the supraoccipital is excavated the braincase anatomy. The epipterygoid appears by two circular sulci that are separated by a vertical, to be absent in this specimen. There is little to no sagittal crest. Two processes project posteriorly from exposure of the prootic on the external braincase wall the supraoccipital, immediately ventral to the slit- ventral to the foramen ovale. The anterior margins like post-temporal fenestrae, and are most clearly of the laterosphenoid capitate processes are oriented observable in NHMUK OR 30393 and NHMUK OR obliquely relative to the midline. The well-formed 25167 (Fig. 6). These processes are not large enough to lateral laterosphenoid bridge is anteroposteriorly exceed the level of the posterior margin of the cranial thick, fully enclosing the ophthalmic foramen medially. table in the adult specimen NHMUK OR 30392, but The lateral surface of the laterosphenoid bridge is they are clearly visible in dorsal view of the juvenile excavated by a broad groove for the maxillary ramus of specimen NHMUK OR 25167. cranial nerve V, and there is no development of a caudal Exoccipital: The exoccipitals have a smooth paroccipital laterosphenoid bridge dorsal to the foramen ovale. surface devoid of any boss. They extend considerably The supraorbital foramen, which usually pierces the lateral to the posterior opening of the cranioquadrate laterosphenoid–quadrate suture at this level, cannot passage, but do not reach the basioccipital tubera be discerned. The pterygoid is broken away ventrally, ventrally (Fig. 3D). Lateral to the post-temporal but it is unlikely that any ascending process of the fenestra, the surface of the exoccipital is strongly pterygoid contributed to the laterosphenoid bridge concave. Overall, the occipital surface formed by the sensu Alligator. Although missing, the suture scar for © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 15 the pterygoid can be traced on the quadrate, and this the 15th maxillary alveolus (Fig. 2D). The ectopterygoid was most likely linear from the large, posterior, semi- forms the medial wall of the last maxillary alveolus lunate basisphenoid exposure to the foramen ovale. only. Anterior to this alveolus, the maxilla is so Only the posteriormost portion of the basisphenoid prominently developed medially that the ectopterygoid rostrum is preserved, which lacks a large lateral fossa, does not about the maxillary tooth row. As commented and is poorly exposed on the lateral braincase wall upon by Brochu (1997), this condition is closer to other anterior to the foramen ovale. The foramina for the alligatoroids than to the plesiomorphic crocodylian palatine ramus of cranial nerve VII are not preserved. condition. The posteroventral process is approximately Palatine: The palatines flare anteriorly, developing a twice the length of the anterior process. This lateral process projecting into the suborbital fenestra. posteroventral process does not reach the tip of the The palatine–maxilla suture enters the suborbital pterygoid at maturity; instead it terminates beyond fenestra at its anteromedial margin. The anterior tip midlength of the pterygoid wing. The ectopterygoid– of the palatine, truncated and with a slightly convex pterygoid flexure is clearly visible in the juvenile skull edge, reaches the level of the seventh interalveolar NHMUK OR 25167, as well as the pterygoids and space (i.e. anterior to the suborbital fenestra), associated ectopterygoids (NHMUK OR 30251) (Fig. without tapering. The palatines do not significantly 8A) of a fully-grown individual that might belong with flare posteriorly, and thus do not form a palatine NHMUK OR 30393. Based on NHMUK OR 30251, shelf, such as the one seen in Alligator; instead, the the ventrolateral surface of the ectopterygoid, near lateral margins of the palatines are roughly parallel the margin of the suborbital fenestra, is pierced by posteriorly. The palatine–pterygoid suture is well two foramina on the right side and three on the left. preserved in NHMUK OR 25166, 30392 (Figs 1, 2) The ectopterygoid has a blunt posterior process on the and CAMSM TN 907; in all specimens it is situated medial surface of the jugal arch (Fig. 7B), and sends far anterior to the posterior corner of the suborbital an ascending branch along the medial surface of the fenestra, but varies in shape from being straight to postorbital bar. Immediately anterior to the sutural prominently ‘W’ shaped. surface for the jugal, on the anterior branch of the left Ectopterygoid: The relationship between the ectopterygoid, there is a large foramen. ectopterygoid and the maxilla was evaluated on Pterygoid: The pterygoid forms the posterior the basis of NHMUK OR 30392, the only specimen margin of the suborbital fenestra. The pterygoid– preserving the posterior region of the maxillary tooth ectopterygoid suture, visible in lateral view, intersects row. The anterior process of the ectopterygoid is the posterolateral/posterior edge of the suborbital pointed, reaching the level of the anterior margin of fenestra. The pterygoid wings are large and robust. Figure 8. Comparisons of the ectopterygoid–pterygoid suture in Diplocynodon hantoniensis and other crocodylians: A, D. hantoniensis (NHMUK OR 30251); B, Caiman yacare (AMNH 97300); C, Crocodylus acutus (FMNH 69884). White arrows indicate the position of the ectopterygoid–pterygoid flexure. Scale bars = 2 cm in all specimens. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 16 J. P. RIO ET AL. Their lateral margins are thickened and expanded dentary–angular suture reaches about the midlength dorsoventrally, with a rugose texture. A distinct of the ventral rim of the fenestra. The angular– sulcus runs parallel and dorsal to the suture with surangular suture intersects the fenestra ventral to the ectopterygoid along the lateral surface of the the posterodorsal angle. pterygoid (Fig. 7C). Two foramina are present within Dentary: The best-preserved dentaries (i.e. NHMUK the fossa on the left side of NHMUK OR 30251: a OR 30396, 30397) each host 20 alveoli. The dentaries large posterior foramen, filled with matrix, and a are gently curved between the fourth and 11th alveoli. much smaller anterior foramen. An equivalent small There is some variation in the posterior extent of the anterior foramen is also present on the right side. dentary symphysis: it extends to the middle of the Anteriorly, the pterygoid is sutured to the palatine. fourth alveolus in NHMUK OR 25178 and 30394 (Figs Dorsally, they form the dorsolateral walls of the 9A, 10A), to the posterior rim of the fourth alveolus nasopharyngeal passage. The two halves of the in NHMUK OR 30397 and to the middle of the fifth pterygoids rise dorsomedially, meeting at the midline alveolus in NHMUK OR 30396 and R 1043. In lateral to surround the airways, forming a dorsoventrally view, the dorsal edge of the dentary gently undulates: tall and transversely very thin wall, immediately the concavities correspond to the second, seventh and below the level of the frontal. The posterior pterygoid 14th alveoli, whereas the convexities correspond to the processes project posterodorsally and slightly first alveolus, the third interalveolar space, and the 12th laterally. They are dorsoventrally tall and posteriorly alveolus. The jaw is also slightly festooned in dorsal prominent, and developed ventrally to the median view, with a main lateral convexity corresponding to eustachian foramen. The choanae open centrally the confluent third and fourth alveoli. Posteriorly, within the pterygoids. In posterior view, the pterygoid between the fourth and tenth alveoli, the jaw is slightly wings extend posteroventrally at around 30° from the concave laterally. The third and fourth alveoli are the horizontal. The dorsal surface of each wing is strongly largest, with the latter slightly the larger of the two, concave. This concavity results from the large rugose and are confluent with one another. Posteriorly, alveoli ‘buttress’ that defines the lateral edge of each five to nine are smaller and approximately equal in pterygoid wing, the dorsomedially directed lamina anteroposterior length. The next largest dentary that encloses the nasopharyngeal duct, as well as the alveolus is the 12th. The seventh interalveolar space posteromedial process. This concavity flattens out is at least a little larger than the surrounding ones in posteriorly towards where the pterygoid wing forms all specimens (for very clear examples, see NHMUK an acute process. OR 30287 and 30321). The Meckelian groove begins at the posterior end of the dentary symphysis, at approximately mid-height. In NHMUK OR 30397, a Mandible foramen perforates the dentary along the Meckelian Preservation: As with the crania, some lower jaws groove, at the level of the ninth dentary alveolus. are completely isolated [NHMUK OR 25188 (Fig. 9), Splenial: The splenials are well preserved in at least NHMUK OR 30394 (Fig. 10), NHMUK OR 30397, NHMUK OR 25178, 30394 and 30396. They are NHMUK OR 30396, CAMSM TN 904], whereas others not involved in the symphysis of the lower jaw and are preserved on a slab along with vertebrae, e.g. their anterior tip is located ventral to the Meckelian NHMUK R 1045. NHMUK OR 30396 is a fused pair groove (Fig. 9E, F). Given the lack of foramina in this of mandibles braced with a metal rod. Only one pair region, the mandibular ramus of cranial nerve V can of mandibles is associated with a cranium – NHMUK be inferred to have left the Meckelian groove at the OR 30393, but very few mandibular features can be anterior end of the splenial. There is a foramen at mid- discerned from this specimen. Across these specimens, height of the splenial, at the level of the 16th dentary nearly all elements of the mandible are preserved, alveolus in NHMUK OR 30394 (Fig. 10F) and possibly with the exception of the coronoid. A list of mandibular in NHMUK OR 25178 too. In NHMUK OR 30396, the measurements is presented in Table 2. splenial does not form the dorsomedial wall of the Mandibular fenestrae and major openings: The posteriormost dentary alveoli; however, in NHMUK external mandibular fenestra is moderately large OR 25178, the splenial approaches the medial wall of and oval-shaped, but the foramen intermandibularis the final three alveoli, and formed the wall at least in caudalis is barely visible in lateral view. The presence the last two. The anterodorsal margin of the foramen of the foramen intermandibularis medius cannot intermandibularis caudalis is open in all specimens be determined due to the absence of the coronoid where preserved, indicating that the splenial formed in all specimens. The dentary–surangular suture the anterior margin of this foramen. intersects this fenestra slightly anterior to the Angular: The angular of NHMUK OR 30394 (Fig. 10) is posterodorsal angle of the opening (very close to the well preserved. Towards its anterior end, the angular angle in NHMUK OR 25178) (Fig. 9C, D), whereas the extends dorsally to the foramen intermandibularis © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 17 Figure 9. Left mandibular ramus of Diplocynodon hantoniensis (NHMUK OR 25188) in dorsal (A, B); lateral (C, D); and medial (E, F) views. Abbreviations: 4, 12, 17, alveolus positions in the dentary; an, angular; art, articular; dt, dentary; emf, external mandibular fenestra; fa, foramen aerum; fo, foramen; mf, Meckelian fossa; oc, occlusal pit; sp, splenial; sp.s, suture scar for the splenial; su, surangular. Scale bar = 10 cm. caudalis, with a tip that is rather rounded and ridge occurs on the ventral margin of the angular, dorsoventrally tall (Fig. 10F). The lateral surface of the extending onto the lateral surface, and defines the angular is strongly ornamented with pits and furrows boundary between the anterior ornamented surface (Figs 9C, 10C). However, they do not continue onto the of the angular from the posteriorly smooth surface, ventral surface, nor do they extend onto the lateral beneath the retroarticular process (Fig. 11A). This surface of the retroarticular process. A longitudinal ridge is very prominent in NHMUK OR 30396, © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 18 J. P. RIO ET AL. Figure 10. Right mandibular ramus of Diplocynodon hantoniensis (NHMUK OR 30394) in dorsal (A, B); lateral (C, D); and medial (E, F) views. Abbreviations: 4, 12, alveolus positions in the dentary; an, angular; art, articular; co.s, suture scar for coronoid; dt, dentary; emf, external mandibular fenestra; fa, foramen aerum; fic, foramen intermandibularis caudalis; fo, foramen; mf, Meckelian fossa; oc, occlusal pit; sp, splenial; sp.s, suture scar for the splenial; su, surangular. Scale bar = 10 cm. NHMUK OR 30397 and CAMSM TN 904 but less markedly to the condition in all Crocodylus (Fig 11C) well-developed in NHMUK OR 30394 and 25178. and Alligator species, which lack a sharp ridge and in This feature probably marks the anterior limit of the which the ventral margin of the angular is very exposed insertion of M. pterygoideus ventralis, which inserts in lateral view. There is a large longitudinal sulcus along in a similar position in living caimans (Bona & Desojo, the ventromedial surface of the angular. This sulcus 2011), imparting the same ridge (Fig. 11B). This differs begins posterior to the foramen intermandibularis © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 19 Table 2. Mandibular measurements of Diplocynodon hantoniensis (mm) Feature NHMUK OR 30394 NHMUK OR 25188 Mandible length 480 469 Tooth row length 265 259 External mandibular fenestra length 62 60 External mandibular fenestra height 29 32 Width across glenoid fossa 42 42 Retroarticular process length 61 62 Mandible height at fourth tooth 39 37 Symphysis length 54 52 caudalis, and terminates approximately at its anterior margin. A series of large foramina occur posterior to this sulcus in all specimens except for NHMUK OR 30397, which is poorly preserved in this region. Whereas there are two foramina in NHMUK OR 30394, there are three in NHMUK OR 25178 and NHMUK OR 30396. The anterior extent of the angular is difficult to fully discern; however, it appears to reach the level of the 17th alveolus based on the articulated mandible NHMUK OR 30396 and the isolated left ramus NHMUK OR 25178. Posteriorly, the angular extends to the posterior end of the retroarticular process, along the ventrolateral margin, terminating in an acute process. Surangular: The anterior processes of the surangular are best preserved in in NHMUK OR 30381 (Fig. 12) and NHMUK OR 30396. They are subequal in length, with the dorsal process extending slightly further anteriorly than the ventral one. This morphology is confirmed by NHMUK OR 25178 (Fig. 9D), in which the dorsal process, despite being slightly incomplete anteriorly, is so thick and robust at its preserved anterior end that it had to be longer than the completely preserved ventral process. NHMUK OR 30394 preserves a broken ventral process of the Figure 11. Comparison of the posterior ramus of the mandible Figure 12. Posterior ramus of the right mandible of in lateral view in A, Diplocynodon hantoniensis CAMSM TN Diplocynodon hantoniensis (NHMUK OR 30381), lateral 904; B, Caiman latirostris Daudin, 1801 MACN V 1420; and C, view. Arrows indicate the anterior extent of the dorsal and Crocodylus siamensis Schneider, 1801 NHMUK 1921.4.1.168. ventral surangular processes. Abbreviations: an, angular; Black arrow indicates the boundary between the ornamented dt, dentary; emf, external mandibular fenestra; sur, and unornamented surface. Scale bar = 5 cm. surangular. Scale bar = 4 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 20 J. P. RIO ET AL. surangular (whose development is implied by the to attach to the medial surface of the articular in living scar it left on the accompanying dentary) and a very crocodylians, e.g. Alligator mississippiensis (Holliday long, tapering, laminar dorsal process (Fig. 10D). The et al., 2013). The medial edge of the retroarticular surangular spur between the dentary and splenial process is sharp, and overhangs the medial surface of is not completely preserved in any one specimen. In the articular as a shelf, as does the medial edge of the NHMUK OR 30394, the spur is mostly preserved and hemifossa. At the level of the transverse ridge, there visible sutured to the medial surface of the splenial is a large medial notch dividing the retroarticular (Fig. 10B). The posteriormost alveoli are not preserved process from the glenoid fossa. in this specimen, but the process most likely reached Pattern of occlusion: There are five alveoli in each at least the 17 th interalveolar space. This can be premaxilla, 17 in each maxilla and 20 in each dentary. confirmed in NHMUK OR 25178 and CAMSM TN All of the alveoli are approximately circular in cross- 904. In both specimens, the surangular spur is not section. None of the juvenile specimens preserve completely preserved. However, the scar for the spur the premaxilla, meaning that it is not possible to on the splenial is adjacent to the last three alveoli. directly assess the changes through ontogeny of the Just above the external mandibular fenestra and up relationships between the fourth dentary tooth and to the glenoid fossa, the dorsal edge of the surangular the region corresponding to the premaxillo-maxillary is flat and gently laterally sloping area (Fig. 9A, suture. However, the four fully grown specimens show B). Posterior to this smooth area, the surangular significant variation, suggesting that ontogenetically approaches the dorsal tip of the lateral wall of the the primitive condition was the one with the tooth glenoid fossa, though does not completely cover it in hosted in a pit, and that in some cases the outer lateral view. A very prominent sulcus occurs on the wall of the pit could be progressively eroded, giving dorsolateral margin of the surangular, at the level of origin to a notch. In particular, NHMUK OR 25166 the glenoid fossa (Fig. 9D). Posteriorly, the tip of the has a deep pit on the premaxilla between the last surangular does not reach the posterior tip of the alveolus and the suture with the maxilla (Fig. 1C, D). retroarticular process in either of the mandibular rami NHMUK OR 30392, 30393 and CAMSM TN 907 all of NHMUK OR 30396, but it is much more posteriorly clearly show a notch corresponding to the premaxillo- developed in NHMUK OR 25178 (Fig. 9D). The maxillary suture (Figs 2D, 3B). Such variability is also surangular–angular suture intersects the articular shown in isolated premaxillae NHMUK OR 30369 and close, but dorsal to the ventral tip (Fig. 9F). 30370, with a pit in the former and a lateral notch in The surangular–articular suture is very straight the latter. The left premaxilla of NHMUK OR 25166 in the posterior wall of the adductor chamber, lacking (the type of Diplocynodon hantoniensis) shows a very laminae and not recessed in a sulcus. A lingual foramen deep and large occlusal pit located lingual to the tooth for the articular artery, infilled with matrix, can be row, close to the suture with the maxilla; a second pit, discerned in all specimens. In NHMUK OR 30396 and much smaller but as deep, is placed lingual to the first CAMSM TN 904, the lingual foramen is situated on maxillary alveolus. The preservation on the right side the surangular–articular suture, whereas it is located of the same skull hinders the detection of the latter solely on the articular in remaining specimens (i.e. pit, but the one on the premaxilla is placed more in NHMUK OR 25178, 30394 and 30397). Posteriorly, the line with the alveoli than the corresponding pit on the surangular–articular suture is situated on the lateral left premaxilla. edge of the glenoid fossa, and is not bowed within the The disposition of the occlusal pits on the maxillae fossa. It also extends onto the dorsolateral surface of indicates that the dentary teeth occluded lingual to the the transverse ridge that separates the retroarticular maxillary teeth, except for those occluding in line after process from the glenoid fossa. At its posteriormost the sixth and seventh maxillary teeth. This is clearly extent, the suture forms the sharp lateral margin of shown in the maxillae of NHMUK OR 25166 (Fig. 1D), the retroarticular process. 25168–9 and 30392 (Fig. 2D), and is confirmed by the Articular: The retroarticular process is short and dentaries of NHMUK R1043, 25178, 30394 and 30397, oriented posterodorsally. The foramen aëreum all of which have occlusal pits labial to interalveolar is large and clearly inset from the margin of the spaces 14 and 15 (Figs 9B, 10B). retroarticular process. A sharp, vertical lamina occurs In the premaxilla, the third and fourth alveoli are for a short distance posterior to the foramen aëreum. the largest, with the latter slightly larger. The fifth On the medial surface, there is a prominent, wedge- alveolus is the smallest, and the first and second alveoli shaped sulcus, dorsally defined by a ridge. This runs are equally intermediate in size. Two large occlusal approximately parallel to the articular–angular suture, pits (one on each side) occur within the premaxillary and is perpendicular to the anteromedial corner of the palate, between the first and second premaxillary articular. This ridge was probably associated with the alveoli, and are separated posteromedially from the attachment of M. pterygoideus dorsalis, which is known incisive foramen by a thin straight wall. Only the left © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 21 occlusal pit is visible in NHMUK OR 30392, as the belong to one individual. We use this specimen as a right is infilled with matrix. In NHMUK OR 25166, basis for a description of the axial column. Diplocynodon the walls of the pit are very thin and worn. hantoniensis appears to have had nine cervical, 15 dorsal In the maxilla, alveoli four and five are the largest, and two sacral vertebrae, as in extant crocodylians and subequal in size, corresponding to the first lateral (Hoffstetter & Gasc, 1969). The number of caudal and ventral convexity of the maxilla. Alveoli six to vertebrae cannot be determined. None of the associated eight are smaller and subequal in size. Alveolus size cervical to sacral vertebrae of NHMUK OR 30402 show increases posteriorly from alveolus eight, reaching a any hint of neurocentral suture, suggesting that they second maximum at position ten and 11, which are belong to an adult individual. All of the vertebral centra subequal. Posteriorly from alveolus 11, the alveoli are fully procoelous, with the exception of the first sacral gradually decrease in size and remain circular. centrum that is concave anteriorly and flat to slightly In the dentary, the first alveolus is largest and the convex posteriorly and the second sacral centrum that second alveolus is considerably smaller. Alveoli three is nearly flat anteriorly and slightly concave posteriorly. and four are subequal and confluent; they are larger Atlas–axis complex: The boomerang-shaped proatlas than alveolus two, and slightly smaller than alveolus NHMUK OR 30289 is incomplete (Fig. 14G–I): the one. Alveoli five to ten are smaller than alveoli three right posteroventral tip is missing and the long and four. Alveoli 11 and 12 are larger than the preceding anterior process is partially damaged. When compared six alveoli. In some specimens, alveolus 13 is as large to the dorsal crest, the ventral tubercle is rather short as 11 and 12, e.g. NHMUK OR 30396. Posteriorly from in lateral view. The height of the dorsal keel cannot alveolus 13, the alveoli are subequal and smaller. be fully determined because it is partly damaged. No atlantal intercentrum is preserved. The odontoid process and the axis are preserved in Dentition connection only in NHMUK R 1045 (a block hosting NHMUK OR 25166 preserves a nearly continuous a fragmentary lower jaw, several cervical vertebrae, series of premaxillary and maxillary teeth in place. ribs and osteoderms), but the large odontoid process All premaxillary and anterior maxillary teeth are NHMUK OR 25186 (Fig. 14E, F) fits well in size, conical and lingually curved, with smooth carinae, and shape and colour with an unnumbered axis (Fig. commonly preserve faint longitudinal striations (Fig. 14A). The anterior surface of the axial centrum is 13A, B). Carinae occur on the medial and lateral edges flat for reception of the odontoid process, whereas of the anteriormost premaxillary teeth, but on the the posterior end forms a condyle. Two deep grooves, anterior and posterior edges of all other premaxillary pierced by foramina, excavate the lateral surface and maxillary teeth (Fig. 13D). of the centrum, delimiting a prominent ridge Posteriorly, the maxillary teeth become shorter, (Fig. 14A). The ventral surface of the centrum is more globular and less lingually curved, though never defined by a sharp midline keel. The unforked axial becoming blunt as they retain modest carinae and hypapophysis does not approach the anterior end of pointed tips (Fig. 13C). A small constriction separates the centrum. In lateral view, the neurocentral suture the tooth crown from the root, as in D. remensis (Martin is clearly visible. The neural canal is taller than et al., 2014) (Fig. 13E, F). wide. Whereas the circular prezygapophyses are Fewer dentary teeth are preserved in place (e.g. very small, postzygapophyses are much larger and NHMUK OR 30396). However, the same pattern is oval-shaped. In axial view, both sets of zygapophyses observed, with anteriorly very conical teeth, with are inclined at an angle of approximately 45° to smooth carinae, becoming shorter and more globular the horizontal. The axial neural arch bears an posteriorly. anteroventrally facing, sub-rounded facet directly ventral to the prezygapophyses, which contacts the odontoid process. The lateral margins of the neural Axial column arches are sculpted with fossae and ridges, and there Cervical, dorsal, lumbar, sacral and caudal vertebrae is no trace of a diapophysis on their lateral surfaces. are all preserved, as are postaxial ribs from these The axial neural spine is tall, with a consistent regions (excluding gastralia). Most vertebrae in the thickness along its length. Its dorsal margin is near- NHM collections are isolated, except for a continuous horizontal (although the anterior sector of the neural series of cervical to sacral vertebrae (NHMUK OR spine is broken off in NHMUK R 1045), and does not 30402), which have been manually assembled. It is develop any posterior crest. The posteriormost tip of unknown whether these associated vertebrae belong to the axial neural spine extends beyond the posterior one or multiple individuals. However, the preservation margins of the condyle and postzygapophyses. of the vertebrae is similar, and there is serial variation A fossa excavates the lateral surface of the axial in size and morphology, indicating that they might neural spine at midlength. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 22 J. P. RIO ET AL. margin is flat and wide. Assuming that the sequence of the associated series of vertebrae NHMUK OR 30402 is correct, the posteriormost hypapophysis is represented by a small elongated knob on the 12 th vertebra posterior to the atlas. The hypapophyses are not preserved in Cv3–5 of NHMUK OR 30402 (Fig. 15), but a raised, broken surface indicates that they were still present. Relatively short hypapophyses are consistently positioned towards the anterior margin of the centrum, and are closely flanked either side by parapophyses. In Cv7, 8 and Dv3, 4 of NHMUK OR 30402, hypapophyses are swept slightly anteriorly. This can also be observed in several well-preserved isolated cervical vertebrae (NHMUK R 1046, NHMUK OR 25177 and NHMUK OR 25210). However, Cv9 and Dv1 of NHMUK OR 30402, although slightly worn, have hypapophyses that are directed almost directly ventrally. The parapophyses in the first postaxial cervical vertebra (Cv3) are poorly preserved, but are situated very close to the hypapophyses. Moving posteriorly through the cervical vertebrae, the parapophyses remain approximately at the same level on the centrum. Parapophyses are preserved and articulated with cervical ribs in Cv4–7. Here, the parapophyseal articular facets are elliptical, with the long axis oriented anteroposteriorly. The parapophyseal facets are directed ventrolaterally, slightly more laterally than the diapophyses. In the tenth postaxial vertebra (Dv3), the parapophyses move dorsally to the base of the transverse process. In between the parapophyses and diapophyses, the lateral surface of the centrum has a prominent sulcus in Cv3–7. A lateral fossa is still present in Cv8 to Dv2, although it is much shallower and located posterior to the parapophysis on the lateral surface of the centrum. Diapophyses have small circular facets on Cv3, which are directed ventrally and slightly laterally. In subsequent cervical vertebrae, the diapophyseal facets become more elliptical, with the long axis oriented anteroposteriorly, although they still project ventrolaterally. The width Figure 13. The dentition of Diplocynodon hantoniensis. across the prezygapophyses is smaller than that A–C, NHMUK OR 25166 in anteroventral view (A), of the postzygapophyses in Cv3–4. Further along anterolateral view, showing the premaxilla–maxilla the cervical series, the zygapophyseal widths are transition (B) and lateral view showing the posterior subequal. The prezygapophyseal facets are elliptical, maxillary dentition (C). D, NHMUK OR 30317, isolated inclined approximately at 45° to the horizontal in tooth from the anterior region of the dentition in mesial axial view. The size of the prezygapophyseal facets view; E–F, unregistered isolated teeth from the posterior increases posteriorly from Cv3 to Cv5 as vertebral region of the dentition in labial view. All scale bars = 2 cm. size increases. The anteriormost cervical vertebrae have anterolaterally orientated prezygapophyses, but Cervical vertebrae: The ventral surfaces of the these become more laterally elongated in subsequent cervical centra lack deep pits (these are also absent cervical vertebrae. The postzygapophyses have from the first dorsal vertebra, Dv1). Posterior to the the same inclination as the prezygapophyses, with hypapophyses, the ventral margin of the centrum elliptical facets. Cervical neural spines are positioned is very sharp in most cervical vertebrae, except in at the posterior end of the neural arch. The dorsally cervical vertebrae (Cv) 4 and 5, in which the ventral incomplete neural spine of Cv3 is anteroposteriorly © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 23 Figure 14. Atlas–axis complex of Diplocynodon hantoniensis. A–D, axis in lateral (A), anterior (B), posterior (C) and dorsal (D) views. E–F, unregistered odontoid process in anterior (E) and posterior (F) views. G–H, proatlas NHMUK OR 30289 in dorsal (G), ventral (H) and ventrolateral (I) views. Abbreviations: apr, anterior process; cn, condyle; hp, hypapophysis; nc, neural canal; ncs, neurocentral suture; nsp, neural spine; odp, odontoid process; odp.s, sutural surface for the odontoid process; poz, postzygapophysis; prz, prezygapophysis; tb, ventral tubercle. All scale bars = 3 cm. long, greater than half the anteroposterior length Sacral vertebrae: The tuberculum and capitulum of the non-condylar centrum. An isolated, but better of the sacral vertebrae (Fig. 18) are approximately preserved, Cv3 (NHMUK R 1046) preserves the same equally well-developed, such that it is not possible to shape, but the neural spine is anteriorly hooked at its see the latter in dorsal view (e.g. NHMUK OR 25179). dorsal tip (Fig. 15A). Cv4 has a similarly wide neural This is also the case in three anterior sacral vertebrae spine, but the neural spines become anteroposteriorly accessioned under NHMUK R 1050. However, the shorter in Cv5–7. In Cv4, the neural spine is oriented tuberculum is slightly shorter than the capitulum primarily dorsally, with a slight anterior deflection. (meaning that the latter is visible) in a fourth anterior In lateral view, it has a concave anterior margin. The sacral vertebra, bearing the same collection number, neural spines of Cv5–6 have a similar orientation, but and in NHMUK OR 30402 (Fig. 18D). lack the concave anterior margin. The neural spines Caudal vertebrae: Relatively few caudal vertebrae are become anteroposteriorly longer in Cv8 and Cv9. preserved and they are disarticulated (NHMUK R 1052 Dorsal vertebrae: The lateral and ventral surfaces of and several unnumbered specimens) (Fig. 19). The the dorsal vertebral centra are featureless, lacking vertebral centrum is very elongated, with tuberosities the lateral fossa seen in cervical vertebrae, as on the ventral surface, immediately anterior to the well as a hypapophysis after Dv3 (Fig. 16). Dorsal posterior condyle, for articulation with chevrons (which vertebrae have prominent transverse processes and are not preserved). A longitudinal sulcus occurs on the their zygapophyses are more shallowly inclined than ventral surface of the centrum, defined on each side by those of the cervical vertebrae (Fig. 17). The width a ridge. Where preserved, the anteroposteriorly narrow across the prezygapophyses is consistently greater neural spine is directed vertically and positioned than that across the postzygapophyses, but the long towards the level of the midlength of the centrum. axes of both are laterally directed. Neural spines are Transverse processes are short and the width across anteroposteriorly long, with an inclined dorsal edge the prezygapophyses is significantly greater than the (i.e. higher at the anterior end and sloping posteriorly) postzygapophyseal width. and laterally flared dorsal tips (Fig. 17). Neural spine Osteoderms: Osteoderms are preserved both as isolated height decreases across the first four dorsal vertebrae, elements and embedded in blocks of matrix along with stabilizing at a consistent height from Dv4. other skeletal elements (Fig. 20). The rectangular © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 24 J. P. RIO ET AL. Figure 15. Cervical vertebrae of Diplocynodon hantoniensis. A–D, cervical vertebra 3, NHMUK R 1046 in left lateral (A), anterior (B), dorsal (C) and ventral (D) views. E–K, NHMUK OR 30402, cervical vertebra 9 (E), 8 (F), 7 (G), 6 (H), 5 (I), 4 (J) and 3 (K) in right lateral view. Abbreviations: cn, condyle; ct, cotyle; dp, diapophyses; hp, hypapophysis; nc, neural canal; nsp, neural spine; pa, parapophysis; poz, postzygapophysis; prz, prezygapophysis. All scale bars = 3 cm. dorsal osteoderms are characterized by a smooth and by robust elements, exemplified by large-sized remains, narrow anterior strip, followed by a broad and pitted including a humerus (NHMUK OR 30206), an ulna area that hosts a medial keel (Fig. 20A). In most of the (NHMUK OR 30236), femora (NHMUK OR 25244, dorsal osteoderms, the anterior margin is straight, but 30210, 30211, 30213, 30399), tibiae (NHMUK OR at least three specimens (NHMUK R 5267 partim, R 25236, 30215, 30216, 30217) and fibulae (NHMUK OR 5214 partim and 46434) show an undulating anterior 30333, 30335). Although no associated limb bones are profile that can be defined as an anterior process available, it seems likely that the forelimb was shorter (sensu Norell & Clark, 1990; Brochu, 1997) (Fig. than the hindlimb, as all the preserved femora are 20C). The ventral osteoderms are represented by two longer than the largest preserved humerus (NHMUK units that are sutured together: the anterior one is OR 30206). The relative anatomical orientations anteroposteriorly narrow, with a smooth anterior half (i.e. medial, lateral, dorsal, ventral, etc.) reflect the and a pitted posterior one (Fig. 20D); the posterior unit sprawling posture of the forelimb, following Meers is entirely pitted and devoid of any keel (Fig. 20E, F). (2003: fig 1), whereas those for the hindlimb are based on Klinkhamer et al. (2017: fig 2). Scapula: The horizontal articular surface for the Appendicular skeleton coracoid is poorly preserved in all scapulae, but is Appendicular remains comprise most of the pectoral teardrop-shaped, with a narrow anterior tip and and pelvic girdles, as well as the proximal (upper) a much larger posterior area, close to the glenoid elements of the limbs. The limb bones are represented surface. The fact that the largest scapula (NHMUK © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 25 Figure 16. Dorsal vertebrae of Diplocynodon hantoniensis (NHMUK OR 30402). A–O, dorsal vertebrae 15–1, respectively, in right lateral view. Abbreviations: dp, diapophysis; hp, hypapophysis; pa, parapophysis; poz, postzygapophysis; prz, prezygapophyis. Scale bar = 5 cm. OR 30247) is large, but not fused to the coracoid, caudalis ventrally (Fig. 21F). The medial surface of the implies that their fusion must have occurred very late blade is less sculpted than the lateral surface, with in ontogeny. The glenoid surface of the scapula faces one broad depression situated slightly posterior to posterolaterally (Fig. 21A). It is large and ovoid, with the blade at midlength: this likely marks the origin a rounded dorsal margin and a modest constriction at of M. subscapularis. Dorsal to the glenoid surface, on its anterior margin. This constriction is related to a the glenoid buttress, is a large tubercle. This scar is prominent fossa anterior to the glenoid surface that prominent even in the smallest scapulae (e.g. NHMUK is pierced by a small foramen (see NHMUK OR 30247 OR 30414) and is the attachment site for M. triceps and 30414) (Fig. 21E, F). In all specimens, regardless longus lateralis (Meers, 2003). of size, the deltoid crest is thin. Coracoid: Two left coracoids are preserved (NHMUK The posterior edge of the scapular blade is straight OR 30359 and 25245), although the distal ends of to slightly convex and the anterior edge is concave. both are missing (Fig. 21G). The ventral surface of the There is a large constriction between the ventral glenoid surface is circular and faces posterolaterally. corpus of the scapula and the distal blade, which A small projection of the glenoid surface overhangs the flares. In dorsal view, the blade is sharp and straight. recess ventral to the glenoid surface. A large coracoid The lateral surface of the scapula blade is smooth foramen is situated anterior to the glenoid surface. and slightly convex proximally, becoming flatter The coracoid blade is directed ventrally and medially. distally. Attachment sites for several muscles can be Although incomplete, it expands distally, but the shape recognized on the lateral surface of the largest scapula and width of the distal end cannot be determined. NHMUK OR 30247 (Fig. 21F). Two distinct, broad Humerus: Two right humeri (NHMUK OR 30219 and depressions extend over the greater part of the blade. 30206) are preserved, along with a partial left humerus The anterior of these two depressions is the larger (NHMUK OR 30206a) (Fig. 22). The proximal extremity origin site for M. deltoideus scapularis. A low ridge is poorly preserved in the largest humerus (NHMUK separates this scar from a posterior depression, which OR 30206). In NHMUK OR 30219, the dorsal border is inferred to be the smaller origin site of M. teres of the articular surface is significantly raised relative major (Meers, 2003). Anterior to the attachment for to the proximal extremity of the humeral shaft. The M. deltoideus scapularis is a depression that occupies medial condyle of the proximal end is larger than the anteriormost edge of the scapula and adjacent to its lateral counterpart. Although slightly eroded, the the deltoid crest. This is the origin of M. deltoideus deltopectoral crests of NHMUK OR 30219 and 30206a clavicularis. The posteriormost edge of the scapula clearly emerge abruptly from the humerus. The blade also has a depression, which is inferred to be straight deltopectoral crest is aligned with the lateral the area of insertion of M. serratus ventralis thoracis margin of the humerus (Fig. 22D, H). The apex of the dorsally, and the site of origin for M. scapulohumeralis deltopectoral crest projects ventrally, perpendicular to © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 26 J. P. RIO ET AL. Figure 17. Dorsal vertebrae of Diplocynodon hantoniensis (NHMUK OR 30402). A–O, dorsal vertebrae 1–15, respectively, in anterior view. Scale bar = 5 cm. the humeral shaft, with no medial deflection. Dorsal insertion of M. teres major and M. latissimus dorsi, to the deltopectoral crest, on the lateral margin which is especially prominent in NHMUK OR 30206 of the humeral shaft, there is a single scar for the (Fig. 22G). In lateral view, a longitudinal ridge can be © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 27 Figure 18. Sacral vertebrae of Diplocynodon hantoniensis (NHMUK OR 30402). The first sacral vertebra in anterior (A), posterior (B), dorsal (D), right lateral (H) views; and the second sacral vertebra in anterior (E), posterior (F), dorsal (C) and right lateral (G) views. Abbreviations: cap, capitulum; cn, condyle; cot, cotyle; nc, nueral canal; nsp, neural spine; poz, postzygapophysis; prz, prezygapophysis. Scale bar = 5 cm. Figure 19. Caudal vertebrae of Diplocynodon hantoniensis. A–D, NHMUK R 1052 in anterior (A), posterior (B), right lateral (C) and dorsal (D) views. Abbreviations: cn, condyle; ct, cotyle; nc, neural canal; nsp, neural spine; poz, postzygapophysis; prz, prezygapophysis; tp, transverse process. Scale bar = 3 cm. traced, extending from the proximal articular border, cranialis from the extensive dorsal origin of M. triceps through the scar for M. latissimus dorsi, then running brevis intermedius (Meers, 2003). Medial to the diagonally across the lateral side of the diaphysis M. teres major scar, there is a broad depression on the (Fig. 22F). This ridge probably defines the boundary dorsal surface of the proximal metaphysis. Based on between M. humeroradialis and M. triceps brevis a comparison with extant crocodylians this might be © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 28 J. P. RIO ET AL. Figure 20. Osteoderms of Diplocynodon hantoniensis. A, unregistered dorsal midline osteoderm. Accessory osteoderms: B, NHMUK OR 46434; C, NHMUK R.5214; D, unregistered, anterior half of bipartite ventral osteoderm; E–F, unregistered posterior halves of bipartite ventral osteoderms. the insertion of M. scapulohumeralis caudalis (Meers, condyle is mediolaterally compressed and has an acute 2003) (Fig. 22G). posterior termination, whereas the lateral condyle is The proximal and distal epiphyses are offset, with wider and rounded. In distal view, the medial edge of the former extending further dorsally and resulting the distal epiphysis is straight, whereas the lateral in a sigmoidal outline to the humerus in medial view. edge is diagonal. In dorsal view, the lateral margins of The medial margin of the proximal extremity extends the distal and proximal extremities are approximately dorsally and medially away from the humeral shaft, in line, and the humeral shaft has a distinct lateral forming a tuberosity that would have articulated with bow between these extremities. The medial distal the glenoid surface of the scapula and coracoid. The extremities are more offset, with the proximal medial medial edge of this tuberosity is scarred, possibly margin directed more medially than the distal margin. marking the origin of M. triceps brevis as in extant A prominent trochlea divides the lateral and medial crocodylians (Meers, 2003; Klinkhamer et al., 2017). hemicondyles of the humerus. A broad depression The medial margin of the humeral shaft is strongly occurs on the ventral surface of the distal extremity, at concave. Posteriorly, the humeral shaft widens the midline, immediately anterior to the condyles. The towards the distal extremity. Scarring on the medial border of the distal articular surface is worn. side of the distal metaphysis may mark the origins Ulna: There are three right ulnae (NHMUK OR 30326, of M. pronator teres and M. flexor digitorum longus 30327, 30389) and one left ulna (NHMUK OR 30242). (Meers, 2003). The medial and lateral condyles of the The proximal epiphysis of the ulna is much larger than distal extremity are not equidimensional. The medial the distal epiphysis (Fig. 23). There is a large, rounded © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 29 Figure 21. The pectoral girdle of Diplocynodon hantoniensis. A–D, left scapula (NHMUK OR 30414) in anterior (A), lateral (B), medial (C) and posterior (D) views, arrow indicates the position of a constriction in the glenoid fossa. Photograph and interpretive drawing of the left scapula (NHMUK OR 30247) in lateral view (E, F). G, left coracoid (NHMUK OR 30359) in lateral view. Abbreviations: cf, coracoid foramen; dc, deltoid crest; dcl, M. deltoideus clavicularis; dsc, M. deltoideus scapularis; gl, glenoid surface; shc, M. scapulohumeralis caudalis; svt, M. serratus ventralis thoracis; tm, M. teres major; tll, M. triceps longus lateralis. Scale bar = 4 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 30 J. P. RIO ET AL. Figure 22. Humeral morphology of Diplocynodon hantoniensis. Right humerus (NHMUK OR 30206) in medial (A, E), lateral (B, F), dorsal (C, G) and ventral (D, H) views. Abbreviations: ab, articular border; di. diaphysis; dist.ep, distal epiphysis; dist.met, distal metaphysis; dpc, deltopectoral crest; fdl, M. flexor digitorum longus; ld, M. latissimus dorsi; mt, medial tuberosity; prox.ep, proximal epiphysis; prox.met, proximal metaphysis; pt, M. pronator teres; shc, M. scapulohumeralis caudalis; tb, M. triceps brevis; tm, M. teres major; tr, trochlea. Scale bar = 4 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 31 Figure 23. The ulna and radius of Diplocynodon hantoniensis. A–E, right ulna (NHMUK OR 30389) in medial (A), proximal (B), lateral (C), ventral (D) and dorsal (E) views. F–G, right radius (NHMUK OR 30401) in dorsal (F) and ventral (G) views. Abbreviations: dist.ep, distal epiphysis; ful, M. flexor ulnaris; ocp, olecranon process; pqd, M. pronator quadratus; prox.ep, proximal epiphysis. Scale bar = 4 cm. olecranon process (Fig. 23D). The proximal articular Radius: The proximal epiphysis of the radius is surface has a subquadrangular shape (Fig. 23B), with quadrangular, with perpendicular ventral, medial and its straight dorsal and lateral margins perpendicular lateral margins, and a rounded dorsal margin. This to one another. The dorsal half of the medial margin of proximal articular surface is bevelled, such that the the proximal articular surface is parallel to the lateral dorsolateral margin projects further dorsally than the margin, whereas the ventral half has a prominent dorsomedial margin. The mediolateral widths of the notch. This notch might mark the posterior extent proximal and distal epiphyses are subequal. Whereas of the attachment of M. pronator quadratus on the the diaphysis widens abruptly towards the proximal medial surface (Meers, 2003). epiphysis, there is a much more gradual increase The diaphysis is mediolaterally compressed across in mediolateral width towards the distal epiphysis. its entire length, with a modest medial bowing. It is Proximally, the diaphysis has a rounded cross-section, narrowest anterior to the midlength of the shaft. In becoming dorsoventrally compressed distally. The lateral view, the ventral edge of the ulna is convex diaphysis is straight and sculptured with ridges that and the dorsal margin is concave. On the dorsolateral mark muscle attachment sites. On the ventromedial surface of the diaphysis, immediately anterior to side, there is a longitudinal ridge, which terminates the proximal epiphysis, is a circular scar (Fig. 23E). above the midlength of the diaphysis. In proximal A very prominent, straight ridge runs parallel to the view, the distal epiphysis has a semi-circular outline, long axis of the ulna along the dorsal surface (best with a flat ventral margin and rounded dorsal margin. seen in NHMUK OR 30242 and 30389). A very small The medial half of the distal epiphysis projects further sulcus extends longitudinally along the lateral surface anteriorly than the lateral half. of the diaphysis, and probably marks the insertion Ilium: Four right (NHMUK OR 30362, 25252a, 25253, of M. flexor ulnaris (Meers, 2003) (Fig. 23C, E). On 30414a) and two left (NHMUK OR 25252b, 30414b) ilia the dorsal surface at the distal end, posterior to the are preserved (Fig. 24). The anterior process of the ilium epiphysis, is a modestly developed ridge that extends is not well-developed and the dorsal margin is rounded, posteriorly, slightly onto the medial surface. The distal with a smooth border and with a very deep posterior epiphysis is strongly compressed mediolaterally. Its tip. The postacetabular process is approximately articular surface is elliptical, facing anteromedially equivalent to the height of the acetabulum. On the and slightly ventrally. In anterior view, its lateral posterior margin of the postacetabular process, the margin is crescentic, and the medial margin has a attachment site for M. ilioischio caudalis is situated small notch around the dorsoventral midpoint. There dorsomedially (Fig. 24A). The acetabulum occupies is also a prominent ventromedial projection on the approximately half of the lateral surface of the ilium. articular surface, from which a small lamina extends The supracetabular crest is narrow, with a rugose onto the diaphysis (see NHMUK OR 30389). texture, ornamented with a series of small parallel © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 32 J. P. RIO ET AL. Figure 24. Right ilium (NHMUK OR 30362) of Diplocynodon hantoniensis in lateral (A), medial (B), dorsal (C), anterior (D) and ventral (E) views. Abbreviations: ace, acetabulum; ace.perf, acetabular perforation; ant.cond, anterior condyle; ilic, M. ilioischio caudalis; il.cr, iliac crest; isch.ped, ischial peduncle of the ilium; post.cond, posterior condyle; postace.pro, postacetabular process; prace.pro, preacetabular process; pub.ped, pubic peduncle. Scale bar = 4 cm. vertical ridges. The inclination of the iliac crest laterally than the pubic peduncle, but they reach the relative to the sagittal plane varies anteroposteriorly: same level ventrally. Although worn in all specimens, it rises to a subvertical orientation anteriorly, whereas the ventral surface of the ischial peduncle appears the inclination of the crest becomes shallower (at to be horizontal and triangular. The iliac margin of approximately 45° to the sagittal plane) towards the the acetabular perforation is well defined. Its dorsal posterior end of the postacetabular process (Fig. 24D). margin is broadly curved, and the acetabulum is In dorsal view, the iliac crest is not linear, but curves deeply concave, especially around its ventral margin. laterally (Fig. 24C). Attachment sites for the transverse processes The articular surface of the pubic peduncle faces of the sacral vertebrae are centrally positioned ventrally. The condyles of the pubic peduncle are on the medial surface of the ilium (Fig. 24B). The asymmetric, with a large anterior condyle and small attachment for the posterior transverse process posterior condyle (Fig. 24E). Between the pubic extends posteriorly along the ventral margin of peduncle and the preacetabular process, the anterior the postacetabular process, leading slightly on to edge of the ilium is concave. The ischial peduncle forms its ventromedial side. As such, the postacetabular the posterior wall of the acetabulum. It projects further process would not have extended much further than © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 33 the posterior sacral vertebra. The attachment site for the anterior sacral vertebrae is sub-triangular and adjacent to the posterior scar. An anteroposteriorly thin, vertical strut (or ‘bridge’) of bone separates these two attachment sites. Ischium: Two partial ischia are preserved (both NHMUK OR 30354) (Fig. 25): a left ischiu and a smaller right element. In both specimens, the tip of the distal blade is broken off and the posterior peduncle is worn. Although worn, the posterior peduncle is larger than the anterior peduncle. The posterior peduncle would have articulated with the ilium to enclose the posterior and ventral margins of the acetabulum. A laterally directed articular surface is preserved on the posterior peduncle of the right ischium, for articulation with the proximal head of the femur. The head of the anterior peduncle is anteroposteriorly compressed, and its articular surface is flat and orientated ventromedially. This peduncle encloses the anterior margin of the acetabulum, which probably had a dorsoventrally elongated elliptical outline. The large, rounded ventral margin of the acetabular perforation is formed by the peduncle bridge of the ischium. The peduncle bridge is mediolaterally constricted at the level of the acetabular perforation. Towards the head of the anterior peduncle, the margins of the bridge flare outwards. A prominent scar occurs ventral to the peduncle bridge, on the anterolateral surface of the ischium. This probably marks the attachment site for one of the flexor tibialis internus group muscles, as in C. porosus (Klinkhamer et al., 2017). In the larger left ischium, this scar is significantly bigger and more rugose, extending further onto the anterior margin of the ischial blade. A prominent ridge is also associated with this scar, extending along the long axis of the ischial blade. A series of muscle attachment scars occur on the posterior margin of the ischium, ventrolateral to the posterior peduncle. The dorsalmost scar probably marks the attachment of M. pubo-ischio-tibialis, whereas the more ventral scars are for the remaining flexor tibialis internus muscles (Klinkhamer et al., 2017). Again, this scar is much more prominent and rugose in the larger left ischium. The ischial blade is directed medially and slightly posteriorly. Whereas the anterior edge of the blade is sharp, the posterior margin is more rounded. Although incomplete in both ischia, the distal blade expands Figure 25. Ischia of Diplocynodon hantoniensis (NHMUK towards its distal end: this expansion occurs along OR 30354). A, right ischium in lateral view; B, left ischium an anteromedial-to-posterolateral axis. However, we in lateral view. Abbreviations: ace.perf, acetabular cannot determine whether the distal end of the blade perforation; ant.ped, anterior peduncle; fti, M. flexor tibialis reached a greater width than the proximal head. internus; isch.bl, ischial blade; ped.br, peduncle bridge; pit, Femur: There are several well-preserved femora. M. puboischio tibialis. Scale bar = 2 cm. Among the more complete are two right (NHMUK OR 30211, 25244) and three left (NHMUK OR to which individuals, all femora are longer than the 30210, 30399 and R.5215) elements. Although it is preserved humeri. The proximal epiphysis of the not possible to determine which elements belong femur is mediolaterally compressed (Fig. 26). The © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 34 J. P. RIO ET AL. Figure 26. Photographs and line drawings of the left femur of Diplocynodon hantoniensis (NHMUK OR 30210) in dorsal (A, C) and ventral (B, D) views. Abbreviations: 4t, fourth trochanter; add, M. adductor; fo, foramen; fte, M. femorotibialis externus; fti, M. femorotibialis internus; if, M. iliofemoralis; pf, popliteal fossa; pife, M. pubo-ischio-femoralis externus; pifi, M. pubo-ischio-femoralis internus. Scale bar = 5 cm. dorsal surface of the proximal metaphysis is broadly muscles in living crocodylians: (1) M. iliofemoralis; (2) concave. Scarring towards the posterior end of the M. femorotibialis internus; and (3) M. femorotibialis proximo-dorsal metaphysis is regarded as evidence externus. M. iliofemoralis has a fleshy insertion along for the tendinous insertion of M. pubo-ischio-femoralis the distal third of the dorsolateral surface of the internus (Fig. 26C). femur in living crocodylians (Tumarkin-Deratzian On the ventral surface of the proximal metaphysis, et al., 2007; Klinkhamer et al., 2017), and it probably two broad fossae occur either side of a discrete knob formed the raised flat surface that is so prominent that projects from the proximal head (Fig. 26B, D). in D. hantoniensis. M. femorotibialis internus and On the proximo-anterior edge of the femur there is a M. femorotibialis externus surround M. iliofemoralis prominent notch covered in striations, which extend anteriorly and posteriorly, respectively. towards the articular border of the proximal epiphysis. The ventral side of the femur is dominated by This is interpreted as the site for the tendinous scarring associated with muscle attachments around insertion of M. pubo-ischio-femoralis externus, as in the fourth trochanter (Fig. 26D). The fourth trochanter C. porosus (Klinkhamer et al., 2017) (Fig. 26C). comprises a short, longitudinal ridge. Immediately In dorsal view, the femur is strongly sigmoidal anterior to the fourth trochanter ridge is a depression in outline. This results from curvature along the that corresponds to the area of insertion for M. pubo- diaphysis, and torsion between the proximal and ischio-femoralis internus (Klinkhamer et al., distal epiphyses. The dorsal surface of the diaphysis is 2017). The prominent ridge that defines the fourth characterised by a centrally positioned, raised surface, trochanter, as well as a large scarred depression which is defined by two longitudinal ridges (Fig. following on from the ridge, are interpreted as the sites 26A, C) The posteriormost of these ridges intersects for tendinous insertions of M. caudofemoralis longus the posterior margin of the diaphysis at the femoral and M. caudofemoralis brevis. midlength, converging with the anteriormost ridge Distally, there is a depression on the ventromedial centrally on the femoral shaft. A fossa, pierced by edge of the diaphysis, which is interpreted as the a foramen, occurs anterior to the raised flat ridge. area of insertion for M. adductor, as in C. porosus These ridges correspond with the insertions of three (Klinkhamer et al., 2017). The distal epiphysis is poorly © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 35 preserved in the largest femora, but well preserved in views. The proximomedial surface of the fibula has a NHMUK R 5215. A very prominent constriction occurs triangular crest, which has a rough, striated texture. at the midpoint of the distal epiphysis, dividing the This texture extends proximally until the border of lateral and medial condyles. The lateral half of the the articular surface. This surface probably marks distal articular surface is dorsoventrally taller than the tendinous insertion of M. interosseous cruris and the medial half. The ventral margin of the lateral half another tendinous attachment for M. fibularis longus is very acute, extending further ventrally than the (Fig. 27H). medial half. The trochanter for the insertion of M. iliofibularis Tibia: Several tibiae are preserved, with two right occurs on the proximolateral surface of the fibula, (NHMUK OR 30215, 30217) and two left (NHMUK near the anterior edge, as in extant crocodylians OR 30216 and 25236) elements amongst the best (Fig. 27G). Above the iliofibularis trochanter, on the preserved. The proximal epiphysis is broader than the proximolateral metaphysis, there is more scarring, distal epiphysis, facing dorsally and slightly posteriorly although less prominent than on the medial side. In Fig. 27A–D). A large sulcus occurs towards the medial extant crocodylians, this is the location of origin for edge of the facet, for the tibial condyle of the femur. M. extensor digitorum longus and M. fibularis brevis. The posterior margin of the proximal articular facet The distal half of the lateral fibular surface has a forms an overhanging triangular lip. The proximal centrally positioned, longitudinal ridge, which likely metaphysis of the tibia is circumscribed by a striated marks the origin of M. extensor digitorum brevis surface indicating the attachment of numerous (Klinkhamer et al., 2017). muscles, but it is not possible to distinguish between The distal epiphysis is compressed in a posteromedial- them in detail. The proximo-medial edge is particularly to-anterolateral direction. This generates a small scarred and in C. porosus this area corresponds with amount of torsion between the proximal and distal multiple tendinous insertions, including: M. flexor epiphyses. The articular facet of the distal epiphysis tibialis internus, M. pubo-ischio-tibialis, M. ambiens faces anteroventrally for articulation with the and M. iliotibialis (Klinkhamer et al., 2017). A pale, astragalus. In distal view, the facet is sub-triangular. discoloured tuberosity occurs on the proximal anterior Its lateral edge is straight, whereas the medial edge surface, and probably marks the origin of M. tibialis- is slightly concave, forming a distinct notch at the anterior. This tuberosity leads onto a longitudinal anteroposterior midpoint. The posterior edge of the ridge where M. tibialis anterior attaches in C. porosus distal metaphysis is defined by a thin ridge, possibly (Klinkhamer et al., 2017). marking the origin of M. pronator profundus. Whereas the medial edge of the tibial diaphysis is concave, the lateral edge is slightly convex. On the posterior surface of the diaphysis, there is a prominent Taxonomic status of previously referred longitudinal ridge (Fig. 27C). This ridge begins material proximally, on the centre of the diaphysis, and moves Several specimens have been referred to Diplocynodon to the medial edge distally. A second ridge occurs in hantoniensis from Palaeogene deposits in western NHMUK OR 30215, at a slight angle to the first ridge, Europe and the USA. If correctly attributed, these merging with its distal end. These ridges probably remains demonstrate that this species had a much correspond to the sites of insertion of M. interosseous greater spatiotemporal distribution. Below we cruris, as in living crocodylians (Klinkhamer et al., critically reassess each of these referrals. 2017). The distal epiphysis is anteroposteriorly compressed, with a notch on the posterior side, producing an Early Eocene of the USA arcuate articular facet for the astragalus (Fig. 27C). Weems (1999) referred a single tooth (USNM This facet faces posteroventrally. The medial portion 496207) and a keeled dorsal osteoderm (unnumbered of the facet projects slightly further distally than the specimen) to D. hantoniensis from the early Eocene lateral counterpart. Nanjemoy Formation, which crops out in the Fisher/ Fibula: Two right [NHMUK OR 30233 (Fig. 27E– Sullivan site in Stafford County, Virginia. The H), 30235] and two broken and fragmentary left referral was based partly on the purported presence (NHMUK OR 30241, 30234) fibulae are preserved. of ‘Diplocynodon stuckeri’ from the middle Eocene of The proximal end is mediolaterally compressed, Wyoming (Mook, 1960). However, this latter material with a dorsomedially orientated, elliptical, articular has since been referred to Borealosuchus wilsoni Mook, facet. The proximal epiphysis is strongly expanded 1959 (Brochu, 1997). The tooth described by Weems posteriorly for the long, external lateral ligament (Fig. (1999) is conical, with carinae on the anterior and distal 27L). It has a notch at its midpoint, which results edges, and the osteoderm is approximately rectangular in a dorsally concave outline in medial and lateral with a modest keel. The morphology of these elements © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 36 J. P. RIO ET AL. Figure 27. The tibia and fibula of Diplocynodon hantoniensis. A–D, right tibia (NHMUK OR 30215) in posterior (A, C) and anterior (B, D) views. E–H, right fibula (NHMUK OR 30233) in lateral (E, G) and medial (F, H) views. Abbreviations: as, astragalar articular surface; edb, M. extensor digitorum brevis; edl, M. extensor digitorum longus; fb, M. fibularis brevis; fl, M. fibularis longus; fo, foramen; fti, M. flexor tibialis internus; ic, M. interosseous cruris; if, M. iliofibularis; lell, long external lateral ligament; pp, M. pronator profundus; s.tc, surface for the tibial condyle of the femur; ta, tibialis anterior. Scale bar = 5 cm. is insufficient for specific or even generic identification. Late Eocene of Switzerland We consider this material from Virginia to represent Pictet et al. (1857) described fragmentary remains an indeterminate crocodylian, pending the discovery of of a crocodylian from the late Eocene of Mormont diagnostic material from this locality. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 37 (=Mauremont), in the canton of Vaud, western curved dentary, are features that are common to all Switzerland. The material consists of five unnumbered Diplocynodon species. The presence of a short dentary cranial fragments, a fragment of the retroarticular symphysis can restrict the referral to one of four species, process and two isolated teeth. Pictet et al. (1857) as it is only present in D. ratelii, D. hantoniensis, referred the material to D. cf. hantoniensis based on D. muelleri and D. remensis, but in D. remensis the the presence of a large occlusal pit (for dentary tooth splenial participates in the symphysis, unlike any 1) on the ventral surface of the premaxilla (described other Diplocynodon species (Martin et al., 2014). If the but not figured). The shapes of both the squamosal rostral tip of the splenial was preserved in the German and supratemporal fenestra were also regarded as specimen, this would allow us to distinguish between identical to D. hantoniensis, along with two isolated D. muelleri (where it passes dorsal to the Meckelian teeth that display similar carinae. We find similarities groove) and D. hantoniensis and D. ratelii (in which in the material with D. hantoniensis, such as the it passes ventral to the groove). No further diagnostic long squamosal prongs and elongate supratemporal characters can be retrieved from the dentary and other fenestra. However, none of the diagnostic features remains are non-diagnostic. Therefore, we assign this of D. hantoniensis recognized in this study can be German occurrence to Diplocynodon sp. identified in the Swiss material. Furthermore, large occlusal pits for the dentary teeth are not unique to D. hantoniensis: they occur in all other alligatoroids Early Oligocene of France and crocodyloids. Carinae on the anterior and posterior A relatively complete skull of an adult crocodylian was edges of teeth are equally undiagnostic, being present referred to Diplocynodon cf. hantoniensis by Vignaud in essentially all non-longirostrine crocodylians. We et al. (1996) from early Oligocene sediments in the also note two potential differences: the rim of the Razac-d’Eymet commune in Dordogne, south-western premaxilla is very sharp in the Swiss material, unlike France. The specimen is unnumbered but described as D. hantoniensis, and the size of the external naris in part of the ‘Guevel collection’, which may be housed proportion to the surface area of the premaxilla is at the University of Poitiers, France. The proportions larger than that of D. hantoniensis. Given that none of the skull match D. hantoniensis, in particular the of the diagnostic features of Diplocynodon (Martin relatively wide rostrum and lack of a prominent et al., 2014) can be identified in the Swiss material, we constriction at the premaxillary–maxillary suture. regard it as Alligatoroidea indet. Unfortunately, the figures are not detailed enough to comment on the sutural relationships of the skull. Early Oligocene of Germany However, there are important differences based on the description, which suggest the material might not Gramann (1958), and later Karl & Müller (2008), be referable to D. hantoniensis. Vignaud et al. (1996: described crocodylian remains from the early Oligocene fig. 1b) described and figured a narrow ectopterygoid ‘Melanian clay’ (=Melanienton), which crops out that runs beside the border of the maxillary alveoli, around the Altenburg IV quarry at Borken, in Hesse, as in D. ratelii. This condition contrasts with central Germany. Gramann (1958) noted the presence D. hantoniensis, which has a unique condition of of an anterior dentary fragment (lacking the splenial), the ectopterygoid (see Description). The presence of a lacrimal, fragments of a left and right angular, teeth, other diagnostic features of D. hantoniensis cannot be portions of two femora and numerous osteodermal discerned from either the description or figures, such and vertebral fragments, which he referred to as the presence of ectopterygoid–pterygoid flexure, D. hantoniensis. No specimen numbers were assigned or a sulcus posterolateral to the external naris. In to the material, but they are deposited in the Institute summary, until further descriptions of this specimen for Geology and Palaeontology at the University are made, it can only be referred to Diplocynodon sp. of Marburg, Germany. Additional osteoderms were later referred to D. cf. hantoniensis by Karl & Müller (2008) (PMLU HS 389-A10.02.05). Of the figured and Phylogenetic analysis described material, only the dentary is taxonomically informative. Three character states can be determined: Data matrix (1) confluence of the 3rd and 4th dentary alveoli; (2) a Diplocynodon hantoniensis was re-scored for the data gently curved dentary between alveoli 4 and 10; and matrix of Martin et al. (2014), which is an expanded (3) dentary symphysis extending to the 3rd/4th dentary version of that presented in Brochu et al. (2012), alveolus. One further character, the position of the containing all valid species of Diplocynodon. This anterior splenial tip relative to the Meckelian groove, resulted in 21 character score changes, as documented is indiscernible from figures in Gramann (1958). in the Appendix. Preliminary analyses resulted in Confluence of the 3rd and 4th alveoli and the gently the highly unusual recovery of the latest Cretaceous © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 38 J. P. RIO ET AL. basal eusuchian Allodaposuchus precedens Nopcsa, implied weighting can be negatively influenced by 1928 as a member of Diplocynodon; consequently, missing data, especially in cases where homoplastic we added several additional allodaposuchid taxa in characters are only scored in a few operational order to remove a potential long-branch attraction taxonomic units (OTUs). The lack of information due to converging characters in the two clades. Two in these characters can lead to a misleadingly low additional species of Allodaposuchus (A. hulki Blanco homoplastic rate when convergent OTUs are grouped et al., 2015, A. palustris Blanco et al., 2014), as well as together (Goloboff, 2014; Tschopp et al., 2018). In Agaresuchus subjuniperus Puértolas et al., 2013 and order to counteract this negative influence of missing Arenysuchus gascabadiolorum Puértolas et al., 2011, data, Goloboff (2014) implemented extended implied were scored based on Blanco et al. (2015), and we also weighting, which downweights homoplastic characters updated scores for Allodaposuchus precedens based not only in relation to their homoplastic rate, but also on Martin et al. (2016). Eight additional characters based on the amount of missing data. As has been (C180–187) from Brochu & Storrs (2012) were also shown by Goloboff et al. (2018), this methodology added to the character list. One new character state seems to perform best, especially when using was added to each of characters 146 and 147 following morphological datasets. Both implied and extended Delfino et al. (2008), to better characterize the unique implied weighting use a concavity constant ‘k’ to morphology of the posterior margin of the otic aperture define the strength of the downweighting: the lower in allodaposuchids. Our revised dataset comprises the k-value, the stronger the downweighting function. 103 taxa scored for 187 characters. The data matrix Although there is no biological reasoning behind the (nexus and tnt files), and character list are provided in choice of a specific k-value (Turner & Zandee, 1995), Supplementary Information 1–3 respectively. it is probably reasonable to exclude extreme values, and to test and compare the impact of various k-values on tree topology (Goloboff, 1995; Tschopp & Upchurch, Analytical protocol 2019). As such, we use a number of different weighting In contrast to previous iterations of this data matrix, strategies. multistate characters were treated as ordered if All analyses were performed using the New they described meristic features, relative positions of Technology search in TNT v.1.1 (Goloboff et al., 2008), characteristics within the skull or obvious morphoclines with all algorithms enabled, and the consensus (following Brazeau, 2011). Characters for which none of tree stabilized five times with a factor of 75. The the states could be identified as a clearly intermediate recovered trees from the first iteration were subjected morphology between two extremes were treated as to a second search using the ‘bbreak’ command, unordered. Thus, characters 17, 40, 49, 55, 63, 82, 88 applying tree bisection and reconnection to find all and 148 were ordered. We conducted the phylogenetic most parsimonious trees (MPTs). Subsequently, a analysis based on maximum parsimony with four complete strict consensus tree was calculated, and different weighting schemes: (1) equal weighting; (2) synapomorphies of nodes and autapomorphies of tips implied weighting with a k-value of 8; (3) extended in the strict consensus tree were retrieved using the implied weighting with a k-value of 8; and (4) extended menu ‘Optimize > Synapomorphies > List common implied weighting with a k-value of 4. Implied weighting synapomorphies’. decreases the impact of variable characters, depending on their homoplastic rate during the tree search in TNT (Goloboff 1993, 2014). Because homoplastic RESULTS characters might or might not bear a phylogenetic signal, or are characters that evolve faster than non- In three of our four analyses, the genus Diplocynodon homoplastic ones, downweighting them in respect was recovered as monophyletic, forming the sister to highly phylogenetically significant ones appears taxon to D. ungeri + D. elavericus, although resolution to be a logical solution (Farris, 1969; Goloboff, 1993; within the genus varied (Fig. 28; Table 3). The Chippindale & Wiens, 1994; Haszprunar, 1998). Indeed, exception is the tree found under extended implied the inclusion of downweighted homoplastic characters weighting with a k-value of 4, in which D. remensis is has been shown to increase phylogenetic accuracy placed outside of Diplocynodon (basal to Leidyosuchus) both in simulations and with real morphological and and Tsoabichi is found deeply nested in Diplocynodon, molecular datasets (e.g. Chippindale & Wiens, 1994; forming the sister taxon to D. ungeri. In this topology, D. Källersjö et al., 1998; Goloboff et al., 2008, 2018; hantoniensis is recovered as sister taxon to D. muelleri Goloboff, 2014). Given that implied weighting, as + (D. elavericus + (Tsoabichi+ D. ungeri)). Three local implemented in the software TNT, provides the only autapomorphies support the distinctiveness and repeatable and user-friendly methodology to date, validity of D. hantoniensis in all four analyses: 66-1, we applied this strategy herein. However, traditional 86-1, 125-1. Although the number and combination © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 39 Figure 28. Results of the phylogenetic analysis. A, strict consensus of 2880 MPTs from the analysis with equal weighting; B, strict consensus of 10 MPTs from the analysis with implied weighting, k = 8; C, strict consensus of 5 MPTs from the analysis with extended implied weighting, k = 8; strict consensus of 5 MPTs from the analysis with extended implied weight, k = 4. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 40 J. P. RIO ET AL. Table 3. Summary of results from the phylogenetic analyses Analysis Protocol Number of MPTs Tree length Analysis 1 Equal weights 2880.0 756 Analysis 2 Implied weights, k = 4 10.0 39.6 Analysis 3 Extended implied weights, k = 8 5.0 37.1 Analysis 4 Extended implied weights, k = 4 5 54.35737 of synapomorphies found for the genus Diplocynodon to find the conflicting topology, and compared the varies between the analyses, all of them are present in relative length increases (following Tschopp et al., D. hantoniensis. 2015). The constraints forced Tsoabichi to be recovered Analysis 1, using equal weighting, found 2880 in Diplocynodon, D. remensis to be positioned basal MPTs with a length of 756 steps (Fig. 28A). The to Leidyosuchus in the first three analyses and genus Diplocynodon can be diagnosed by four Diplocynodon to be fully monophyletic for analysis autapomorphies: 15-0, 34-4, 149-1 and 181-1. 4. In addition, we performed tree searches for analyses Some trees also find the following four possible 1–3 in which we forced all Diplocynodon species, autapomorphies for Diplocynodon: 42-2, 69-1, 82-2,and except D. remensis, to form a monophyletic clade, with 153-1 (see also Discussion). Tsoabichi as a wildcard taxon (i.e. it could be found Analysis 2, using implied weighting (k = 8), within or outside Diplocynodon). Search strategies recovered 10 MPTs with a length of 39.56116 steps. were the same as in the unconstrained analyses, Diplocynodon is slightly more resolved than in the although we refrained from the second iteration of tree strict consensus tree found under equal weighting bisection and reconnection, because the first iteration (Fig. 28B), but the position of D. hantoniensis in the generally already found the shortest trees. Since we genus is the same. Seven autapomorphies were found only compare tree length, this test should provide for Diplocynodon: 15-0, 34-4, 69-1, 82-2, 149-1, 153-1 accurate results without this second step. and 181-1. All of these autapomorphies were also The resulting topologies of the constrained analyses found by at least some MPTs under equal weighting, 1–3 place Tsoabichi as the sister taxon to D. ungeri, but one synapomorphy found with equal weighting as in the unconstrained analysis 4, but D. remensis is (42-2) was not recovered here; this is likely because of recovered in Borealosuchus instead. Relative length the better resolution at the base of Diplocynodon in increases vary considerably between equal-weighting this second analysis. and implied-weighting approaches, averaging to 0.48% The third analysis (Fig. 28C), using extended implied (Table 4). weighting with a k-value of 8, recovered five MPTs The topology resulting from the constrained with a length of 37.09033 steps. Resolution within analysis 4 recovered D. remensis in the same relative Diplocynodon is the best of all four analyses. D. darwini position as in the unconstrained analyses 1–3, and is the most basal species, and D. hantoniensis remains found D. deponiae as the most basal member of the in the same position as when using equal and genus. This is contrary to the unconstrained analysis implied weighting. Seven synapomorphies support 3, which found D. darwini as the most basal member of Diplocynodon as a distinct genus: six of them were the genus. However, the other unconstrained analyses found by all MPTs (15-0, 34-4, 82-2, 149-1, 153-1 and found D. deponiae in a polytomy with D. darwini and 181-1) and one of them by some (69-1). a clade uniting the other species of Diplocynodon. The fourth analysis, using extended implied weighting Therefore, this difference probably just indicates with a k-value of 4, found five MPTs with a length of the lack of data to resolve the relationships between 54.35737 steps. The recovered topology of the strict these basal species. The relative length increase of the consensus tree is different from the other analyses (Fig. constrained analysis 4 amounts to 0.14%. 28D), with Diplocynodon remensis basal to Leidyosuchus Using the single constraint for a monophyletic and Tsoabichi deeply nested in Diplocynodon, as Diplocynodon that excludes D. remensis and with the sister taxon to D. ungeri. The clade including all Tsoabichi as a wildcard taxon, analyses 1–3 all Diplocynodon species (except D. remensis) and Tsoabichi recovered D. remensis in Borealosuchus, as in the other was united by the following synapomorphies, which constrained searches. None of the analyses recovered were all found by at least one other analysis as well: Tsoabichi in Diplocynodon. Relative length increase 15-0, 34-4, 82-2, 149-1, 153-1 and 181-1. was less than half compared to the more constrained In order to explore these conflicting topologies, we analyses (Table 4) and reached lower values in performed constrained searches to force the analyses analyses 2 and 3 than in the constrained analysis 4. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 41 Although generally low, relative length increases with a weaker downweighting function than analysis are, therefore, considerably higher when constraining 4, recovers the same monophyletic Diplocynodon as analyses 1–3 to find the unexpected topology of the analyses under implied weighting with an equal analysis 4 than in the opposite case, but values k-value and under equal weighting, supports this decrease significantly if Tsoabichi is found outside interpretation. Further investigation into the indices the genus. These results indicate that a monophyletic reveals that the group including Tsoabichi has two Diplocynodon appears to be the most parsimonious more characters with overlaps than Diplocynodon solution, although it remains unclear if D. remensis alone, but still lower values. These are characters 75 should be included or not. and 76, which are only scored in D. darwini within When comparing the Overlap Indices (Tschopp Diplocynodon. In total, only 24% (character 75) and et al., 2015, 2018) of the three topologies found for 23% (character 76) of the OTUs are scored for these Diplocynodon, both the AOI and COI obtain higher characters, which is much less than the average (62%). values for a unique genus excluding Tsoabichi, but Of the eight potential synapomorphies found by the equal values regardless as to whether or not D. four unconstrained analyses, Tsoabichi could only be remensis is included (Table 5). The Overlap Indices scored for three. In one of these three synapomorphies simply quantify the anatomical overlap in a specific (82-2), Tsoabichi was scored differently (82-1; this group among all characters (AOI) and among the reversal was recovered as a local autapomorphy of comparable characters only (COI) and can thus not be the species in Diplocynodon in analysis 4) and the regarded as a support value for the analysed group of other two are either shared (42-2) or ambiguous (149- OTUs (Tschopp et al., 2018). However, the combination 1) synapomorphies, meaning that they both occur in of less overlap with a stronger downweighting strategy, species outside Diplocynodon as well (Tschopp et al., as occurred in our unconstrained analysis 4, indicates 2015). that this result might be affected by the negative In summary, whereas we can probably exclude the impact of missing data (Tschopp et al., 2018). The fact possibility that Tsoabichi greenriverensis Brochu, that analysis 3, which used extended implied weighting 2010 is a species of Diplocynodon, we cannot currently Table 4. Relative tree length increases using different constraints. Abbreviations: NA, not applicable Constraint Relative length increase Average Analysis 1 Analysis 2 Analysis 3 Analysis 4 D. remensis basal to Leidyosuchus, and Tsoabichi 0.92% 0.29% 0.22% NA 0.48% nested in Diplocynodon. D. remensis outside a monophyletic Diplocynodon, 0.40% 0.11% 0.10% NA 0.20% with Tsoabichi as a floating taxon. Diplocynodon monophyletic NA NA NA 0.14% 0.14% Table 5. Overlap indices for different compositions of Diplocynodon found in the phylogenetic analyses Diplocynodon Diplocynodon without D. remensis Diplocynodon without monophyletic but including Tsoabichi D. remensis and Tsoabichi Group members (M) 9 9 8 Characters (C) 187 187 187 Possible anatomical overlaps per 8 8 7 character (M-1) Characters with overlap (Co) 158 160 158 Total overlaps in group (Om) 873 812 761 (Om/C) 4.67 4.34 4.07 (Om/Co) 5.53 5.08 4.82 Overlap index for all characters. 58% 54% 58% AOI = (Om/C)/(M-1) Overlap index for comparable 69% 63% 69% characters. COI = (Om/Co)/(M-1) © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 42 J. P. RIO ET AL. be certain whether or not Diplocynodon remensis is a + (D. remensis + (D. hantoniensis + (D. elavericus + D. member of the genus. ungeri))). Analysis 4 similarly does not support a sister relationship between D. tormis and D. muelleri. Here, D. tormis, D. ratelii and D. muelleri are recovered as DISCUSSION successively nested species. The analyses using equal weighting, implied phylogenetic relationships of Diplocynodon weighting (k = 8) and extended implied weighting Our results do not alter the generic diagnosis of (k = 8), produced trees with the same stratigraphical Diplocynodon presented by Martin et al. (2014). incongruence observed by Martin et al. (2014). Here, However, we provide a categorization of the the late Palaeocene species Diplocynodon remensis synapomorphies resulting from our analyses into is recovered as the sister taxon to D. hantoniensis unambiguous, shared and ambiguous. Only one + (D. elavericus + D. ungeri). However, our fourth unambiguous synapomorphy is recovered for analysis, with the greatest downweighting of Diplocynodon, i.e. known solely in that genus: a homoplastic characters, recovered D. remensis outside rounded dorsal margin of the iliac blade, which is of Diplocynodon, basal to Leidyosuchus. This result posteriorly very deep. However, this character cannot is interesting considering the anatomical similarities currently be scored for D. elavericus, D. remensis, between Leidyosuchus and D. remensis (Martin D. ungeri or D. tormis. Four shared synapomorphies et al., 2014); for example, in both taxa the splenial are identified, i.e. present in all Diplocynodon species participates in the dentary symphysis, which is (where preserved), but also in other taxa: (1) nasals unknown in any other Diplocynodon species. However, excluded, at least externally, from the naris; (2) Martin et al. (2014) stressed that the similarities axial hypapophysis located towards the centre of the between these taxa are non-exclusive and outlined centrum; (3) one row of postoccipital osteoderms; and several anatomical differences between them (as well (4) ventral armour formed of paired ossifications. We as with Borealosuchus: see constrained analyses). also identify three ambiguous synapomorphies, i.e. Nonetheless, our results suggest that there is some present in some Diplocynodon species, but also in uncertainty in the placement of D. remensis in other taxa: (1) lingual foramen for the articular artery Diplocynodon. and alveolar nerve perforating the surangular/angular suture (absent in D. muelleri but present, for example, in Alligator); (2) frontoparietal suture linear between Comparisons of D. hantoniensis with other the supratemporal fenestrae (concavo-convex in D. Diplocynodon species ratelii, but linear also in Paleosuchus); and (3) parietal Diplocynodon hantoniensis has a particularly and squamosal approach each other on the posterior broad rostrum, which among the known species wall of the supratemporal fenestrae (absent in of Diplocynodon is closest in shape to D. darwini, D. elavericus and present in Brachychampsa montana D. deponiae, D. elavericus and D. muelleri. In contrast, Gilmore, 1911). D. ratelii, D. ungeri and D. tormis have notably The topology presented here is similar to that in narrower, more pointed rostra. Furthermore, those Martin et al. (2014), with some differences. Although taxa with a narrow rostrum appear to have a very the relationship between D. deponiae and D. darwini modest canthus, which is absent in D. hantoniensis. cannot be stabilized in analyses 1 and 2, they are The outline of the naris is square in D. hantoniensis, recovered as the sister taxa to all other Diplocynodon with straight, anteriorly diverging lateral margins. species in analyses 3 and 4, as in Martin et al. (2014). This contrasts with the strongly rounded narial Whereas in analysis 3, D. deponiae and D. darwini margins seen in D. ratelii, D. ungeri and D. tormis. The are successively nested species, which are sister to all presence of a deep notch situated lateral and slightly other Diplocynodon species, in analysis 4 they form posterior to the external nares is characteristic of a sister pair to all other Diplocynodon species except Alligator, but a similarly deep notch is observed in D. remensis (see above). Unlike the earlier version of D. hantoniensis and D. tormis. the analysis, D. hantoniensis is recovered as the sister There is a large amount of inter- and intraspecific species of D. elavericus and D. ungeri. The latter two variation in cranial ornamentation in Diplocynodon. species were not included in the analysis of Martin Diplocynodon hantoniensis, appears to be the only et al. (2014). Furthermore, we do not recover a clear species with preorbital ridges (NHMUK OR 30392, sister taxon relationship between the two Spanish CAMSM TN 907). These ridges are not as tall, narrow species, D. muelleri (early Oligocene) and D. tormis or elongate as those in some extant Crocodylus (middle Eocene). In analysis 1 their relationships species, e.g. C. porosus Schneider, 1801. A small step are not resolved; in analysis 2 and 3 D. tormis is the immediately anterior to the orbits, and crossing sister to the group comprised of D. ratelii, D. muelleri the frontals, is absent or very weakly developed in © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 43 adult D. hantoniensis, as in D. ratelii (e.g. MNHN SG condition. We regard this unique condition as a local 539) and D. darwini (e.g. HLMD Me 7500). However, a autapomorphy of D. hantoniensis. step has been described in D. remensis (Martin et al., In all extant alligatorids, the squamosal and parietal 2014) and was figured in the description of D. tormis have a large sutural contact on the posterior wall of the (Buscalioni et al., 1992: fig. 2). A step was not described supratemporal fenestra, anterior to the orbitotemporal in D. muelleri, but figures in Piras & Buscalioni (2006: canal (Brochu, 1997) (Fig. 30A). In contrast, extant figs 2 and 3a) indicate its presence. The holotype of Crocodylus, Tomistoma and Gavialis express an D. deponiae does not preserve the dorsal surface of the alternative condition, in which the quadrate widely rostrum, and the best preserved referred specimens separates the squamosal and parietal, forming the floor are broken along the frontal [e.g. SMF Me 2609, IRSNB of the orbitotemporal foramen (Fig. 30B). Diplocynodon R 261 (Delfino & Smith, 2012)]. A juvenile specimen ratelii possesses an intermediate condition, in which (HLMD-Be-147) apparently lacks a step and so it is the squamosal approaches the parietal, but a narrow inferred to be absent in adult D. deponiae too. wedge of the quadrate separates them (Brochu, 1997) Most species of Diplocynodon exhibit an ectopterygoid (Fig. 30C). Previously, this character was scored as that contacts the walls of the posteriormost maxillary unknown in D. hantoniensis (Brochu et al., 2012; alveoli, which is the plesiomorphic condition in Martin et al., 2014), but close inspection of the walls Crocodylia (Brochu, 1997) (Fig. 29C, D). Only of the supratemporal fenestra in NHMUK OR 30393 D. deponiae (Delfino & Smith, 2012) and D. tormis reveals that the intermediate condition is also present (Buscalioni et al., 1992) seem to express the alternative in D. hantoniensis (Fig. 30D). This condition is now character state, whereby the maxilla separates the recorded in almost all Diplocynodon species, except posteriormost alveoli from the ectopterygoid (Fig. D. ungeri, in which it is unknown, and D. elavericus 29B). Although D. hantoniensis also expresses the (Martin, 2010), which seems to express the derived plesiomorphic condition, it shows a slight variation alligatorid condition. However, in the latter species, from other taxa. Here, the posteriormost maxillary the supratemporal fenestra is poorly preserved. alveolus is bordered medially by the ectopterygoid, Outside of Diplocynodon, this condition is known only but anteriorly the ectopterygoid narrows abruptly in Brachychampsa montana, so this feature could be a to an acute point, separated from the toothrow local autapomorphy of Diplocynodon. by the maxilla (Fig. 29E). This differs from other Flexure of the ectopterygoid–pterygoid suture Diplocynodon species, e.g. D. ratelii (MNHN SG 539), is retained into adulthood in D. hantoniensis (Fig. whereby the ectopterygoid narrows gradually to an 8A). This feature was previously regarded as an acute point (Fig. 29D). This condition resembles that unambiguous synapomorphy of crown caimans seen in some Caiman crocodilus chiapasius (Bocourt, (Brochu, 1999) (Fig. 8B). However, it appears to 1876) specimens (FMNH 73694, 73701, 73721) (Fig. occur more widely, as it is present in Allodaposuchus 29F), which do not show the typical alligatoroid precedens (Martin et al., 2016), Thoracosaurus Figure 29. Comparison of the ectopterygoid–maxilla suture near the posterior maxillary toothrow in several crocodylians. A, generalized crocodylian skull in ventral view, showing the enlarged area in B–F; B, Alligator mississippiensis (Daudin, 1802) (AMNH 71621); C, Crocodylus rhombifer Cuvier, 1807 (AMNH R 154087); D, Diplocynodon ratelii (MNHN SG 539); E, Diplocynodon hantoniensis (NHMUK OR 30392); F, Caiman crocodilus chiapasius (FMNH 73694). Scale bar = 2 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 44 J. P. RIO ET AL. Figure 30. Comparisons of the posterior wall of the supratemporal fenestra in several crocodylians. A–C, enlarged left supratemporal fenestrae of Alligator mississippiensis (NHMUK uncatalogued) (A), Crocodylus porosus (NHMUK uncatalogued) (B) and Diplocynodon ratelii (MNHN SG 539) (C). D, both supratemporal fenestrae of Diplocynodon hantoniensis (NHMUK OR 30393). Abbreviations: pa, parietal; pr, prootic; qd, quadrate; sq, squamosal. White arrows indicate the position of the quadrate-parietal suture. Scale bar = 1 cm. macrorhynchus de Blainville, 1855 [based on character to the suborbital fenestra. In contrast, D. hantoniensis scores in Brochu (1999)] and possibly in D. deponiae also has an acute process of the ectopterygoid in the (Delfino & Smith, 2012). In the latter case, Delfino & pterygoid, which we consider an autapomorphy of the Smith (2012) expressed some doubt that IRSNB 261, species (Fig. 8A). which formed the basis of their description, was a The choanae of D. hantoniensis are heart-shaped, mature individual of D. deponiae. As a result, we cannot resulting from a posterior projection of the pterygoid, be confident that ectopterygoid–pteryoid flexure is which aligns with the recessed choanal septum (Figs retained into adulthood in D. deponiae, and thus we 6B, 31A). Furthermore, they are ornamented with an re-scored this character as missing in this taxon. All anterolaterally directed ridge on each side. This ridge other species of Diplocynodon lack the ectopterygoid– is even developed in juvenile specimens, becoming a pterygoid flexure. The condition in D. hantoniensis can prominent ventrally directed lamina bound by a sulcus only be confidently observed in a large isolated pair of at maturity. Heart-shaped choanae with anterolaterally pterygoids (NHMUK OR 30251). Here, the condition is developed ridges are also present in D. remensis slightly different to that of Caiman, which has a large (Martin et al., 2014: fig 6K) and, tentatively, in D. lateral process of the pterygoid immediately posterior ratelii (MNHN SG 539) (Fig. 31D). The lateral walls of © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 45 Figure 31. Comparisons between the structure of the choanae, in ventrolateral view, in: A, Diplocynodon hantoniensis (CAMSM TN 907); B, Caiman yacare (MACN I-15144-3603); C, Alligator mississippiensis (NHMUK 1873.2.21.1); D, Diplocynodon ratelii (MNHN SG 539); E, Crocodylus intermedius Graves, 1819 (NHMUK 1851.8.25.29); F, Paleosuchus trigonatus (Schneider, 1801) (NHMUK 1868.10.8.1). Scale bar = 2 cm. the choana are broken off in the latter specimen, but characters has been based on the assumption that the deep sulci similar to D. hantoniensis are suggestive of crocodylian postcranial skeleton is morphologically a similar ridge. Wu et al. (2001) also described ridges conservative, being less phylogenetically informative lateral to the choana in Leidyosuchus canadensis than cranial and mandibular components (see: Lambe, 1907, although restricted to some of the Godoy et al., 2016). However, recent studies of axial largest individuals. This contrasts with the condition and appendicular skeletal components in extant in D. hantoniensis, in which they appear throughout crocodylians have demonstrated that there are broad ontogeny. The ridges in D. hantoniensis, D. remensis differences in crocodylian postcrania, which have yet and D. ratelii appear homologous to those of several to be incorporated into phylogenetic studies (Chamero caimanines (Figs 31B, F), although the morphology is et al., 2013; Iijima et al., 2018). Furthermore, recent clearly different. In extant Caiman (Fig. 31B), these studies of notosuchian (Pol et al., 2012; Godoy et al., ridges are shorter, restricted to the posterolateral 2016), basal neosuchian (Martin et al., 2016) and choanal margins and commonly folded medially into crocodylian (Iijima & Kobayashi, 2019) anatomy have the choana. In Paleosuchus (Fig. 31F), ridges can started to demonstrate that there is a potentially richer encircle the choana and become almost completely suite of postcranial characters, and that these might everted. The ridges are also similar to those developed impact upon crocodylomorph phylogenetic relationships. in Alligator (Fig. 31C), but in that taxon they form a The quantity and quality of preserved postcranial near continuous posterior wall to the choana and do material belonging to Diplocynodon hantoniensis not extend very far anteriorly. affords comparisons with extant and fossil taxa, and the identification of potential postcranial characters. In particular, the appendicular material The potential for new phylogenetic is especially well preserved and represented by characters from the postcrania several ontogenetic stages, allowing us to distinguish Postcranial character sampling is low in Crocodylia, as between interspecific and ontogenetic variation. is the case for all of Crocodylomorpha (Godoy et al., 2016; In addition to the recognition of numerous muscle Mannion et al., 2019). For example, in the data matrix scars (see Description), below we briefly outline a used here, just 46 of 187 characters (25%) represent number of potential new characters pertaining to the postcranial skeleton. The dearth of postcranial the appendicular skeleton. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 46 J. P. RIO ET AL. Figure 32. Comparisons of humeral torsion and axial rotation of the proximal extremity of the humerus in Crocodylia, in medial (left) and ventral (right) views. The left humerus of Crocodylus porosus (QM J 48127) (A, B), Kambara sp. (mirror image of the right humerus) (QM F 56060) (C, D), Diplocynodon hantoniensis (NHMUK OR 30206) (E, F), Borealosuchus sternbergii (USNM 6533) (G, H), Asiatosuchus germanicus (SMF Me 1801) (I, J) and Gavialis gangeticus (Gmelin, 1789) mirror image of the right humerus (USNM 576261) (K, L). Abbreviations: dpc, deltopectoral crest. Arrows point to the apex of the deltopectoral crest. Vertical cross bars on arrows show the potion of the lateral edge of the humerus. All scale bars = 2 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 ANATOMY AND PHYLOGENY OF DIPLOCYNODON 47 Only two humeral characters appear regularly in phylogenetic analyses of crown group crocodylians. These pertain to the shape of the deltopectoral crest and the presence of a separate or single common insertion for M. teres major and M. dorsalis scapulae (Brochu et al., 2012: C27 and C28). In addition, character 36 in the matrix used herein describes the slenderness and relative length of the forelimb and hindlimb. Observations of D. hantoniensis and other crocodylians have led us to recognize two other potentially significant features in the humerus. Stein et al. (2012) recognized differences in humeral torsion between the early Eocene Australian mekosuchine, Kambara, with that of Crocodylus johnstoni Krefft, 1873 and C. porosus. Humeral torsion describes the dorsoventral offset of the proximal and distal extremities (i.e. the metaphysis and epiphysis) of the humerus, which is most noticeable in medial view. A high degree of torsion results in a strongly sigmoidal outline of the humerus (Fig. 32A), whereas a low degree of torsion results in a relatively straight outline (Fig. 32C). Stein et al. (2012) noted that humeral torsion appears to be associated with a rotation of the lateral margin of the proximal extremity and resultant medial rotation of the deltopectoral crest (i.e. axial rotation of the proximal extremity). This results in movement of the apex of the deltopectoral crest from the lateral edge, to the centre of the humerus, when viewed ventrally (Fig. 32B). This also exposes the ventral surface of the proximal extremity in medial view.Whereas these two features appear to be linked to one another in Crocodylus and Kambara, an examination of a larger sample of crocodylians reveals that these features are not always associated. For example, D. hantoniensis exhibits a high degree of dorsoventral offset of the proximal and distal humeral extremities, without the axial rotation of the proximal end (Fig. 32E, F). This might be the plesiomophic condition in Crocodylia, as it is observed in the stem brevirostrine Borealosuchus sternbergii (Gilmore, 1910) (USNM 6533) (Fig. 32G, H) and the basal crocodyloid Asiatosuchus germanicus Berg, 1966 (SMF Me 1801) (Fig. 32I, J). The same condition is also observed in a number of other alligatoroids, e.g. D. ratelii (MNHN SG 628), Alligator (AMNH 71621) and Caiman (AMNH 97300). In contrast, Gavialis appears to lack significant dorsoventral offset of the proximal and distal extremities. When viewed medially, the distal half of the humerus of Gavialis is almost straight (Fig. 32K). However, the proximal end, beginning around the level of the deltopectoral crest, shows extreme axial rotation (Fig. 32L). A comparison of the humeri of C. porosus at different ontogenetic stages reveals variation in the second character (axial rotation of the proximal extremity) (Fig. Figure 33. Comparisons between the humeri of adult 33A, B). Humeral torsion appears relatively consistent Crocodylus porosus (QM J48127) (mirror image of left in this growth series (Fig. 33C, D), but the juvenile humerus) (A, C) and juvenile Crocodylus porosus (AMNH specimens show less axial rotation of the proximal 7115) (B, D). Arrows point to the apex of the deltopectoral extremity than adult specimens. As a result, the latter crest. Vertical cross bars on arrows show the potion of the should be compared only between adult specimens. lateral edge of the humerus. All scale bars = 2 cm. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51 Downloaded from https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz034/5525732 by UNIVERSITAT AUTONOMA DE BARCELONA user on 11 November 2019 48 J. P. RIO ET AL. CONCLUSIONS sheds light on the early history of caimanines. Proceedings of the Royal Society B 285: 20180843. The basal alligatoroid Diplocynodon hantoniensis from Brazeau MD. 2011. Problematic character coding methods the late Eocene (Priabonian) of southern England is fully in morphology and their effects. Biological Journal of the described for the first time in 150 years. This includes a Linnean Society 104: 489–498. detailed description of the often neglected postcranial Brinkmann W, Rauhe M. 1998. Diplocynodon ratelii POMEL, skeleton, including a reconstruction of numerous muscle 1847 (Crocodylia, Leidyosuchidae) aus dem Unter-Oligozän attachment sites, allowing future comparisons with other von Céreste (Südfrankreich). Neues Jahrbuch für Geologie crocodylians. We also recognize humeral torsion and axial und Paläontologie, Abhandlungen 209: 295–321. rotation of the proximal humeral extremity as two new Brochu CA. 1997. A review of ‘Leidyosuchus’ (Crocodyliformes, independent characters, which show notable variation in Eusuchia) from the Cretaceous through Eocene of North Crocodylia that can be discretized in future phylogenetic America. Journal of Vertebrate Paleontology 17: 679–697. analyses. Diplocynodon hantoniensis can be diagnosed by Brochu CA. 1999. Phylogenetics, taxonomy, and historical a unique combination of characters and its geographic biogeography of Alligatoroidea. Journal of Vertebrate Paleontology 19: 9–100. range is currently restricted to the late Eocene of the Brochu CA. 2011. Phylogenetic relationships of Necrosuchus UK. Using a revised phylogenetic data matrix, including ionensis Simpson, 1937 and the early history of caimanines. all valid species of Diplocynodon, we show that implied Zoological Journal of the Linnean Society 163: S228–S256. and extended implied weighting improves tree resolution Brochu CA, Storrs GW. 2012. A giant crocodile from the producing largely congruent tree topologies with equal Plio-Pleistocene of Kenya, the phylogenetic relationships weighted trees. of Neogene African crocodylines, and the antiquity of Crocodylus in Africa. Journal of Vertebrate Paleontology 32: 587–602. ACKNOWLEDEGEMENTS Brochu CA, Parris DC, Grandstaff BS, Denton RK Jr, We would like to thank Paul Barrett, Sandra Chapman, Gallagher WB. 2012. A new species of Borealosuchus Susannah Maidment and Lorna Steel for providing (Crocodyliformes, Eusuchia) from the Late Cretaceous–Early Paleogene of New Jersey. Journal of Vertebrate Paleontology access to the Diplocynodon hantoniensis materials held 32: 105–16. in the NHMUK, and Matt Riley for providing access to Buscalioni AD, Sanz JL, Casanovas ML. 1992. A new material in the CAMSM. We also acknowledge those species of the eusuchian crocodile Diplocynodon from who have facilitated collections’ visits to study additional the Eocene of Spain. Neues Jahrbuch für Geologie und Diplocynodon remains (Ronan Allain, Krister Smith and Paläontologie, Abhandlungen 187: 1–29. Torsten Wappler). We also thank Michael Stein for sharing Chamero B, Buscalioni AD, Marugán-Lobón J. 2013. photographs of the humerus of Kambara from the QM. We Pectoral girdle and forelimb variation in extant Crocodylia: gratefully acknowledge the Willi Hennig Society, which the coracoid–humerus pair as an evolutionary module. has sponsored the development and free distribution of Biological Journal of the Linnean Society 108: 600–618. TNT. JPR is funded by a NERC PhD studentship. PDMs Chandler MEJ. 1961. Flora of the Lower Headon Beds of research was supported by a Royal Society University Hampshire and the Isle of Wight. Bulletin of the British Research Feollowship (UF160216). ET holds a Theodore Museum (Natural History), Geology Series 5: 93–157. Roosovelt Memorial Fund and Division of Paleontology Chippindale PT, Wiens JJ. 1994. Weighting, partitioning, Postdoctoral Fellowship from the Richard Gilder Graduate and combining characters in phylogenetic analysis. School at the American Museum of Natural History. Systematic Biology 43: 278–87. Cray PE. 1973. 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The London (1→?), C43 (1→?), C49 (1→0), C62 (1→0), C65 Geological Journal, and Record of Discoveries in British and (0→1), C66 (0→1), C84 (?→1), C86 (?→1), C90 Foreign Paleontology 1: 1–7. (?→0), C94 (0→1), C96 (?→0), C125 (0→1), C144 Woodward AS. 1885. On the literature and nomenclature of (?→1), C146 (0→1), C147 (1→2), C162 (0→1), C173 British fossil Crocodilia. Geological Magazine 2: 496–510. (?→0). SUPPORTING INFORMATION Additional Supporting Information may be found in the online version of this article at the publisher's web-site. Supporting Information 1. Character taxon matrix nex file. Supporting Information 2. Character taxon matrix TNT file. Supporting Information 3. Character list. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, XX, 1–51