1 Triassic clam shrimps (“Conchostraca”; Branchiopoda: Diplostraca) from 2 Mallorca: Taxonomic description and interregional comparisons 3 4 Frank Scholze 1 & Rafel Matamales-Andreu 2,3 1 5 Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Burgweg 11, 07749 6 Jena, Germany. E-mail: Frank.Scholze@uni-jena.de 2 7 Institut Català de Paleontologia Miquel Crusafont (ICP), C/ de les Columnes s/n, Edifici Z 8 (ICTA-ICP), Campus de la Universitat Autònoma de Barcelona, 08193 Cerdanyola del 9 Vallès, Barcelona, Catalunya, Spain. E-mail: rafel.matamales@icp.cat 3 10 Museu Balear de Ciències Naturals, Carretera Palma–Port de Sóller Km. 30, 07100 Sóller, 11 Mallorca, Illes Balears, Spain. 12 13 14 Abstract 15 We describe four upper Lower Triassic to lower Middle Triassic clam shrimp–bearing 16 intervals from Mallorca, which include the clam shrimp species Hornestheria sp. aff. 17 Hornestheria sollingensis and several other forms of carapace valve morphology: 18 Hornestheria? Morphotype 1, Hornestheria? Morphotype 2, and other undetermined carapace 19 valves. All of this material was obtained from red-bed units cropping out in the Serra de 20 Tramuntana mountains of Mallorca (western Mediterranean). Except for a few 21 morphologically similar carapace valves of Middle Triassic age from China, Hornestheria is 22 known only from the type locality of its type species, Hornestheria sollingensis Kozur et 23 Lepper, in the Solling Formation (Middle Buntsandstein Subgroup) in the German part of the 24 Central European Basin. According to its original definition, the larval carapace valve of 25 Hornestheria Kozur et Lepper is characterized by a radial sculpture, but this characteristic 26 apparently is only variably developed. Due to both a limited number of previously known 27 occurrences of Hornestheria and its poorly known carapace valve morphology, open 28 nomenclature is applied to the taxonomy herein. The studied specimens were freshly collected 29 from outcrop sections composed of greyish-green to greyish-red laminated claystones and 30 siltstones that accumulated in a fluvial facies. The clam shrimp specimens are accompanied 31 by remains of insects and fishes, invertebrate and tetrapod ichnofossils, and micro- 32 /macroplant remains, all of which either have been described by previous workers or are 33 currently part of a separate study. 34 35 Key words: Crustacea, Spinicaudata, Hornestheria, Pangaea, Balearic Islands, western 36 Mediterranean 37 38 Introduction 39 The traditional term ‘conchostracans’ designates a group of small-sized branchiopod 40 crustaceans, which are known from Devonian to recent times (e.g., Tasch 1969; Hegna & 41 Astrop 2020). They have a chitinous, bivalved carapace, composed of two symmetric valves 42 laterally enclosing their crustacean body. However, it is difficult to link their fossil forms to 43 the zoological classification scheme of their recent relatives (i.e., clam shrimps and 44 waterfleas) due to the often poorly preserved morphological characteristics of fossil carapace 45 valves. Therefore, fossil ‘Conchostraca’ are treated as a paraphyletic taxonomic group 46 corresponding to a composite of the extant monophyletic suborders Spinicaudata, 47 Laevicaudata, Cyclestherida, and Cladocera (e.g., Rogers & Hegna 2020). In both fossil and 48 recent taxonomic systematics, these four suborders comprise the order Diplostraca within the 49 class Branchiopoda. Occurrences of modern clam shrimps in continental deposits are known 50 from all continents other than Antarctica (Frank 1988). Fossil clam shrimps are mostly found 51 in continental bodies of water, but can also be found in both brackish and shallow marine 52 deposits (e.g., Webb 1979; Liu & He 2000; Scholze et al. 2020). 53 Both modern and fossil clam shrimps are (palaeo-)geographically widely distributed 54 from the Devonian to the present, because of the very high dispersal potential of their minute, 55 drought and/or frost resistant, wind-transportable dormant eggs. Their eggs can also be 56 attached to other animals or survive passage through the digestive system of predators, thus 57 adding to their geographic dispersal mechanisms (e.g., Vanschoenwinkel et al. 2008). As 58 noticed by previous workers (e.g., Schneider et al. 2020), clam shrimp habitats changed 59 during the late Palaeozoic from perennial lakes to seasonal and temporary lakes, ponds, and 60 puddles, coincident with a nearly global palaeoclimate aridification that began at that time. 61 Like other pioneering organisms, many clam shrimp species (including all modern species of 62 spinicaudatans and laevicaudatans) became adapted to thrive in ephemeral freshwater pools, 63 as well as in water bodies with elevated salinities because of evaporation, where their 64 carapace valves often accumulated in mass occurrences that are useful for biostratigraphy. 65 Here, we study the clam shrimps from the lower Middle Triassic continental units of 66 Mallorca. The deposits of the study area are known for their occurrences of insects (Calafat & 67 Sáez 1987; Calafat 1988; Zessin 2008a, 2008b; Aristov & Zessin 2009; Juárez-Ruiz & 68 Wachtler 2015) and tetrapod trace fossils (Juárez-Ruiz & Wachtler 2015). Additionally, 69 records of clam shrimps (“Euestheria sp.”), shark egg capsules (“Palaeoxyris sp.”), a fish of 70 unknown affinities, and arthropod trace fossils also have been mentioned by Calafat & Sáez 71 (1987), Calafat (1988), and Zessin (2008b). Additionally, occurrences of conifers and other 72 micro- and macroplant remains have been reported in detail by previous workers (e.g., 73 Álvarez-Ramis et al. 1995; Grauvogel-Stamm & Álvarez Ramis 1996; Diez et al. 2010; 74 Juárez-Ruiz & Wachtler 2015). All of these records open a window into a local biota that 75 succeeded the aftermath of the end-Permian (e.g., Joachimski et al. 2012; Shen & Bowring 76 2014; Biswas et al. 2020) to the Early Triassic (e.g., Galfetti et al. 2007; Stanley 2009; 77 Hochuli et al. 2016; Zhang et al. 2019) mass extinction event, which is known to be the most 78 profound biotic crisis in the history of multicellular life on Earth (e.g., Benton 1995, 2018; 79 Stanley 2016; Delfino et al. 2020). From the upper Lower Triassic to the lower Middle 80 Triassic, fossil clam shrimps are known from only a few of localities apart from the ones 81 studied herein, such as France (e.g., Gall & Grauvogel-Stamm 2005), Germany (e.g., Kozur et 82 al. 1993; Mahler & Sell 2015), Poland (e.g., Żyła et al. 2013), Tunisia (Soussi et al. 2017), 83 and Argentina (e.g., Tassi et al. 2015). Comparing these occurrences with our new data from 84 Mallorca contributes to a better understanding of the Early–Middle Triassic post-crisis fauna 85 within a wider palaeogeographic realm than has been previously possible. Additionally, it is 86 hoped that our present study will motivate future working groups to improve the general 87 knowledge on transitional Lower Triassic–Middle Triassic clam shrimp biostratigraphy 88 through additional future studies, as clam shrimps have great potential for both fine-scale 89 biozonation and regional to interregional biostratigraphic correlations (e.g., Kozur & Weems 90 2010; Scholze et al. 2016; Schneider et al. 2020). 91 92 Geologic setting 93 The study area is located in the Serra de Tramuntana, a mountainous area in the northwest of 94 the island of Mallorca (western Mediterranean). The studied sections, comprising a Lower(?)– 95 Middle Triassic interval of predominantly siliciclastic red-beds (‘Buntsandstein facies’), crop 96 out along the coastal cliffs between Cala d’Estellencs and Punta de son Serralta (Fig. 1A). 97 Gómez-Gras (1993) recognised four (herein called A–D) main lithostratigraphic units in these 98 outcrops: (A) a basal sandstone unit representing braided rivers, (B) a lower sandstone and 99 siltstone unit corresponding to meandering rivers, (C) a middle sandstone unit of meandering 100 or braided river deposits, and (D) an upper siltstone and sandstone unit (Fig. 1B) deposited in 101 tidal flats transitional to the overlying shallow marine deposits of the so-called ‘Muschelkalk 102 facies’ (e.g., Matamales-Andreu et al. 2020). The lithostratigraphic units A–D have not been 103 formally described so far. Diez et al. (2010) provided age attributions based on their review of 104 palynomorphs found in some beds of the units B, C, and D, attributing the lowest two to the 105 Aegean Substage (lower Anisian Stage, lower Middle Triassic), and the upper one to an 106 indeterminate substage within the Anisian. 107 Clam shrimps studied in the present work were all collected in situ from four different 108 stratigraphic horizons, all of them consisting of greenish or reddish grey, finely laminated 109 claystones/siltstones, possibly representing small, dysoxic pools formed on fluvial floodplains 110 near meandering channels or in abandoned channels of the braided rivers during low-water 111 intervals. The four sites have been named (in ascending stratigraphic order) “Punta Roja” 112 (middle part of Unit B; Fig. 1C), “Pedra Alta” (upper part of Unit B; Fig. 1D), “Punta Negra 113 1” (base of Unit C; Fig. 1E) and “Punta Negra 2” (top of Unit C; Fig. 1F). Figure 1B shows a 114 synthetic stratigraphic log that indicates the location of the four sites. 115 116 Material and methods 117 Systematic, bed-by-bed collection of freshly exposed claystone and siltstone slabs from the 118 studied outcrop sections was carried out during a field campaign in 2019 (which was 119 authorized through an excavation permit granted by the Consell de Mallorca under the ref. 120 305-19, code 0313JUL2019ARQ.305-19.estellencs). About 47 slabs were obtained, each 121 containing between one and 20 clam shrimp carapace valves, resulting in a total collection of 122 112 individual carapace valves. The best preserved individuals (25 in total) were selected for 123 both microscope drawing and photography of their carapace valves, using a LEICA MZ12 124 stereo-microscope with a WILD drawing mirror tube, and a BRESSER MikroCamII digital 125 camera. Multiple photographs were taken of each of these clam shrimp carapace valves at 126 different focus distances, which were later vertically stacked automatically using the software 127 ADOBE Photoshop CS4. To make the microscope line drawings by hand with the drawing 128 mirror tube, a cold light source equipped with a flexible rubber light guide was used for 129 variable illumination of the specimens. The vector graphic software COREL DRAW 20 was 130 used for digitizing the microscope line drawings. 131 The microscope drawings were used for precisely measuring a set of carapace valve 132 parameters (e.g., total valve height [H], valve length [L], dorsal margin length [l], larval valve 133 height [h], total number of growth lines), according to the procedure and terminology 134 proposed by Scholze & Schneider (2015). In order to make a taxonomic determination of the 135 clam shrimp material, carapace valve parameters such as the total size values (valve length 136 and height) and their ratio values (total valve height/total valve length, dorsal margin 137 length/total valve length), as well as the positions of points of maximum curvature at the outer 138 margins of the carapace valve, were used for comparison with literature data, which primary 139 includes Triassic clam shrimps (e.g., Reible 1962; Gall 1971; Chang et al. 1976; Kozur & 140 Seidel 1983a; Kozur & Weems 2010). 141 During the stereomicroscopic investigation, it was observed that some microstructures 142 (i.e., remnants of ornamentation on the carapace valves) were partially preserved on a few 143 individuals. Initial attempts to photograph these ornamentation patterns under higher 144 magnifications, utilizing a KEYENCE VHX 6000 digital microscope failed due to ineffective 145 illumination through the inner lens of this digital microscope. Therefore, selected individuals 146 were permanently coated with gold films, and then photographed using a PHILIPS XL30 147 Environmental Scanning Electron Microscope (ESEM) with an acceleration voltage of 10 kV 148 and up to 600x magnification. The resulting descriptions on observed microstructures could 149 serve as additional criteria in clam shrimp taxonomy, as generally discussed in previous 150 studies on other clam shrimp occurrences (e.g., Orlova 1978; Li 2017). 151 All specimens figured herein are permanently stored at Museu de Mallorca in the city 152 of Palma (Mallorca, Balearic Islands, Spain). 153 154 Results 155 The studied clam shrimp carapaces are preserved as a dark greyish to slightly patchy 156 yellowish brown-coloured substance with irregular thicknesses, which represent either 157 remnants of organic materials and/or a derivate of the primary (chitinous) shell substance. An 158 overview of their occurrences in their respective fossiliferous sites, along with a summary of 159 the coordinates, lithostratigraphy, and the taxonomy of the recorded clam shrimp occurrences, 160 is given in Table 1. 161 For the following morphologic description of the clam shrimp carapace valves, the 162 terminology proposed by Scholze & Schneider (2015) is used. The systematic hierarchy (see 163 below Subchapter ‘Systematic palaeontology’) follows Kozur & Weems (2010), Hegna & 164 Astrop (2020), and Hegna & Olesen (2020), except for taxonomically indeterminate 165 individuals (see Subchapter ‘Indeterminate clam shrimps’). 166 167 Table 1. Summary of the (?)upper Lower Triassic–lower Middle Triassic clam shrimp 168 occurrences from the present study. Name of the Coordinates Lithostratigraphy Identified clam shrimp taxa fossil site Punta Negra 2 39°39’41”N, Uppermost part of Hornestheria sp. aff. 02°28’33”E Unit C Hornestheria sollingensis Punta Negra 1 39°39’42”N, Lowermost part of Indeterminate 02°28’31”E Unit C Pedra Alta 39°39’41”N, Upper part of Unit B Hornestheria? Morphotype 1; 02°28’28”E Hornestheria? Morphotype 2 Punta Roja 39°39’54”N, Middle part of Unit B Indeterminate 02°28’39”E 169 170 171 Systematic palaeontology 172 Class Branchiopoda Lamarck, 1801 173 Subclass Phyllopoda Preuss, 1951 174 Order Diplostraca Gerstaecker, 1866 175 Superfamily Lioestheriacea Raymond, 1946 emended Holub et Kozur, 1981 176 Family Xiangxiellidae Shen in Chang et al., 1976 177 178 Remarks. Xiangxiellidae was defined as a subfamily in Chang et al. (1976) and attributed 179 therein to the family Vertexiidae Kobayashi, 1954. Carapace valves of Vertexiidae have 180 concavely recurved growth lines along the upper posterior margin below the dorsal margin, 181 and sometimes a spine-like sculpture on its larval carapace valve (Kobayashi 1954, Astrop & 182 Hegna 2015). These two characteristics are absent in Xiangxiellinae, which excludes this 183 taxon from the family Vertexiidae sensu strico. Instead, Xiangxiellidae became elevated to 184 family rank by Kozur & Weems (2010), which is followed herein. 185 186 Genus Hornestheria Kozur et Lepper in Kozur et Weems, 2010 187 188 Type species. Hornestheria sollingensis Kozur et Lepper (in Kozur & Weems, 2010); from 189 the Frohrieper Berg Quarry (East of the town of Lauenförde, Lower Saxony, Germany), 190 Solling Formation (Middle Buntsandstein Subgroup, Buntsandstein Group), uppermost 191 Spathian (uppermost Olenekian, uppermost Lower Triassic) (Kozur & Weems 2010: p. 395). 192 Hornestheria sollingensis was used in stratigraphic charts (e.g., Bachmann & Kozur 2004) 193 before the taxonomic characteristics for its genus and species levels were formally published 194 by Kozur & Weems (2010: appendix 1). 195 196 Remarks. A diagnosis of this genus was proposed by H.W. Kozur and J. Lepper (in Kozur & 197 Weems 2010), but metric values for total carapace valve length and height were not provided. 198 Besides a low convexity of the carapace valves and their almost straight dorsal margin, other 199 characteristics of Hornestheria included the location of points of maximum curvature of the 200 anterior margin in the horizontal midline of the carapace valve and of the posterior margin 201 below the horizontal midline (Kozur & Weems 2010). The larval carapace valve (= umbonal 202 area of the valve free of growth lines) bore a rather enigmatic radial element, which was either 203 “often quite distinct” (Kozur & Weems 2010: p. 395) or unrecognizable, depending on the 204 respective preservation state of individual carapace valves. 205 Based on the description of Hornestheria in Kozur & Weems (2010), the radial 206 element on the larval carapace valves of Hornestheria most closely resembles that of 207 Lioestheria Depéret et Mazeran, 1912, which has both nodular and radial sculptures on its 208 larval carapace valves (e.g., Kozur et al. 1981). In contrast, however, the size of the larval 209 carapace valve in adult individuals of Lioestheria is greater than 1/3 of the total carapace 210 valve length (Holub & Kozur 1981), and this is much larger relatively than the size of the 211 larval valve seen in the carapace of the genus Hornestheria. This difference provides a clear 212 distinction between Hornestheria and Lioestheria sensu Holub & Kozur (1981). 213 Prominent sculptures are also found on the larval carapace valves of the clam shrimp 214 genus Cornia Lutkevich, 1937, which is widely distributed in the Lower Triassic (e.g., Kozur 215 & Seidel 1983a; Kozur & Mock 1993; Tassi et al. 2013; Scholze et al. 2016). This genus is 216 characterized by an umbonal spine with a circular base located on the larval carapace valve. 217 The spine can be variably preserved (Kozur 1983), being either perpendicular or obliquely 218 embedded in the sediment or being broken, due to variable deformation (e.g., sedimentary 219 compaction) of the rock. According to the description for Hornestheria provided by Kozur & 220 Weems (2010), the generally radial shape of its larval sculpture makes it consistently different 221 from the circular shape of the base of the umbonal spine in Cornia. 222 A further characteristic of Hornestheria in the sense of H.W. Kozur and J. Lepper (in 223 Kozur & Weems 2010) is the presence of certain types of microsculpture (ornamentation) 224 located on the growth bands of the carapace valves. The ornamentation of the genus 225 Hornestheria was originally described as a dense, fine reticulation, which became replaced by 226 fine, short, radial lirae in the outer margins in adult individuals (Kozur & Weems 2010). 227 However, it should be noted that the figures of H. sollingensis in Kozur & Weems (2010: fig. 228 A2) show only indistinct reticulation. 229 In lateral view, the carapace valves of Hornestheria are morphologically similar to 230 Euestheria Depéret et Mazeran, 1912 and Magniestheria Kozur, 1982. However, in contrast 231 to Hornestheria, the carapace valves of Euestheria have a more strongly convex shape with 232 its highest convexity in the umbonal area. The highest convexity in carapace valves of 233 Magniestheria is located in its central area. Both Euestheria and Magniestheria differ in their 234 ornamentation from Hornestheria. The ornamentation of Euestheria is either finely dotted to 235 weakly reticulated (Kozur & Seidel 1983a) or very finely pitted, having pits of round to 236 round-oval shapes with diameters of about 10 μm (Sell 2018), that appear generally to be 237 more regular and smoother than the ornamentation seen in Hornestheria. The ornamentation 238 of Magniestheria is indistinct, varying between coarse pits and very short, irregular, radial 239 lirae (Kozur & Seidel 1983a); their expression is less intense than in Hornestheria. 240 241 Occurrence. Uppermost Spathian (uppermost Olenekian; upper Lower Triassic) and Anisian 242 (lower Middle Triassic) in Germany (Kozur & Weems 2010), China (see discussions on 243 “Protomonocarina” in Kozur & Weems 2010: p. 395), and Mallorca (the present study). 244 245 246 Hornestheria sp. aff. Hornestheria sollingensis Kozur et Lepper (in Kozur & Weems 2010) 247 (Figures 2–7) 248 249 Material. DA21/03/02/58, DA21/03/02/68, DA21/03/02/72, DA21/03/02/75, 250 DA21/03/02/77, DA21/03/02/78, DA21/03/02/80, DA21/03/02/81. 251 252 Description. Carapace valve medium-sized to large, elongated-oval to oval shape; dorsal 253 margin straight, short to long; larval carapace valve very small; umbo located submedial and 254 marginal to inframarginal; anterior and posterior margins sharply curved to very sharply 255 curved; points of maximum curvature along the anterior and posterior margins located in the 256 median-dorsal to median-ventral region, and along the ventral margin in the median to 257 median-posterior region; ornaments fine, indistinct or irregularly reticulated. 258 259 Dimensions. Height (H) of the carapace valve between 1.6–3.9 mm; length (L) of the valve 260 between 2.5–3.9 mm; 0.57