ORIGINAL RESEARCH published: 24 July 2020 doi: 10.3389/feart.2020.00248 Upper Paleozoic to Lower Mesozoic Tetrapod Ichnology Revisited: Photogrammetry and Relative Depth Edited by: Martin Daniel Ezcurra, Pattern Inferences on Functional Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Prevalence of Autopodia Argentina Reviewed by: Eudald Mujal 1,2† , Lorenzo Marchetti 3† , Rainer R. Schoch 1† and Josep Fortuny 2* † Silvina De Valais, 1 Staatliches Museum für Naturkunde Stuttgart, Stuttgart, Germany, 2 Institut Català de Paleontologia Miquel Crusafont, CONICET Research Institute Cerdanyola del Vallès, Spain, 3 Urweltmuseum GEOSKOP/Burg Lichtenberg (Pfalz), Tallichtenberg, Germany in Paleobiology and Geology (IIPG), Argentina Steven W. Salisbury, In recent years photogrammetry has become an essential tool in the study of tetrapod The University of Queensland, Australia footprints. Morphological analyses of footprints are interpretative; thus, researchers Verónica Krapovickas, should use as much information as possible in order to eventually provide an objective Consejo Nacional de Investigaciones conclusion. In this regard, photogrammetry is an extremely helpful tool to avoid potential Científicas y Técnicas (CONICET), Argentina biases and to better present ichnological data. We review the use of this technique *Correspondence: in several Permian and Triassic tetrapod ichnological studies, with considerations on Josep Fortuny (1) ichnotaxonomy, (2) track-trackmaker correlation, (3) locomotion and/or behavior, josep.fortuny@icp.cat † (4) substrate induced effects, and (5) preservation of the fossil record and heritage. ORCID: Eudald Mujal Furthermore, based on the available three-dimensional (3D) data on Permian and orcid.org/0000-0002-6310-323X Triassic material, we present a first qualitative interpretation of relative depth patterns Lorenzo Marchetti and the related functional prevalence (most deeply impressed area) within footprints. We orcid.org/0000-0002-1047-7887 Rainer R. Schoch identified three main groups: (1) anamniote, captorhinomorph/parareptile tracks (medial- orcid.org/0000-0002-0312-2877 median functional prevalence), (2) diapsid tracks (median functional prevalence), and (3) Josep Fortuny orcid.org/0000-0003-4282-1619 synapsid tracks (median-lateral functional prevalence). The use of 3D photogrammetric models brings new light to the tetrapod footprint record, helping to better understand Specialty section: tetrapod communities throughout the late Paleozoic (and the end-Guadalupian and This article was submitted to Paleontology, end-Permian extinctions) and the tetrapod recovery during the early Mesozoic. a section of the journal Keywords: photogrammetry, tetrapod ichnology, ichnotaxonomy, locomotion, preservation, Permian, Triassic Frontiers in Earth Science Received: 20 December 2019 Accepted: 05 June 2020 INTRODUCTION Published: 24 July 2020 Citation: Tetrapod footprints are abundant vertebrate remains in upper Paleozoic to lower Mesozoic Mujal E, Marchetti L, Schoch RR terrestrial successions (Klein and Lucas, 2010; Lucas, 2019). Their study facilitates the and Fortuny J (2020) Upper Paleozoic reconstruction of past ecosystems, especially where skeletal remains are absent, scattered, or poorly to Lower Mesozoic Tetrapod preserved. Tetrapod tracks of late Carboniferous to Middle Triassic age have been intensively Ichnology Revisited: Photogrammetry and Relative Depth Pattern Inferences studied since the 19th century, and especially during the 20th century, with the publication of on Functional Prevalence an extensive literature that includes research articles, books, and monographies (among many of Autopodia. Front. Earth Sci. 8:248. other publications: Kaup, 1835; Geinitz, 1861, 1863; Curioni, 1870; Geinitz and Deichmüller, 1882; doi: 10.3389/feart.2020.00248 Pabst, 1895, 1908; Marsh, 1894; Maidwell, 1911; Nopcsa, 1923; Gilmore, 1927; Moodie, 1929, 1930; Frontiers in Earth Science | www.frontiersin.org 1 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology Abel, 1935; Peabody, 1948; Heyler and Lessertisseur, 1963; articles (e.g., Matthews, 2008; Petti et al., 2008; Falkingham, 2012; Haubold, 1970, 1971a,b; Gand, 1987). These works often Mallison and Wings, 2014; Belvedere et al., 2018; Falkingham include morphological analyses, usually comprising (1) a graphic et al., 2018). Historical photogrammetry, in which 3D digital interpretation (mostly represented by line and shadow drawings) models are created from scans of analog photographs taken of footprints and (2) a quantification of the track and trackway before photogrammetry even existed (e.g., Falkingham et al., proportions by measuring different features (for measurement 2014; Lallensack et al., 2015), is now also being used. Only standards: Haubold, 1971b; Leonardi, 1987). Such analyses recently, several works dealing with tetrapod footprints from form the basis of ichnotaxonomy that follows the rules of the Permian and Triassic (and, to a lesser degree, the upper the International Code of Zoological Nomenclature (ICZN; Carboniferous) used photogrammetry to better interpret the International Commission on Zoological Nomenclature, 1999). morphology of the tetrapod footprints and locomotion (e.g., Based on these morphological analyses, hundreds of ichnotaxa Mujal et al., 2015, 2016b, 2017a; Marchetti et al., 2017a, 2019c,d; (grouped in ichnogenera and ichnospecies, and sometimes Citton et al., 2018; Lagnaoui et al., 2019; Mujal and Marchetti, in ichnofamilies as well) have been erected. Nevertheless, in 2020; Mujal and Schoch, 2020; and references therein). several studies the conditions of the original substrate (e.g., The study between non-avian dinosaur ichnology and composition, granulometry, original moisture, rheology) where Permian–Triassic tetrapod ichnology is slightly different, though footprints were impressed and the behavior of the trackmakers they both follow the same conventions and use the same have not been fully considered, though such factors are the methodologies to correctly interpret the footprint morphology. main constraints (together with the autopodia anatomy) of In fact, non-dinosaur tetrapod footprints are generally smaller the final track morphology (Falkingham, 2014; Gatesy and in comparison with most dinosaur footprints and show a Falkingham, 2017; Belvedere et al., 2018; Marchetti et al., 2019a). higher degree of complexity, whereas dinosaur tracks (with Therefore, several ichnotaxa have been erected on the basis of some exceptions related to small-sized dinosaur ichnotaxa) show extramorphological features, which represent variations not due more complex depth patterns and thus difficulties to trace to the foot anatomy (Peabody, 1948; Haubold, 1996). This has footprint outlines (e.g., Lallensack, 2019). In this work, we resulted in an oversplitting of ichnotaxa and also, in particular review different ichnofossil records in which photogrammetry cases, in an oversimplification (McKeever and Haubold, 1996; was used, ranging from the lower Permian (Cisuralian) to Marchetti et al., 2019c,d). This is why footprints useful for the Middle Triassic. We review previously published works ichnotaxonomy need to be selected by means of morphological on tetrapod ichnology including 3D models. In addition, in preservation, which is the preservation of features derived from order to provide a wider overview and for further comparisons, the foot anatomy (Marchetti et al., 2019a). we also create new 3D models of some tracks and ichnotaxa, At the end of the 20th century, new techniques to create from which photogrammetric studies have not been carried 3D digital models started to be applied in tetrapod ichnology, out. We synthesize examples with the aim to demonstrate revolutionizing this field. These techniques allowed a better how photogrammetry helped in the ichnological analyses, not understanding of the processes of footprint formation, and only for newly uncovered fossils but also in ichnotaxonomic thus of producers’ locomotion, by creating virtual 3D models revisions. We identify up to five fields in which photogrammetry of this mechanism (e.g., Gatesy et al., 1999; Falkingham and is of great use: (1) ichnotaxonomy, (2) track-trackmaker Gatesy, 2014). 3D digital modeling rapidly expanded in dinosaur correlations, (3) locomotor and/or behavioral considerations, ichnology by digitizing actual fossil footprints in order to (4) substrate induced effects, (5) preservation of the fossil provide more objective interpretations and measurements (e.g., record and heritage. Moreover, for the first time, we discuss Leonardi and Mietto, 2000; Bates et al., 2008a,b, 2010; Petti the footprint relative depth patterns of the most important et al., 2008). However, 3D modeling has not been widely used upper Carboniferous, Permian, and Triassic tetrapod groups in the study of Permian and Triassic tetrapod ichnofossils, using 3D digital models and compare them to the phylogenies with few exceptions including 3D models, mostly used as of these groups. complementary representations of footprints (e.g., de Klerk, 2002; Petti et al., 2009). In the beginning, the use of 3D techniques, often based on laser scanning, was very limited, MATERIALS AND METHODS especially due to the cost and limited portability of the scanners; however, this completely changed with the rapid advances Elaboration of 3D Photogrammetric of photogrammetric techniques (Matthews, 2008; Remondino Models et al., 2010; Falkingham, 2012; Mallison and Wings, 2014; Photogrammetry plays a prominent role in tetrapod ichnological Falkingham et al., 2018). In fact, because of its low cost and analyses and is essential in some cases (e.g., in the identification efficiency, photogrammetry rapidly spread out in ichnological of the true shape of footprints when other techniques fail). studies, especially in dinosaur ichnology. The introduction of Furthermore, a single 3D model can be used for different 3D models brought more objectivity to the analyses of a given purposes. Photogrammetry provides important information in morphology. Several studies focused in the best procedure to ichnotaxonomy, in track-trackmaker correlations, in elucidating obtain the raw data for 3D photogrammetric models (i.e., digital the locomotion of the producers, in the reconstruction of photos), as well as in the methodologies to create models taphonomic processes and of paleoenvironmental settings, as well and/or how they should be studied and presented in scientific as in the digital preservation of this heritage. Frontiers in Earth Science | www.frontiersin.org 2 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology Herein, we provide a brief resume of how we created (1.2) Agisoft Photoscan (standard version 1.1.42 ): Generation 3D photogrammetric models. Noteworthy, the majority of the of the dense point cloud and the triangular mesh, and application examples reviewed in this work are uncollected specimens. In this of the original texture (i.e., color) of the specimen. case, the photographs for the 3D models were obtained during (2) MeshLab (open access; v.1.3.2 and v.2016.123 ): (1) fieldwork, so the data collection was usually constrained by the generation of the triangular mesh based on the dense point cloud; natural light conditions. The following work flow is modified (2) application of the original texture (i.e., color) of the specimen; from Mujal (2017). (3) cleaning (removing) of the generated odd points; (4) aligning, Overall, photogrammetry relies on obtaining digital 3D scaling, and orientation of the 3D model (Figure 1D). Note that models of an object. The photogrammetric models are built from the two first steps are unnecessary if Agisoft Photoscan is used. photos of the specimens (Figure 1). For detailed explanations (3) ParaView (open access; v.3.98.1 to v.4.1.04 ): Generation on the procedures of photo acquisition, we refer to Matthews of the depth map by using a color scale and application of (2008), Falkingham (2012), and Mallison and Wings (2014). the contours (Figure 1E). In order to provide a homogeneous In the examples presented here, these procedures were set view of the whole track sample, in this work we have used the according to the (field) conditions of each specimen. First of “Rainbow Desaturated” filter for the elevation map. Otherwise, all, the track-bearing surface must be cleaned. Then, centimetric the number of contours is set according to the complexity of and/or millimetric scales, as well as white paper squares with each specimen; the number of lines in each model generally varies specific black geometric forms (being easily recognizable by between 30 and 60. the modeling software), are placed around the footprint or trackway to be modeled (Figure 1A). Afterward light conditions Depth Pattern Analysis are evaluated, as specimens must not have too much shadow We conducted a qualitative analysis aiming to identify and nor too many bright zones during the photo acquisition. classify depth patterns among tetrapod ichnotaxa from the upper If necessary, a (weak) shadow is projected over the entire Carboniferous to the Middle Triassic (see section “Relative Depth specimen to be modeled in order to obtain homogeneous light Patterns on Permian to Triassic Tetrapod Footprints” below). The conditions. Once light conditions are suitable, photographs can database includes new data and a review of the published 3D data be taken in these ways: (1) photos of the specimen from all (de Klerk, 2002; Mujal et al., 2015, 2016a,b, 2017a,b, 2018b; Citton its perspectives and angles, i.e., moving along a cupola- or et al., 2016, 2018, 2019a,b, 2020; Meade et al., 2016; Milàn et al., umbrella-like form around the specimen while photographing 2016; Marchetti et al., 2017a,b, 2018a,b, 2019b,c,d; Francischini (Figure 1B), and/or (2) photos orthogonal to the surface, et al., 2018, 2020; Lagnaoui et al., 2019; Marchetti, 2019; Cavin covering the entire area to be modeled and taking into account and Piuz, 2020; Cisneros et al., 2020; Farman and Bell, 2020; that two consecutive photos must overlap at least one third Mujal and Marchetti, 2020; Mujal and Schoch, 2020; Reolid (Figure 1C). On average, 30 photos per footprint/pes-manus et al., 2020). Note that the number of tracks and localities from couple were taken, but this number varies according to the the upper Carboniferous is notably lower than those from the size and complexity of each specimen (e.g., some models are lower Permian. Nevertheless, most of the upper Carboniferous built up with 50, 60, and 70 photos, or in specific cases, such ichnotaxa are also present in the lower Permian (Lucas, 2019, and as whole trackways, with more than 200 photos). The use references therein). of camera tripods is recommended, although in most cases In order to avoid misinterpretations, here we define the (for specimens in the field) they cannot be used due to the functional prevalence of footprints as the part of the footprint location of the samples. Different digital cameras were used, that is more deeply impressed than the other parts of the same all being characterized by having at least 8.1 Megapixel of footprint. This character must be consistent along a trackway resolution. The cameras we used are Sony DSC-T200, device and/or between tracks assigned to the same ichnotaxon, and standard lens 35–175 mm F3.5–F4.4; Sony DSC-H50, device under different substrate (as well as different paleoenvironments) standard lens 31–465 mm F2.7–F4.5; Panasonic DMC-FZ18, and locomotory conditions. device standard lens 28–504 mm F2.8–F4.2; Canon PowerShot The relative depth patterns of footprints are independent SX410 IS, device standard lens 4.3–172 mm F.3.5–F6.3; Canon from all the ichnotaphonomic effects as defined by Marchetti EOS 70D, lenses 18–135 mm and 10–18 mm; focal lengths et al. (2019a). Therefore, it should be related to the functional are usually of 4, 5, or 6 mm. Once the photos were obtained, anatomy of the trackmakers’ postcranial skeleton (or, more they were processed by a workstation, with recommended specifically, autopodia). Medial, median, and lateral refer to the minimum requirements: i7 processor (or equivalent-updated) inner, middle, and outer parts, respectively, of a footprint as with minimum 32GB RAM and graphic card GeForce RTX defined by Leonardi (1987). To characterize the track-bearing 2060 equivalent or updated. Photos were taken (in JPG format) substrate (e.g., mudstone, sandstone, limestone), we evaluated without any size reduction in different software, most of which composition, granulometry, and sedimentary structures. Such are freely available: information, when available (e.g., some tracks are currently only (1.1) Visual SfM (open access; v.0.5.22 to v.0.5.251 ): documented by artificial casts), was integrated with data from the Generation of the dense point cloud on which the 3D model is based (Figures 1B,C). 2 www.agisoft.com 3 meshlab.sourceforge.net 1 4 www.ccwu.me/vsfm/ www.paraview.org Frontiers in Earth Science | www.frontiersin.org 3 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 1 | Photogrammetry workflow (from Mujal, 2017). (A) Set of photos to generate the 3D model with different centimetric scales. (B,C) Relative position of the photos as seen in Visual SfM software; photos were obtained changing the perspective in each one (B) and moving the camera laterally (C). (D) 3D model with the original texture of the specimen as prepared with MeshLab. (E) Depth color map and contours of the 3D model as prepared with ParaView. (A,C–E) Correspond to the same modeling process. The example used is a right manus-pes couple of Limnopus isp. from the Pyrenean Basin (corresponding mold and replica are IPS82608). geological successions that yielded the studied specimens. For the ◦ Lower–Middle Triassic ichnoassociations from the sake of clarity, and as further explained in section “Relative Depth Buntsandstein facies unit analyzed by Mujal et al. Patterns on Permian to Triassic Tetrapod Footprints” below, (2016a, 2017a), who included 3D models. the substrate features, if not relevant for the analysis, are not specified nor discussed. - Catalan Basin (northeastern Iberian Peninsula): Middle Triassic ichnoassociations from the Buntsandstein facies Tetrapod Ichnoassociations From the (early–middle Anisian; Fortuny et al., 2011, who did not include 3D models) and middle Muschelkalk facies (late Lower Permian to the Middle Triassic Anisian–middle Ladinian; Mujal et al., 2015, 2018b, who Here, we list the localities yielding the tetrapod ichnofossil record included 3D models). (for which 3D photogrammetric models were created) reviewed - Lodève Basin (southern France; for units datations see in section “Results and Discussion” below; further details on such Michel et al., 2015): localities are found in the Supplementary Data S1. The label of each analyzed specimen is mentioned through the present work and/or in the corresponding figure caption (see also institutional ◦ Asselian (Cisuralian) trackways of Ichniotherium abbreviations below). For further specimen numbers we also refer cottae from the Usclas Saint Privat or the Tuillières- the reader to the publications herein cited. Loiras formations (previously studied by Heyler and Lessertisseur, 1963; recently reanalyzed by Mujal and - Pyrenean Basin (northeastern Iberian Peninsula): Marchetti, 2020, who included 3D models). ◦ Artinskian (Cisuralian) ichnoassociations from the ◦ Artinskian (Cisuralian) ichnoassociations from the Rabejac Formation (e.g., Gand, 1987; Heyler and Lower Red Units (time-equivalent to the Peranera Gand, 2000; Gand and Durand, 2006, who did not Formation) (see Gisbert, 1981; Voigt and Haubold, include 3D models). 2015; Mujal et al., 2016a, 2018a) analyzed by Mujal et al. ◦ Guadalupian tracks of Merifontichnus from the La (2016b), who included 3D models. Lieude Formation (e.g., Gand et al., 2000, who did not ◦ Guadalupian–Lopingian ichnoassociation from the include 3D models). Upper Red Unit analyzed by Mujal et al. (2017b), who ◦ Middle Triassic archosauromorph tracks from included 3D models. red-bed deposits (e.g., Demathieu, 1985; Frontiers in Earth Science | www.frontiersin.org 4 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology Demathieu and Demathieu, 2004; Gand et al., 2007, and SMNS, Staatliches Museum für Naturkunde Stuttgart, references therein, who did not include 3D models). Stuttgart, Germany. UCMP, University of California, Berkeley, CA, United States. - Orobic Basin (northern Italy): Kungurian (Cisuralian) UG, Faculty of Geoscience and Geography, Göttingen trackway of Amphisauropus kablikae from the Pizzo del University, Göttingen, Germany. Diavolo Formation of the Orobic Basin (see Marchetti UGKU, Urweltmuseum GEOSKOP/Burg Lichtenberg (Pfalz), et al., 2015; Marchetti, 2016) analyzed by Marchetti et al. Tallichtenberg, Germany. (2017a), who included 3D models. UM-LOD, Lodève collection at Université de Montpellier, - Hessian Basin (Germany): Lopingian ichnoassociations Montpellier, France. from eolian units of the Cornberg Formation (see USNM, Smithsonian – National Museum of Natural History, Schmidt, 1959) analyzed by Marchetti et al. (2019c), who Washington, DC, United States. included 3D models. YPM, Yale Peabody Museum of Natural History, New Haven, - Central European Basin (Germany): Ladinian (Middle CT, United States. Triassic) tracks from the base of the Antrhakonitbank at the Vellberg Fossil-Lagerstätte, Lower Keuper (Erfurt Formation; see Schoch and Seegis, 2016) analyzed by Mujal RESULTS AND DISCUSSION and Schoch (2020), who included 3D models. - Lochmaben and Dumfries basins (Scotland, Herein, we present a review of research on tetrapod tracks United Kingdom): Lopingian ichnoassociations from involving 3D photogrammetric models. Taken together, the eolian units of the Cornockle and Locharbriggs formations analyzed examples cover both a long age interval (Cisuralian (see Anonymous, 1828; Jardine, 1853; McKeever and to Middle Triassic) and a wide range of substrates and Haubold, 1996) analyzed by Marchetti et al. (2019c), who paleoenvironments (eolian, fluvial, lacustrine, and coastal included 3D models. settings). Subsequently, we provide a first analysis based on the - Arizona (United States): Kungurian (Cisuralian) relative depth pattern of tetrapod footprints, suggesting a link ichnoassociations from the eolian units of the De with the functional prevalence of autopodia. Chelly and Coconino formations (see Gilmore, 1927; Haubold et al., 1995a) analyzed by Marchetti et al. (2019b), Photogrammetry: A Toolkit for who included 3D models. Ichnologists - Karoo Basin (South Africa): Guadalupian to Lower Following we identify different ichnological fields in which Triassic ichnoassociations from continental fluvial photogrammetry is useful, independently from the analyzed units of the Abrahamskraal, Teekloof, and Balfour ichnotaxon, age, substrate, and paleoenvironment. formations (see Smith and Botha-Brink, 2014; Klein et al., 2015) analyzed by Marchetti et al. (2019d), who Ichnotaxonomy included 3D models. Tetrapod ichnotaxonomy is primarily based on the morphology of footprints (distinguishing manual and pedal impressions for quadruped trackmakers) and, to a lesser extent, on the Institutional Abbreviations trackway pattern. In ideal conditions, the footprint morphology AM, Albany Museum, Grahamstown (Makhanda, as of 4 and the trackway pattern are considered to reflect anatomical October 2018), South Africa. traits of the trackmaker (e.g., Olsen, 1995; Carrano and Wilson, DUMFM, Dumfries Museum and Camera Obscura, Dumfries, 2001; Belvedere et al., 2018; Marchetti et al., 2019a). In fact, Scotland, United Kingdom. each ichnotaxon could be potentially correlated with a specific EMVG, ‘Casa del Tempo’, Ecomuseo della Val Gerola, Sondrio, trackmaker group, often at family or higher level groups Lombardy, Italy. (Falcon-Lang et al., 2010). IPS, Institut Català de Paleontologia Miquel Crusafont, Anatomy-consistent morphology is the very base of Sabadell, Catalonia, Spain. ichnotaxonomy (e.g., Baird, 1957; Haubold et al., 1995b; MF-LOD, Lodève collection at Musée Fleury, Lodève, France. Haubold, 1996; Marchetti et al., 2019a). Autopodial anatomy MNA, Museum of Northern Arizona, Flagstaff, is recorded during the trackmaker’s locomotion, i.e., AZ, United States. footprints record a dynamic behavior. Therefore, before MNHN.F.LOD, Lodève collection at Muséum National doing ichnotaxonomical inferences, the process of autopodia d’Histoire Naturelle, Paris, France. impression in the sediment should be understood (e.g., NHMUK PV, British Museum of Natural History, Gatesy et al., 1999; Falkingham and Gatesy, 2014; Gatesy and London, United Kingdom. Falkingham, 2017; Mujal and Schoch, 2020). In addition, the NMK, Museum of Natural History in the Ottoneum, effects of the substrate conditions at the time of impression Kassel, Germany. should be considered (see also section “Relative Depth Patterns RAM, Raymond M. Alf Museum of Paleontology, Claremont, on Permian to Triassic Tetrapod Footprints” below), because CA, United States. they can alter significantly the morphological traits linked to RMS, National Museum of Scotland, Edinburgh, anatomy (Falkingham, 2014). Moreover, a number of other Scotland, United Kingdom. effects occurring after the track recording, such as trace fossil Frontiers in Earth Science | www.frontiersin.org 5 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology and sedimentary structure superimpositions and the sediment to an originally water-saturated substrate), and the digit erosion, should be correctly interpreted (e.g., Marty et al., impressions were recognized only after the study of 2009). Nonetheless, as shown in section “Relative Depth the false color depth maps. The 3D model that Mujal Patterns on Permian to Triassic Tetrapod Footprints” below, et al. (2016b) published better illustrates the orientation we observe that 3D models help to identify the anatomy- of the footprints, allowing observation of the trackway related ichnotaxa morphology through the analysis of shape arrangement and thus providing correct measurement of and relative depth patterns of imprints, even if the track the trackway parameters. In the same way, some digital morphologies are apparently different due to different substrates. impressions (e.g., from the pedal track in Mujal et al., Therefore, 3D models help in comparing track morphologies 2016a; Figure 2C) were only recognized through the study useful for ichnotaxonomy, thus avoiding oversplitting or of the 3D model, because their relief is relatively low. All overlumping of ichnotaxa that would have no faunistic and these features are diagnostic of Varanopus; nonetheless, hence no evolutionary, biomechanic, biostratigraphic, and given the relatively poor preservation of the tracks, an paleobiogeographic meaning (e.g., Marchetti et al., 2019a,d; open nomenclature was preferred by Mujal et al. (2016b), Mujal and Marchetti, 2020). which we follow here. 3D models help to identify important anatomy-consistent - The Hyloidichnus isp. specimens (not collected) studied features of both new and already known ichnotaxa that may by Mujal et al. (2016b) could only be identified after remain unseen to the naked eye. Indeed, this is the case of analyzing the 3D model of a manus-pes set (Figure 2D). several ichnotaxa (including different morphologies and sizes) The impression of pedal digit IV was first identified from previously published specimens. Herein we review and in the false color depth map provided by Mujal summarize examples from some of our studies (see section et al. (2016b): such impression is partially masked “Tetrapod Ichnoassociations from the Lower Permian to the (overprinted) by rain drop impressions and flow ripples. Middle Triassic” above and Supplementary Data S1) showing In fact, it is a very shallow impression that could the use of 3D photogrammetric models in recognizing relevant not be distinguished on photographs due to the light ichnotaxonomic features: conditions. After this first identification, further tracks from the same surface and stratigraphically close levels - The complete shape of pedal impressions in (not collected) were also classified as Hyloidichnus isp. MNHN.F.LOD83 from the Cisuralian of southern by Mujal et al. (2016b). France re-analyzed by Mujal and Marchetti (2020) was - The type material of Laoporus (YPM 2143, 2144, USNM identified through the study of the 3D model that these V 8422) from the Kungurian Coconino Fm. of the authors carried out (Figure 2A). Impressions of pedal United States was recently re-analyzed by Marchetti et al. digits IV and V (roman numbers and red arrows in (2019d), who concluded that the supposed diagnostic Figure 2A) are unclear when observed in the original features of this ichnogenus (subequal pes and manus with specimen, but a consistent depth pattern was observed digit impressions of subequal length) were actually due in the false color depth maps. The 3D model also to the locomotion on sandy inclined planes (dune foreset facilitated the identification of two trackways, which surfaces). The 3D model created by Marchetti et al. (2019d) were also unclear from the previous analysis by Heyler allowed to identify both the generally deeper, parallel, and and Lessertisseur (1963). The actual length of several sub-equal digit drag traces and the actual digit imprints, digit impressions, as well as the shape of sole and palm which are generally shorter, shallower, and with a different impressions, is masked by the presence of digit scratches orientation (arrows in Figure 2E). Therefore, the inferred and other irregularities of the surface; nevertheless, the digit impressions of Laoporus are actually digit drag traces false color maps allowed for the discrimination of all parallel to the slope and they actually represent the digit these features. Because of the correct recognition of these sliding during locomotion. Therefore, this ichnogenus is features, Mujal and Marchetti (2020) assigned the tracks considered a nomen dubium and this material is assigned of MNHN.F.LOD83 to Ichniotherium cottae, being the to cf. Varanopus isp. based on anatomy-consistent features, first unambiguous report of this ichnogenus from the i.e., the actual length of digit impressions and their French Lodève Basin. relative proportions (for further details, see Marchetti et al., - In some Limnopus isp. specimens (some of the originals 2019d) (Figure 2E). were not collected; e.g., mold and replica IPS82608), from - The holotype of Chelichnus duncani (DUMFM 5) from the the Artinskian of the Catalan Pyrenees studied by Mujal Lopingian of Scotland, United Kingdom, was re-analyzed et al. (2016b), the actual shape of the tracks (including with the aid of photogrammetry by Marchetti et al. (2019c). also diagnostic features of the ichnogenus, such as the The 3D model shows a ridge between the digit tip and number of digit impressions and their relative length the digit base impressions. This was an important feature and proportions) was inferred from the false color depth for the assignment of this material to cf. Dicynodontipus maps, which also revealed the presence of deformed pedal and for its attribution to therapsid tracks. C. duncani is impressions (Figure 2B). considered a nomen dubium due to its incompleteness, - The tracks referred to cf. Varanopus (original not collected; especially as regards the pes imprints. mold and replica IPS82607) by Mujal et al. (2016b) - The study of the false color depth maps on 3D (Figure 2C) are notably deformed (most probably due models was central for the ichnotaxonomic revision of Frontiers in Earth Science | www.frontiersin.org 6 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 2 | 3D models [except (A,D), which include a photo instead] and corresponding depth maps of Cisuralian (A–E) and Lopingian (F) ichnotaxa. (A) Tracks and trackways of Ichniotherium cottae (MNHN.F.LOD83), Lodève Basin; arrows indicate expulsion rims (white) and pedal digit V impressions (red); 1 and 2 indicate detailed 3D models of pes tracks showing digits IV and V. (B) Right manus-pes couple of Limnopus isp. (mold and replica IPS82608), Pyrenean Basin; arrows indicate parts of the pes track (red) and a hole on the substrate that apparently looked like a digit impression (white). (C) Left manus-pes couple of cf. Varanopus (mold and replica IPS82607), Pyrenean Basin. (D) Left manus-pes couple of Hyloidichnus isp., Pyrenean Basin. (E) Trackway in upslope progression of cf. Varanopus (YPM 2143), Coconino Formation; arrows indicate sliding and true orientation of digit impressions. (F) Right manus-pes couple of Karoopes gansfonteinensis (GF-TR 1), Karoo Basin. Frontiers in Earth Science | www.frontiersin.org 7 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 3 | 3D models [except (F), which includes a photo instead] and corresponding depth maps of Lower–Middle Triassic archosauromorph tracks from the Pyrenean Basin. (A,B) Left (A) and right (B) manus-pes couples of Prorotodactylus mesaxonichnus. (C,D) Manus (C) and pes (D) tracks of a left couple of Chirotheriidae indet. (similar to Protochirotherium; mold and replica IPS82616). (E) Chirotheriidae indet. left track similar to Protochirotherium. (F) Large partial right pes track of Chirotheriidae indet. Arrows indicate anatomical (red) and sedimentary (e.g., digits collapse) (white) features; see text for discussion. the Guadalupian–Lower Triassic tetrapod tracks from anamniote ichnotaxa, which are instead more deeply South Africa (Marchetti et al., 2019d). This is true impressed medially. especially for the erection of the ichnotaxon Karoopes - Several anatomical traits of Prorotodactylus mesaxonichnus gansfonteinensis, which is characterized by tracks more from the Lower–Middle Triassic of the Catalan Pyrenees deeply impressed laterally (e.g., GF-TR 1, 2, 3, SAM-NN described by Mujal et al. (2017a), including impressions 3) (Figure 2F). This is an important feature different of pedal digits IV and V, as well as the proximal from some morphologically similar captorhinomorph impression of pedal digit III, were first recognized in 3D tracks such as Hyloidichnus and Merifontichnus. The same models (red arrows in Figures 3A,B). This allowed the is observed in material assigned to cf. Capitosauroides measurement of the actual length of the digit impressions isp. (GF-TR 9, GFTS 1, SAM-PK-K 7878a, b). Because and of the digit divarication. In addition, the equivalent of the deeper lateral impression of these tracks, an depth patterns observed in specimens with an apparently assignment to Batrachichnus or Limnopus (anamniote different morphology (e.g., Figures 3A,B) allowed to assign ichnogenera) could be discarded by Marchetti et al. this material to the same ichnotaxon. (2019d), although the overall morphology is quite similar, - The large archosauromorph tracks from the Lower–Middle especially in incomplete imprints. Therefore, as we Triassic of the Catalan Pyrenees described by Mujal further discuss in section “Relative Depth Patterns on et al. (2017a) were identified as Chirotheriidae footprints Permian to Triassic Tetrapod Footprints” below, the resembling Protochirotherium through the analysis of the relative depth pattern allows the clear distinction of 3D models. Several digit impressions are partially collapsed Karoopes and Capitosauroides from captorhinomorph and (white arrows in Figures 3C–E), and some parts of the Frontiers in Earth Science | www.frontiersin.org 8 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology impressed surface are missing due to natural erosion collections, even if possibly the lifetime of the replica material (Figures 3C–E); in one case a pes track preserves only three is also limited. digit impressions (Figure 3F). Nonetheless, the false color maps allowed Mujal et al. (2017a) a correct interpretation Track-Trackmaker Correlation of the digits outline and their relative position, showing the The analysis of relative depth patterns of footprints is strictly typical morphology of chirotheriid ichnotaxa. linked to morphofunctional features of the possible trackmakers (see section “Relative Depth Patterns on Permian to Triassic Generally, despite different types of preservation and Tetrapod Footprints” below), which can be inferred from the even in different (or slightly different) trackway parameters, analysis of articulated and complete appendicular skeletons. examining the whole set of 3D models we observe a consistent In addition, the whole set of movements performed by (“homogeneous”) relative depth pattern within tracks of a single the trackmaker’s limbs (and, potentially, by the whole body ichnotaxon, even if impressed in different substrates. This is of the trackmaker) can also be elucidated by the study because the tracks assigned to an anatomy-based ichnotaxon of depth patterns (e.g., Gatesy et al., 1999; Falkingham are impressed by the same type of trackmakers, which probably and Gatesy, 2014; Gatesy and Falkingham, 2017; Marchetti had autopodia with analog structure and morphology (e.g., et al., 2017a; Mujal et al., 2017a; Mujal and Schoch, 2020). Voigt et al., 2007; Marchetti et al., 2017a; Mujal et al., 2017a; Therefore, a thorough study of these features is of fundamental Mujal and Schoch, 2020). importance for the track-trackmaker correlations. Following, According to the recommendations by Belvedere et al. (2018) different case-studies discussions are presented based on and Falkingham et al. (2018), 3D models, when possible, above considerations: should be used in the definition of ichnotaxa, being tools to recognize ichnotaxobases (see also Bertling et al., 2006). - The large tracks of Ichniotherium cottae preserved in When depth patterns are considered in ichnotaxonomy (i.e., MNHN.F.LOD83 (Figure 2A) are attributed to large basal contributing in ichnotaxobases), the use of 3D models is diadectomorph reptiliomorphs, being different from those recommended. Henceforth, a further consideration must be of smaller specimens of I. cottae and of I. sphaerodactylum raised: several holotypes of tetrapod ichnotaxa, from different (Mujal and Marchetti, 2020), which were produced by geological periods correspond to tracks and trackways that more derived taxa of the group (Voigt et al., 2007). In were documented but not collected. In these cases, high fact, the comparison of 3D photogrammetric models of fidelity molds and replicas were created instead. Here the different specimens (see Figure 5 by Mujal and Marchetti, question regarding such type material is highlighted. According 2020) showed that the large French I. cottae tracks display to the ICZN rules (International Commission on Zoological a more deeply impressed medial area, whereas smaller Nomenclature, 1999), a holotype must correspond to an original Ichniotherium tracks are more deeply impressed in their fossil stored in an adequate repository (museum, research median part (Mujal and Marchetti, 2020). In this case, the institute, university, etc.) for its safe keeping. However, this differences in the relative depth of tracks, together with is not always feasible (especially in the case of trace fossils), the relative position (rotation) of each track respect to the and in most cases, the uncollected type material remains trackway midline, are a key feature to distinguish the most unprotected, and sooner or later will be lost due to erosion probable trackmakers. Therefore, when trackways are not (Lucas and Harris, 2020). available, 3D models of isolated tracks may still provide Molds and casts of holotype specimens (i.e., “plastotypes”) information for the track-trackmaker correlation (see also are stored in institutions, but they can eventually deteriorate Mujal and Schoch, 2020). (as it could also occur with original specimens) as the life time - The ichnogenus Amphisauropus shows tracks more deeply of the mold/cast material is limited and suffers degradation impressed in their medial part, as suggested by Voigt through time (see also section “Preservation and Heritage” (2005) and Avanzini et al. (2008). The 3D model of below). In this case further molds from the original fossil should the Amphisauropus kablikae trackway (EMVG 2), from be created through time, but the preparation of new molds the Kungurian of the Italian Southern Alps and created from the first cast would easily imply a loss of fidelity with by Marchetti et al. (2017a), confirms this hypothesis. respect to the original fossil. As already suggested by Adams According to the analysis by Marchetti et al. (2017a) et al. (2010) [being further supported by Mallison and Wings of the false color depth maps (Figure 4A), the most (2014), as well as by Belvedere et al. (2018), and Lucas and deeply impressed area is placed at the base of digits Harris (2020)], we support the use of digital (3D) models as I and II, where it is possible to observe a large type material, i.e., digitypes (see further discussion on physical circular pad impression (see red arrows and zoom inset replicas by Lucas and Harris, 2020). 3D models have great in Figure 4A). This is exactly the same area that is advantages in the study, as well as in their sharing and repository. subject to the stronger strain within the tarsus and A recent study has partially applied such solution: Lockley carpus of the seymouriamorph Seymouria sanjuanensis. et al. (2019) erected a new ichnotaxon of Pleistocene marine Therefore, this constituted an important feature for the turtle tracks, the holotype of which remains in the field; the correlation of Amphisauropus to the seymouriamorph authors created a 3D model of the holotype trackway and group (Marchetti et al., 2017a). Of note, a similar medial printed part of it in order to appropriately store it in fossil functional prevalence, with digits I and II and their Frontiers in Earth Science | www.frontiersin.org 9 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 4 | 3D models [except (E,F,G), which include photos instead] and corresponding depth maps of Cisuralian (A,E), Lopingian (B,C,D), and Ladinian (Middle Triassic) (F,G) ichnotaxa. (A) Trackway of Amphisauropus kablikae (EMVG 2), Orobic Basin; arrows indicate the deepest parts of tracks (red) and the steepest parts of the tail impression (white); zoom of one of the manus-pes couples (white square) showing the deeper medial side of tracks. (B) Right manus-pes couple of Brontopus giganteus (KW-TR 1), Karoo Basin. (C) Left manus track of Dolomitipes accordii (BE 3), Karoo Basin. (D) Left manus-pes couple of Dolomitipes isp. (UG-NN 1), Hessian Basin. Asterisks (*) in (C,D) indicate equivalent anatomical part. (E) Left manus track of Characichnos isp. produced by a Limnopus-trackmaker, Pyrenean Basin. (F,G) Right manus tracks of an indeterminate ichnotaxon attributed to capitosaur stereospondyl trackmakers by Mujal and Schoch (2020); (F) is a track natural cast not recovered and (G) is preserved in SMNS 97003. bases being much deeper than the rest of the footprint, that trackmakers were dinocephalian therapsids, which is also observed in 3D models of temnospondyl tracks have a broad metacarpal V. The more deeply impressed (Mujal et al., 2016a) (Figure 2B). manus compared to pes imprints helped in the attribution - The identification of footprint depth patterns was central of the ichnogenus Dolomitipes (BE-TR 1, AM 5744/5) in the track-trackmaker correlation of the Guadalupian– (Figures 4C,D) to dicynodont therapsids, which are Lower Triassic therapsid tracks from South Africa carried characterized by a marked postural dichotomy. The out by Marchetti et al. (2019d). The very broad and more deeply impressed lateral side of footprints helped more deeply impressed lateral part of the palm of the Marchetti et al. (2019d) in the attribution of the ichnotaxa ichnogenus Brontopus (KW-TR 1; Figure 4B) suggests Karoopes (GF-TR1, 2, 3, SAM-NN 3) (Figure 2F) and Frontiers in Earth Science | www.frontiersin.org 10 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology Capitosauroides (GF-TR 9, GFTS 1, SAM-PK-K 7878a, b) see Mancuso et al., 2016, and references therein), the tetrapod to therapsid synapsids. locomotion could be difficult, and it was influenced by the - In the tetrapod ichnoassemblages from the Lower–Middle force of gravity not perpendicular to the trampled surface. This Triassic of Port del Cantó and Tossal de Pollerini caused asymmetric trackway pattern, footprint morphology, ichnosites (Pyrenean Basin), Mujal et al. (2017a) observed and footprint depth in trackways not perpendicular to the a similar depth pattern between different ichnotaxa: the slope, which were caused by the trackmaker trying to oppose posterior (proximal) part of digit I impression displays the force of gravity. While these effects are long known (e.g., an elevated relief (Figure 3). This is related to the Leonardi, 1987; Fornós et al., 2002; Hunt and Lucas, 2005; Loope, collapse of the footprint wall in its side (white arrows in 2006; Krapovickas et al., 2015), only recently have they been Figure 3C–E). In other cases, this area is less impressed thoroughly interpreted with the aid of the 3D models (Marchetti (red arrows in Figures 3A,B). Such features could be et al., 2019b,c). The most accurate tool to evaluate these linked to the nature of the footprint producers, all of asymmetries is certainly the study of false color depth maps and which are considered to be archosauromorphs (Mujal contours from 3D models. Commonly, the downslope side of the et al., 2017a). Alternatively, this could be the result of asymmetric trackway is more deeply impressed and shows higher a similar gait of the trackmakers under similar substrate expulsion rims. This was observed by Marchetti et al. (2019b,c) conditions. In any case, the impressions of digits II, III, in several specimens, including trackways of Amphisauropus and IV from both manus and pes tracks are usually (MNA-V 3442A; Figure 5D), Ichniotherium (UCMP 42945), cf. the deepest (though exceptions exist: e.g., Mujal et al., Tambachichnium (USNM-V 11502), Dolomitipes (UG-NN 1; 2017a), denoting a trend toward a tridactyl functional Figure 4D), and Pachypes (DUMFM-NN 2). prevalence of autopodia. This is the case of chirotheriid In all the 3D models of Prorotodactylus mesaxonichnus ichnotaxa, as well as of other archosauromorph tracks, such manus tracks from the Lower–Middle Triassic of the Catalan as Prorotodactylus, Rotodactylus, and Rhynchosauroides Pyrenees created by Mujal et al. (2017a), a broad and relatively (the later also referred to lepidosauromorph trackmakers) high expulsion rim between the impressions of digits IV and (Mujal et al., 2015, 2017a, 2018b). V is observed (white arrow in Figures 3A,B). This feature, clearly visible in the false color maps (but often unseen at Locomotion and Behavior naked eye), indicates that the forelimbs applied a postero-lateral Differential depths within a track, as well as subtle differences in pressure during the taking off phase (Mujal et al., 2017a). steepness of impressions, are often indicative of the functional Hence, the manus was slightly rotated outwards and/or the prevalence and progression direction of the trackmakers trackmaker performed a lateral movement of the trunk (as also (Figures 4, 5). Footprints and trackways may record the whole suggested by the sinuous tail impressions of some trackways; set of limb movements (or at least partially) (e.g., Gatesy and Mujal et al., 2017a). Falkingham, 2017; Mujal and Schoch, 2020). In these cases, 3D The ichnogenus Characichnos is interpreted as the trace fossil models help to elucidate the timeline of footprints impression. of swimming tetrapods. Characichnos generally consists of digit The 3D model of the Amphisauropus kablikae trackway in drag traces on the substrate, therefore, not fully representing the EMVG 2 was used in the identification of the locomotion pattern anatomy of the trackmaker’s autopodia (except for number of by Marchetti et al. (2017a) (Figure 4A): the tail impression digits and width) but rather its behavior. In fact, Characichnos displays different steepness of the lateral margins along the tracks are associated with different ichnotaxa and are attributed trackway (white arrows in Figure 4A). This, combined with the to very different trackmakers, in a very long stratigraphic interval relative position of the tail impression respect to the tracks, (e.g., Thomson and Droser, 2015; Vila et al., 2015; Mujal et al., indicates the direction of movement of the tail and the body 2016a). Nevertheless, in some cases Characichnos may show of the trackmaker during locomotion. Moreover, the continuous features similar to the associated ichntoaxa, possibly due to digit drag traces indicate the precise position of the limbs during specific morphofunctional characteristics of the trackmaker. This the entire step cycle. The 3D model provided additional clues was observed by Mujal et al. (2016b), who performed 3D models to Marchetti et al. (2017a) on the digits that were closer to the of Characichnos isp. (although not figured there) associated with substrate while lifted and thus generated deeper drag traces. It Limnopus isp. tracks (mold and replica IPS82604) from the should be taken into account that the observed depth patterns Artinskian of the Catalan Pyrenees. The 3D models reveal the and different steepness along the trackway are small, often same depth pattern in trace fossils assigned to both ichnogenera. indiscernible to the naked eye. In some cases, only impressions of digits I and II are preserved The Permian tetrapod footprints from eolian as scratches, and the false color depth map revealed that these paleoenvironments were commonly preserved on the foreset scratches are associated with two digit tip imprints corresponding dune surfaces (e.g., Gilmore, 1926, 1927; Schmidt, 1959; to digits III and IV (Figure 4E). This trait enhanced the McKeever and Haubold, 1996). Since these surfaces were correlation between the two track morphologies (Limnopus and inclined and the sand was likely dry at the time of impression Characichnos, walking and swimming tracks, respectively; see (as indicated by the sand avalanches and expulsion rims; the Mujal et al., 2016a). This provides additional support to the sand was probably dampened shortly after the impression or interpreted medial functional prevalence of the trackmakers’ in the first phases of burial, allowing the preservation of tracks; forelimbs, with digits I and II more deeply impressed (or in Marchetti et al., 2019b; for further details on track preservation, this case, producing deeper drag traces) than digits III and IV Frontiers in Earth Science | www.frontiersin.org 11 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 5 | 3D models [except (A), which includes a photo instead] and corresponding depth maps of Cisuralian (A–D) and Lopingian (E) ichnotaxa. (A,B) Right manus-pes couples of Dimetropus leisnerianus, Pyrenean Basin; note that tracks in (A) are partially eroded. (C) Trackway in upslope progression of Varanopus curvidactylus (MNA-V 3327), Coconino Formation. (D) Trackway in downslope progression of Amphisauropus isp. (MNA-V 3442A), De Chelly Formation. (E) Trackway in upslope progression with significant lateral component of Dolomitipes isp. (NMK-PAL 183), Hessian Basin. Arrows in (C–E) indicate sliding and true orientation of digit impressions. Frontiers in Earth Science | www.frontiersin.org 12 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology (see section “Relative Depth Patterns on Permian to Triassic of the digits, causing problems in ichnotaxonomy (Marchetti Tetrapod Footprints” below). et al., 2019b,c; Francischini et al., 2020). In order to correctly The set of tracks from the Ladinian of southern Germany, interpret this substrate-induced effect and recognize the actual referred to temnospondyls producers by Mujal and Schoch digit impressions, the analysis of the 3D models is very helpful. (2020), could not be assigned to a specific ichnotaxon due to the Some notable examples of this are observed for trackways of paucity and preservation of the material. Nevertheless, the 3D Amphisauropus, Varanopus, and Dolomitipes with different models of footprints revealed a consistent depth pattern within orientations compared to the slope studied by Marchetti et al. footprints of apparently different shape due to their different (2019b,c) (RAM-NN 1, UCMP V 42944, MNA-V 3327, MNA-V size and preservation mode (Figures 4F,G). This allowed Mujal 3442A, NMK-PAL 183, YPM 2143) (Figures 2E, 5C–E). and Schoch (2020) to decipher a consistent locomotor pattern of the trackmakers (interpreted as a bottom-walking locomotion in Preservation and Heritage subaqueous conditions), hence implying that they most probably Collecting and/or excavating tetrapod footprints (and especially correspond to the same trackmaker group. complete trackways) is often difficult or impossible. Therefore, an alternative solution in order to preserve the fossil record must Substrate Induced Effects be found. Traditionally, the use of molds and replicas of tracks The 3D model of MNHN.F.LOD83 from the Asselian of and complete trackways has fulfilled this necessity (e.g., Heyler Lodève, southern France (Mujal and Marchetti, 2020), reveals the and Lessertisseur, 1963; Gand et al., 2000; Mujal et al., 2016a,b, presence of shallow and relatively wide expulsion rims around 2017a; Marchetti et al., 2019d; Lucas and Harris, 2020). However, most of the tracks (white arrows in Figure 2A). Such structures such molds and casts usually do not preserve the whole record of remain difficult to identify by the naked eye due to their low relief: a site but a small part of it and even if adequately stored, can be they were not reported by Heyler and Lessertisseur (1963), and deteriorated, depending on the lifetime of the mold/cast material Mujal and Marchetti (2020) noticed them only after the analysis nature, even becoming useless, as all material has a lifetime before of the 3D models. The identification of expulsion rims allowed degradation (mostly being of silicone, gypsum, acrylic resin, etc.). for better inference of the substrate conditions, despite tracks The preservation of the tetrapod ichnofossil record may preserved as plaster casts and the original substrate is lost. The present some difficulties that should be considered. This is expulsion rims, together with the relative depth patterns of the commonly due to difficult access to the localities (either because tracks, denote a peculiar fact: those from the pes tracks are higher they are in private properties or due to orographic characteristics, (in this case, deeper, as they are casts) than those of the manus i.e., remote areas) that may lead to the loss of specimens due to tracks, whereas manus tracks are much better impressed than pes erosion and weathering. This problem has been faced by several tracks. This indicates an impression of the hindlimbs relatively of our works, in which photogrammetry has been a useful tool stronger than of the forelimbs (but more focalized to a part of the to overcome the impossibility to collect/cast/move specimens autopodia in the case of the hindlimbs; see further discussion in (e.g., Mujal et al., 2016a,b, 2017a,b; Marchetti et al., 2019b,d; Mujal and Marchetti, 2020). Mujal and Marchetti, 2020): A manus-pes set of Dimetropus leisnerianus from the Artinskian of the Catalan Pyrenees (not collected) was strongly - Manipulation of large casts can be harmful for their eroded (smoothed) by the water flow probably soon after the preservation, as they can be damaged during analysis. impression of the footprints (Mujal et al., 2016a) (Figure 5A). Nevertheless, 3D photogrammetric models facilitate the Indeed, in the field, tracks were identified as shallow irregular analysis, as tracks can be (virtually) moved and observed impressions within a surface featured by unidirectional flow in different perspectives. This was indeed the case of ripples. The identification of such impressions as D. leisnerianus MNHN.F.LOD83 (see Mujal and Marchetti, 2020). was possible thanks to the 3D model: the false color depth map - Photogrammetry is advantageous through time: previous shows the depth pattern that characterizes this ichnospecies, non-optimal photogrammetric results may become useful with a broad and rounded sole impression and relatively thin thanks to newer versions of software. In fact, herein digit impressions; the same can be observed in other manus- we have re-run a model using the same photos of pes couples found by Mujal et al. (2016b) within the same a Limnopus isp. manus-pes set (IPS73724) and results stratigraphic succession (Figure 5B). show that the new 3D model is more detailed than The morphology of Permian tracks from eolian the previous one (which due to its poor resolution was paleoenvironments was strongly influenced by the substrate discarded by Mujal et al., 2016a) and also ichnologically (Marchetti et al., 2019b,c). This is because the track-bearing informative (Figure 6A). surfaces were foreset dune surfaces; thus, they were relatively - Creation of molds (and replicas) can be impossible due dry sand slopes that caused difficulty in trackmaker progression to difficult access to localities (e.g., remote areas only and very common digit sliding and sand avalanches. These accessible by foot, crossing creeks and relatively dense digit drag traces were generally parallel to the slope and were forests) and to the situation of the track-bearing surfaces generated when the trackmaker’s foot slid downslope with (in high slopes, being even vertically exposed), as is the case respect to the touch-down position. This was observed in of Catalan Pyrenees localities. In the case of relatively small trackways directed parallel, perpendicular, and transverse to tracks (e.g., 2–5 cm long), molds may still be possible to the slope and masks the original orientation and morphology create (e.g., Mujal et al., 2016b,a, 2017a), although those Frontiers in Earth Science | www.frontiersin.org 13 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 6 | (A) Photo (top) and depth maps of the 3D models (new, middle; old, bottom) of a right manus-pes couple of Limnopus isp. (IPS73724), Pyrenean Basin, Cisuralian; note that the surface is much smoothed in the new model, allowing better elucidation of footprint features. (B) Captures of the 3D model obtained with drone of the Coll de Terrers outcrop, Pyrenean Basin, Guadalupian–Lopingian; left image contains the position of each photo. (C) 3D model with the depth map superimposed on transparency of a large surface containing abundant Prorotodactylus mesaxonichnus tracks, including the trackway paratype (mold and replica IPS93867), Pyrenean Basin, Early–Middle Triassic. of relatively large trampled surfaces are often of difficult Pollerini and Coll de Terrers sites (Mujal et al., 2017a,b) transport and are easily damaged, implying further careful an unnamed aerial vehicle was used to take photos lab work (EM and JF, pers. obs. 2016). Therefore, 3D for the creation of high-resolution 3D photogrammetric photogrammetry is the optimal tool for the preservation models of: (1) the whole outcrops (Figure 6B) (for of such record. Recently, during fieldtrips to Tossal de similar documentation of tracksites, see Citton et al., 2017; Frontiers in Earth Science | www.frontiersin.org 14 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology Romilio et al., 2017; Petti et al., 2018) and (2) relatively the sequence of events and, as a result, to correctly identify the large (∼2–5 m2 ) track-bearing surfaces (with sets of footprint outline (see Marchetti et al., 2013; Mujal et al., 2015). around 100 photos per surface) (Figure 6C). Besides Although paleoenvironmental settings (and paleoclimatic providing new ichnological and sedimentological data conditions) constrain the distribution of tetrapod ichnotaxa (currently under analysis), these new models constitute a (e.g., Mujal et al., 2016a), we observe that the same ichnotaxon digital repository of this paleontological heritage. can be present in a relatively wide range of substrates (e.g., - The most important Guadalupian–Lower Triassic tetrapod Mujal et al., 2018a). Also, the absolute depth of footprints ichnosites from South Africa (Marchetti et al., 2019d) from the same ichnotaxon (or even along the same trackway) are surfaces left in place on private property. This is can vary consistently on different substrates, from very deep, potentially harmful for the preservation of this heritage to intermediate, and to very shallow. Nevertheless, all the because of erosion and weathering while the tracksite diagnostic (anatomical) features of ichnotaxa (e.g., digit number, remains unprotected. This is the case of Klein Waterval morphology and relative length, palm/sole impression shape and Bethel sites, which are located in canyons/rills and and size, relative depth within a footprint) do not vary, even covered by sediment at the time of field work (LM, pers. if their recognition relies on the effects caused by substrate obs. 2018). Some latex casts of portions of these surfaces rheology and behavior (see Gatesy and Falkingham, 2017; Mujal are preserved in repositories, but their quality is often not et al., 2018a; Marchetti et al., 2019a). The whole set of 3D adequate for a new study. Therefore, the creation of 3D photogrammetric models (i.e., both reviewed from previous photogrammetric models of the most important trackways publications and newly presented, see Figures 2–7) show a and surfaces from all these localities, as Marchetti et al. consistent relative depth pattern within pes and manus imprints (2019d) did, is the ideal way to preserve such heritage for of each ichnotaxon, independently from the absolute depth the scientific community (for further discussion see also of each footprint. Therefore, the original substrate conditions Reolid et al., 2020). (rheology) and environmental settings do not play a major role in the relative depth pattern (i.e., the diagnostic 3D features) Relative Depth Patterns on Permian to observed within tracks of the same ichnotaxon, whereas their Triassic Tetrapod Footprints absolute depth may vary consistently (see also discussion in Herein we present and discuss the relative depth patterns Mujal et al., 2018a). of tetrapod tracks and their implications for the functional Our qualitative interpretation of footprint relative depth prevalence of autopodia in a phylogenetic framework. patterns allows us to identify the functional prevalence of the autopodia, which results from specific morphofunctional features Substrates and Paleoenvironments of the trackmaker, thus further linking the ichnotaxa to the The localities of the herein reviewed tetrapod footprints potential trackmakers (see section “Relative Footprint Depth encompass a wide range of substrates and paleoenvironments Patterns and Functional Prevalence of Autopodia” below). 3D (see section “Tetrapod Ichnoassociations from the Lower models elucidate potentially biased observations related to the Permian to the Middle Triassic” above and Supplementary absolute depth of footprints: false color depth maps help to Data S1). Most of the localities belong to siliciclastic successions minimize the differences between shallowly and deeply impressed commonly including red-beds, but also dark mudstones. Such tracks of the same ichnotaxon (e.g., see Figures 5H,I of Mujal successions usually represent alluvial, fluvial, and lacustrine and Marchetti, 2020). As highlighted in the introduction of paleoenvironments. Besides, a few of them represent coastal the present work, there is a major difference between upper (e.g., sabkhas and tidal flats) and eolian paleoenvironments. Carboniferous to Triassic tetrapod ichnology and non-avian In a few cases, the track-bearing substrate is a carbonate dinosaur ichnology. For instance, a deeply impressed theropod mudstone, usually of microbial origin, in coastal or restricted dinosaur track may show four digit impressions, whereas a marine settings. shallow one would be just tridactyl, which could eventually More specifically, the substrate preserving footprints, either lead to oversplitting and misinterpretation of ichnogenera (for in concave epirelief or convex hyporelief (or both, if part a thorough study of such case, see Razzolini et al., 2014). On and counterpart strata/slabs are found) includes: (1) mudstones the contrary, within upper Carboniferous, Permian, and Triassic (claystones and siltstones), either massive or laminated; (2) tetrapod tracks the anatomy-related morphological features are sandstones, ranging from very fine-grained to coarse-grained, the same in deep and shallow imprints (only very shallow either massive or with parallel lamination and/or flow and wave tracks may show less digit and/or palm/sole impressions) and/or ripples; (3) fine-grained and well-sorted (eolian) sandstones with in imprints impressed on mudstones, siltstones, carbonates, large-scale cross lamination, wind ripples, and sand avalanches; and dune sandstones. In fact, in different substrates or (4) fine grained limestones (i.e., carbonate mudstones), either substrate rheological properties the relative depth pattern of massive or laminated. Footprint surfaces are either flat or tracks assigned to the same ichnotaxon is generally constant undulated; other structures, such as desiccation cracks, raindrops (especially if trackmakers maintain the same locomotion; e.g., and wrinkles, as well as other vertebrate and/or invertebrate Mujal et al., 2018a; Marchetti, 2019; Mujal and Schoch, 2020). trace fossils, may also be present. In many cases, other trace Even on originally inclined surfaces, which caused additional fossils and structures cross, overprint, and/or mask the analyzed asymmetries to the footprint depth, the relative depth pattern tetrapod footprints. In such cases, 3D models help to elucidate within footprints of the same ichnotaxon is still recognizable Frontiers in Earth Science | www.frontiersin.org 15 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 7 | Depth patterns among Cisuralian to Middle Triassic tetrapod ichnotaxa shown by selected 3D models and corresponding depth maps. In the left part of each section, schematic unscaled left manus-pes couples showing the parts with functional prevalence (black areas) of each group are depicted. (A) 1 left couple (partial pes track) of Limnopus isp. (mold and replica IPS82606), Pyrenean Basin, Cisuralian; 2 left couple of Ichniotherium sphaerodactylum (UGKU 130), Tambach (Continued) Frontiers in Earth Science | www.frontiersin.org 16 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 7 | Continued Formation (Germany), Cisuralian; 3 left couple (partial manus track) of Hyloidichnus isp. (MF-LOD 2001.1447.1), Lodève Basin, Cisuralian; 4 left couple of Merifontichnus thalerius (UM-LOD 16), Lodève Basin, Guadalupian; 5 right couple of Pachypes loxodactylus (DUMFM-NN 2), Dumfries Basin, Lopingian; 6 left couple of Procolophonichnium nopcsai (NHUMK R3173), Karoo Basin, Early Triassic. (B) 1 left manus track of Prorotodactylus mesaxonichnus (mold and replica IPS93869), Pyrenean Basin, Early–Middle Triassic; 2 left couple (partial pes track) of Chirotherium barthii (mold and replica IPS17899), Catalan Basin (Buntsandstein), Middle Triassic; 3 right pes track of Rhynchosauroides isp. (IPS73720), Catalan Basin (middle Muschelkalk), Middle Triassic; 4 right pes track of Isochirotherium isp. (IPS81873), Catalan Basin (middle Muschelkalk), Middle Triassic; 5, 6 left couples of Chirotherium barthii (MF-LOD 2001.1438.1 and MF-LOD 2001.1437.1, respectively), and right pes track of Rotodactylus isp. (arrow in 6), Lodève Basin, Middle Triassic. (C) 1 and 2 pes tracks of Dimetropus leisnerianus (1, right imprint, MNHN.F.LOD59; 2, left imprint –undertrack–, UM-LOD 260), Lodève Basin, Cisuralian; 3 right couple of a Dicynodontipus-like morphotype, Pyrenean Basin, Lopingian; 4 left pes of cf. Capitosauroides (SAM-PT-K 7870), Karoo Basin, Lopingian; 5 right couple of Dolomitipes icelsi (AS-TR Q), Karoo Basin, Lopingian; 6 left couple of Dolomitipes accordii (AS-TR 2), Karoo Basin, Lopingian. Scale bars represent 10 mm [in 1 and 3 of (B)], 20 mm [in 1, 3, 5, and 6 of (A), and 4 of (C)], 50 mm [in 2 and 4 of (A), 2, 4, 5, and 6 of (B), and 1 and 2 of (C)] and 10 cm [in 3, 5, and 6 of (C)]. (e.g., Marchetti et al., 2019b). In fact, ichnotaxonomy is based on Synaptichnium), dinosauromorph tracks (e.g., Atreipus, footprint outline morphology and relative depth patterns of the Eubrontes, Evazoum, Grallator), relatively small-sized non- impressions (i.e., anatomical features of autopodia) (Voigt, 2005). chirotheriid ichnotaxa such as Dromopus, Prorotodactylus, Hence, relative depth patterns are recognizable in any substrate. Rotodactylus, and Rhynchosauroides, as well as larger non- In this sense, only footprints with substantial extramorphological chirotheriid ichnotaxa such as Apatopus and Paradoxichnium. variations (that completely mask/overprint anatomical features; Of note, the Rhynchosauroides trackmakers may correspond see Peabody, 1948; Haubold, 1996) are excluded from such to archosauromorphs or lepidosauromorphs, hence broadly analyses (see section “Substrate Induced Effects” above). corresponding to neodiapsid reptiles. Interestingly, also the varanopid ichnogenus Tambachichnium belongs to this group. Relative Footprint Depth Patterns and Functional Recently, varanopids have been interpreted as diapsids instead Prevalence of Autopodia of synapsids (Laurin and Piñeiro, 2018; Ford and Benson, 2020; As a whole, we preliminary identify three main groups but see discussion in MacDougall et al., 2018). Based on the of upper Carboniferous–Middle Triassic ichnotaxa (selected, present analysis of relative depth patterns, different options could representative specimens are shown in Figure 7) characterized by explain why varanopids are found within the group of diapsid a different functional prevalence: tracks: (1) perhaps they reflect a very basal position of varanopids within synapsids; (2) a functional/convergent signal rather than 1) Anamniote and captorhinomorph/parareptile tracks: a phylogenetic signal for this group. However, the latter option medial-median functional prevalence (Figures 2A–E, seems unfeasible as, in general terms, functional prevalence is 4A,E–G, 5C,D, 6A, 7A). correlated with phylogenetical signal. The first option is herein 2) Diapsid tracks: median functional prevalence preferred; nonetheless, it should be remarked that the present (Figures 3, 6C, 7B). analysis of depth patterns is preliminary, including only one 3) Synapsid tracks: median-lateral functional prevalence ichnotaxon (Tambachichnium) correlated to varanopids, not (Figures 2F, 3B–D, 5A,B,E, 7C). precluding that further sampling could reveal differences across The first group of tracks (Figure 7A) includes ichnogenera varanopid evolution. produced by temnospondyl and lepospondyl amphibians Tracks of the second group are most commonly produced (Batrachichnus, Limnopus, Matthewichnus), seymouriamorphs by autopodia with semidigitigrade to semiplantigrade postures, (Amphisauropus), diadectomorphs (Ichniotherium), and and digits II, III, and IV are usually the deepest, being captorhinomorph/parareptiles (Erpetopus, Hyloidichnus, in many cases the only impressed parts of the autopodia. Merifontichnus, Notolacerta, Procolophonichnium, Pachypes, Noteworthy, some chirotheriid tracks also display a relatively Varanopus). The medial (inner) part of these tracks (usually digits deep impression of the proximal pad of pedal digit V, being I and II, and the corresponding portion of the palm/sole) are similar to that of digits II, III, and IV (see 5 and 6 in generally the most deeply impressed parts. Depth patterns may Figure 7B). Trackways are usually narrow, though most of vary between manus and pes tracks, the medial-lateral decrease the Dromopus and Rhynchosauroides specimens and some in relief is usually more evident in the pes tracks. Besides, the Prorotodactylus specimens display relatively low pace angulations trackways are often broad, with relatively low pace angulations (Mujal et al., 2017a). In the small-sized ichnotaxa, a certain (with the only exception of some captorhinomorph/parareptile degree of overstepping of the pes respect to the manus tracks such as Erpetopus and Varanopus, which show also impressions is common, ranging from a slight lateral position relatively high pace angulation and primary pes-manus overstep). of the pes respect to the manus to a complete overstepping The second group of tracks (Figure 7B) includes of the pes, being positioned to the anterolateral side of the ichnogenera produced by diapsid reptiles, being manus tracks. Generally, the tracks of this group show a trend archosauromorph tracks the most representative group. It toward a tridactyl functionality that can be mesaxonic (e.g., includes ichnogenera from the Chirotheriidae ichnofamily Chirotherium, Isochirotherium, Parachirotherium, Sphingopus, (e.g., Brachychirotherium, Chirotherium, Isochirotherium, Atreipus), as observed in ornithopod and theropod dinosaur Parachirotherium, Protochirotherium, Sphingopus, tracks (e.g., Castanera et al., 2013; Lallensack, 2019), or ectaxonic Frontiers in Earth Science | www.frontiersin.org 17 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology (e.g., Dromopus, Rhynchosauroides, Rotodactylus), as observed in trace fossils and (2) evaluate how the three identified most modern diapsids. groups evolved through time and space. Of note, these The third group of tracks (Figure 7C) is attributed to the analyses also have to consider the exceptions/differences synapsid group (i.e., including therapsids), with the exception of observed within the groups, as well as differences between varanopid synapsid tracks (i.e., Tambachichnium; see discussion ecosystems (i.e., different paleoenvironments in which the above). This group of tracks includes ichnogenera from the footprints are preserved and the related biases, as not all Pennsylvanian–Cisuralian (e.g., Dimetropus), the Guadalupian the paleoenvironments are equally preserved in the fossil (e.g., Brontopus), the Lopingian (e.g., Capitosauroides, record). In addition, future studies of tetrapod tracks from Dicynodontipus, Dolomitipes, Karoopes), and the Triassic (e.g., any age interval and (ichno-) taxonomic group should also Capitosauroides, Dicynodontipus, Pentasauropus, Therapsipus) test this analysis. For instance, in a preliminary overview, (see also Figure 9 by Marchetti et al., 2019d). These tracks often we observed that dinosaur tracks from different lineages (see show a more deeply impressed digit IV, or in some cases even examples in Castanera et al., 2013; Salisbury et al., 2016; digit V, as well as the parts of palm/sole impressions proximal to Lallensack et al., 2017) appear to fit with group 2, which digits IV and V. In addition, these tracks are generally suggestive encompasses diapsids. Furthermore, the identification of of trackmakers with a plantigrade posture, with relatively broad these groups, when possible, should be accompanied with sole and palm impressions and short digit imprints. Complete additional data regarding the ichnotaxa considered, including overstep of pes respect to manus tracks is uncommon and only general footprints’ morphologies, relative proportions, and observed in the ichnotaxa Dicynodontipus and Karoopes. trackway parameters. In fact, depth patterns are considered The relative depth patterns of footprints become more ichnotaxobases (e.g., Voigt, 2005; Marchetti et al., 2017a). Also, significant when mapped onto a cladogram of tetrapods such classification must be based on a representative sample, (Figure 8). Although the discussion on the phylogeny at the avoiding assumptions or interpretations derived from the study amniote and reptile origin is currently debated (e.g., Laurin and of a small number of specimens. Piñeiro, 2017, 2018; MacDougall et al., 2018; Ford and Benson, Of interest, despite the different types of substrates (even 2020), the three groups that we identify are very wide. We plotted for the same ichnotaxon) and the wide age range of each of our groups on two cladograms based on MacDougall et al. (2018) the three identified groups, a substantially consistent relative (see Figure 8A) and Ford and Benson (2020) (see Figure 8B), depth pattern of tracks is observed within the groups (Figure 7). both of which including a possible new position of the varanopid Indeed, all these groups cross major biotic crises, such as the group, being out of Synapsida. Group 1 apparently forms the end-Guadalupian and end-Permian mass extinctions. This would primitive condition of upper Carboniferous to Middle Triassic indicate that the functional prevalence of each group is preserved tetrapods, as it occurs in both the putative lissamphibian stem- and inherited despite environmental and climatic (catastrophic) group (temnospondyls) as well as amniote stem lineages (e.g., changes. Henceforth, evidences of such events may be found in seymouriamorphs and diadectomorphs), which are at the base broader context analyses, including quantitative studies of the of both cladograms represented (Figure 8). Interestingly, group 1 functional prevalence of autopodia, encompassing a relatively does not only encompass anamniotes and stem amniotes, but also wide stratigraphic range. extends to parareptiles and captorhinids, which form successive A further question to explore is the relationship of these sister taxa of diapsid reptiles (Reisz et al., 2011; MacDougall groups with the postcranial skeleton record of the supposed et al., 2018) (Figure 8A). This suggests that the depth pattern producers, which may help to identify the anatomic and of group 2 (diapsid tracks) derived from the depth pattern of functional reasons of these depth patterns. Also, it should be group 1 (probably from captorhinomorphs), independently from checked if the herein proposed groups of footprints according the evolution of the synapsid footprint depth pattern (group 3), to their functional prevalence keep a relationship and/or which forms a separate modification from the basal tetrapod present a correlation with the morphofunctional categories of footprint depth pattern (Figure 8). Further studies should focus temporal skull fenestra in amniotes (Wernerburg, 2019; Ford on the question why the pattern of group 1 is so widespread and Benson, 2020). Indeed, in a general point of view, a similar across early tetrapods and what this means in functional and distribution of high-rank taxonomic groups exists between these locomotory terms. two different grouping. Some recent studies nested the parareptiles within diapsids (Ford and Benson, 2020). This would imply a separation of group 1 in two sub-groups: (1) anamniotes, stem amniotes, CONCLUSION and captorhinids and (2) parareptiles (Figure 8B). A possible explanation for this is a morphological reversal between group Photogrammetry has been proved to be an extremely useful tool 2 (Diapsida) and group 1 (Parareptilia) to the ancestral in the study of upper Paleozoic and Triassic tetrapod ichnofossils. depth pattern condition. However, this would represent a less Nevertheless, this technique has not been commonly applied in parsimonious chain of events. All in all, based on available this ichnological record, especially when compared with dinosaur interpretations of early amniotes, we concur in general terms with footprints. Through the present work, we review and highlight the interpretation of MacDougall et al. (2018). the benefits of using photogrammetry. Indeed, it is proved that The present work is the first step to (1) create a general 3D models can be key elements in ichnotaxonomy. Footprint framework to analyze upper Paleozoic to Triassic tetrapod relative depth patterns, when appropriately compared with other Frontiers in Earth Science | www.frontiersin.org 18 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology FIGURE 8 | Simplified cladograms of tetrapods showing the relationship of the three footprint groups of relative depth patterns. (A) Tree based on MacDougall et al. (2018); note the dashed lines of varanopids: the line from the reptile lineage was recovered by these authors, the line from the synapsid lineage corresponds to the classical interpretation (here preferred; see text for discussion). (B) Tree based on Ford and Benson (2020); note the split of group 1, implying a less parsimonious chain of events. tracks and skeletal remains, refine the correlation of tracks with sedimentary processes (e.g., ripples, collapse of the substrate in the potential trackmakers. False color depth maps also help to inclined surfaces), as well as from weathering of the footprints. unravel the formation and superimposition of structures that Furthermore, 3D models are a suitable solution to preserve the can mask footprints. Such structures may result from both ichnological record, especially when original specimens cannot the locomotion of the same (or other) trackmakers and from be collected and thus are subjected to disappearance. Frontiers in Earth Science | www.frontiersin.org 19 July 2020 | Volume 8 | Article 248 Mujal et al. Photogrammetry in Tetrapod Ichnology A review of the available 3D data of Permian and Triassic Erasmus+ program of the Universitat Autònoma de Barcelona. (and upper Carboniferous to a lesser degree) tetrapod footprints LM was funded by the Alexander von Humboldt Foundation. JF allowed us to preliminary identify three main groups of was supported by the Spanish Agencia Estatal de Investigación tracks linked to specific relative depth patterns derived from and the European Regional Development Fund of the European the functional prevalence of autopodia: (1) anamniote and Union (AEI/FEDER EU, project CGL2017-82654-P). JF is captorhinomorph/parareptile tracks (medial-median functional member of the consolidated research group 2017 SGR 86 GRC prevalence); (2) diapsid tracks (median functional prevalence); of the Generalitat de Catalunya. EM and JF are members of the and (3) synapsid tracks (median-lateral functional prevalence). project “Evolució dels ecosistemes durant la transició Paleozoic– This qualitative analysis shows a substantially consistent depth Mesozoic a Catalunya” (ref. CLT009/18/00066), financially pattern within each group, despite being preserved in different supported by the Departament de Cultura (Generalitat de substrates and corresponding to a wide stratigraphic interval. All Catalunya). This research received support from the CERCA in all, this indicates that 3D models may show features linked to program (ICP) from the Generalitat de Catalunya. the potential trackmakers that remain unseen by the naked eye. ACKNOWLEDGMENTS DATA AVAILABILITY STATEMENT We thank all curators and researchers for allowing access to All datasets generated for this study are included in the specimens presented in this work, stored in their corresponding article/Supplementary Material. institutions, which include Muséum National d’Histoire Naturelle (Paris), Institut des Sciencies de l’Evolution de Montpellier (at Université de Montpellier, Montpellier), Musée AUTHOR CONTRIBUTIONS Fleury (Lodève). We thank all the land owners who granted access to the specimens in situ. We especially thank Sergio Llácer (ICP) JF, EM, LM, and RS contributed on the conception and design for the elaboration of several 3D models for this work. We also of the study and performed the ichnological analysis. EM and thank Rafel Matamales-Andreu (ICP) for the help in acquisition LM organized the database. EM wrote the first draft of the of photos and elaboration of 3D models from Erillcastell, and manuscript and prepared the figures, with comments from all Jaume Balagué (Terra Dron) for the acquisition of aerial and authors. LM, RS, and JF wrote sections of the manuscript. All the close-range photos, and data processing from Erillcastell and authors contributed to manuscript revision, read, and approved Coll de Terrers. We acknowledge three reviewers who helped to the submitted version. substantially improve an earlier version of the manuscript, as well as Martin D. Ezcurra for editorial handling. FUNDING SUPPLEMENTARY MATERIAL EM received funding from the SYNTHESYS Project http: //www.synthesys.info/ (FR-TAF-3621 and FR-TAF-4808) The Supplementary Material for this article can be found which is financed by European Community Research online at: https://www.frontiersin.org/articles/10.3389/feart. Infrastructure Action under the FP7 “Capacities” Program; 2020.00248/full#supplementary-material Secretaria d’Universitats i Recerca (Departament d’Economia, Generalitat de Catalunya; grant number 2013 CTP 00013); the DATA SHEET S1 | Reviewed localities. REFERENCES Bates, K. T., Manning, P. L., Vila, B., and Hodgetts, D. (2008a). Three-dimensional modelling and analysis of dinosaur trackways. Palaeontology 51, 999–1010. Abel, O. (1935). Vorzeitliche Lebensspuren. Jena: Fischer. doi: 10.1111/j.1475-4983.2008.00789.x Adams, T. L., Strganac, C., Polcyn, M. J., and Jacobs, L. L. (2010). High Bates, K. T., Rarity, F., Manning, P. L., Hodgetts, D., Vila, B., Oms, O., et al. 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Strat. 115, 277–290. use, distribution or reproduction is permitted which does not comply with these terms. Frontiers in Earth Science | www.frontiersin.org 23 July 2020 | Volume 8 | Article 248