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Erpetosuchus, a Crocodile-Like Basal Archosaur from the Late Triassic of Elgin, Scotland

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Erpetosuchus, a Crocodile-Like Basal Archosaur from the Late Triassic of Elgin, Scotland Blackwell Science, LtdOxford, UKZOJZoological Journal of the Linnean Society0024-4082The Linnean Society of London, 2002September 2002 1361 2547 Original Article CROCODILE-LIKE ARCHOSAUR FROM LATE TRIASSIC M. J. BENTON and A. D. WALKER Zoological Journal of the Linnean Society, 2002, 136, 25–47. With 10 figures Archosaurian anatomy and palaeontology. Essays in memory of Alick D. Walker. Edited by D. B. Norman and D. J. Gower Erpetosuchus, a crocodile-like basal archosaur from the Late Triassic of Elgin, Scotland MICHAEL J. BENTON and ALICK D. WALKER Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK Erpetosuchus, a small archosaurian reptile from the Late Triassic of Scotland and North America, has often been implicated in the ancestry of crocodilians. A restudy of the type specimen, using new high-fidelity casts, as well as examination of new, hitherto undescribed material, allows a detailed description and restoration of Erpetosuchus granti from the Lossiemouth Sandstone Formation (late Carnian, Late Triassic). This small reptile is known only from the front end of its body; a complete skull, cervical vertebral column, anterior dorsals and ribs, shoulder girdle, and forelimb. The skull shows a number of unusual features: a reduced row of only 4–5 teeth on the anterior part of the maxilla, a large antorbital fenestra set in a deep fossa whose margins are marked by distinct sharply angled ridges, a jugal that is divided into a lateral and a ventral portion by a sharp ridge, a deeply recessed tympanic area, the angular and surangular marked by a strong ridge running back from the ventral margin of the mandibular fenes- tra, and teeth oval in cross-section and lacking anterior and posterior carinae and marginal serrations. The remains suggest that Erpetosuchus was a light, cursorial animal that may have fed on insects. A cladistic analysis of cruro- tarsan archosaurs indicates that Erpetosuchus is the closest sister group of Crocodylomorpha among known basal archosaurs. It shares with them a deep recess in the cheek region framed by the quadrate and quadratojugal which slope forward side-by-side at an angle of 45° above horizontal, and reach the upper margin of the lower temporal fenestra. In Erpetosuchus the recess is entirely lateral, while in crocodylomorphs, the recess penetrates medially as well, since the quadrate/quadratojugal bar meets the side wall of the braincase. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 25–47. ADDITIIONAL KEYWORDS: Archosauria – Crurotarsi – Crocodylomorpha – crocodilians – osteology – skull – phylogenetic. INTRODUCTION and its affinities have remained uncertain, to the extent that it is generally assigned to its own family, When Newton (1894) described the new reptile Erpe- Erpetosuchidae. The recent report of a specimen of tosuchus granti from the Lossiemouth Sandstone For- Erpetosuchus from North America (Olsen et al., 2000) mation (Late Triassic) of Lossiemouth, near Elgin, in indicates that a re-study of the original material, and north-east Scotland, he presented something of an some previously unpublished specimens from Lossie- enigma to the world. Newton clearly recognized that mouth, will be timely. Erpetosuchus was an archosaur, perhaps close to the Alick Walker began to study the type specimen of phytosaurs and the aetosaurs, but also showing some Erpetosuchus granti, housed in the Natural History similarities with crocodilians. The phylogenetic posi- Museum, London in 1963, and he continued to accu- tion of this reptile has been debated ever since (Broom, mulate notes and sketches from that time until the 1906; Watson, 1917; Huene, 1936; 1956; Romer, 1956; 1980s. He published preliminary restorations of the Walker, 1968, 1970; Krebs, 1976; Olsen et al., 2000), skull in lateral view (Walker, 1970; Benton & Walker, 1985), but did not complete his planned re-description of the material. M.J.B. has re-examined all the mate- Correspondence. Mike Benton. rial, including Newton’s casts and A.D.W.’s casts and E-mail: [email protected] notes, and the present paper is a collaborative effort. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 25–47 25 26 M. J. BENTON and A. D. WALKER Institutional abbreviations: AMNH, American inside. Newton had his casts coloured, brown for the Museum of Natural History, New York; BGS GSM, bone and pink for intervening rock. Newton’s casts are Geological Survey Museum, Keyworth; BMNH, still in superb condition, over 100 years after they Natural History Museum, London; NMS, National were made, and they show remarkably fine detail, Museums of Scotland (formerly Royal Scottish such as the sculpturing on the frontal bone, sutures Museum, RSM). between skull bones, and the microscopic sculpture of the tiny scutes (Figs 1B−D, G, 4A). Newton also had some plaster models of the skull and mandible of STUDYING THE ELGIN REPTILES Erpetosuchus sculptured, for purposes of display and exchange (Fig. 1E, H), but the illustrations in his Most of the fossil reptiles from the Permian and Tri- paper (Newton, 1894) were taken directly from the assic sediments at Elgin are preserved as hollows in gutta percha casts, and they are accurate. the rock (Benton & Walker, 1981, 1985). Bone material Later, when A.D.W. began his studies of the Elgin is occasionally present, but it is usually soft and poorly reptiles in the 1950s and 1960s, he used synthetic preserved, and is sometimes heavily mineralized. materials, polyvinyl chloride (PVC) and Vinagel, to Early studies, from the 1840s to the 1880s, relied on take further casts from the natural rock moulds traditional preparation methods; the sandstone was (Figs 1F, I, 4B). PVC has the advantage over gutta removed mechanically, so far as was possible, to percha of not going hard, and it retains its flexibility. expose the bones, or the remnants of the bones. Then, In comparisons of Newton’s casts made in the 1890s in the 1890s, E. T. Newton at the Geological Survey with those made by A.D.W. in the 1960s, it is clear that Museum in London pioneered the use of effective cast- some fine detail has been lost in the rock moulds, prob- ing materials. Earlier and later efforts using plaster of ably as each cast is taken. For example, some of the Paris were not hugely successful, since the natural PVC casts of the cervical scutes show less detail of the rock moulds are complex, and a flexible material has sculpture patterns than is shown by the gutta percha to be used. casts. However, A.D.W. worked carefully to gain access Newton (1894: 574) described the original appear- to areas of the skeleton of Erpetosuchus that Newton ance of the type specimen of Erpetosuchus when he had not reached, and by careful cleaning of the hol- received it (Fig. 1A): lows, he was able to obtain more detail of the verte- When the block first came to me it had been broken across in brae, for example. In addition, a clean PVC cast shows two directions, and one piece was wanting. The two portions better detail of suture lines and surface sculpturing remaining formed an irregular cube, showing on one side some when examined under the microscope than one of the small cavities, which proved to be parts of the vertebral column older gutta percha casts. that had been broken across. Another surface...exhibited sev- It should be noted that the remark made by Olsen eral holes...and an outline which seemed to indicate a trans- et al. (2000: 635), that ‘most of the cranial sutures verse section of the skull...After carefully probing to find in shown in Walker’s reconstruction are not evident on which direction the bones lay,...it was decided to split the the casts examined by us’ is a reflection on their casts, smaller block from end to end, so as, if possible, to open longi- and not on Newton’s or A.D.W.’s casts or interpreta- tudinally a cavity which seemed to be part of a skull. This oper- tions. Recently made casts are tenth- to fifteenth- ation was successful beyond expectation... generation, and the holotype mould has demonstrably Newton revealed the entire skull and mandible, with lost some of the subtleties of detail that are clearly the associated cervical and anterior dorsal vertebral recorded in Newton’s original gutta percha casts and column and ribs, some rows of dorsal scutes above, and A.D.W.’s PVC casts. In this work, we have used the the entire pectoral girdle and forelimb on each side, an original casts. ‘exceedingly pretty little reptile’. For cast-making, Newton chose gutta percha, a rub- ber-like hydrocarbon that is obtained from the latex of DESCRIPTION Malaysian trees of the Family Sapotaceae (Palaquium, SUBDIVISION ARCHOSAURIA COPE, 1869 Payena, etc.). In his day, gutta percha (the name is INFRADIVISION AVESUCHIA BENTON, 1999 getah percha derived from the Malay words , gum, and , ORDER CRUROTARSI SERENO & ARCUCCI, 1990 a tree producing it) was used for making golf balls, but SUBORDER SUCHIA KREBS, 1976 it was its toughness that attracted Newton. He found FAMILY ERPETOSUCHIDAE WATSON, 1917 he could pour the latex into the cavities in the rock, let ERPETOSUCHUS GRANTI NEWTON, 1894 it harden, and then remove it while it was still flexible. In time, it hardened to a firm, if somewhat delicate, 1894 Erpetosuchus granti Newton; Newton, pp. 574– cast, which retained enough flexibility that he could, 586, pl. 53. for example, manipulate processes of the skull to look 1895 Erpetosuchus granti Newton; Traquair, p. 276. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 25–47 CROCODILE-LIKE ARCHOSAUR FROM LATE TRIASSIC 27 A BC D E G F I H Figure 1.
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