Postcranial skeleton of Postosuchus kirkpatricki (Archosauria: Paracrocodylomorpha), from the Upper Triassic of the United States

Postosuchus kirkpatricki is a Late Triassic (Norian) ‘rauisuchid’ archosaur from North America. The initial description of the Postosuchus type material included elements from two poposaurids. This confusion has prevented adequate description of the material. Recent examination of the type material and other specimens of Postosuchus, and of related taxa, has helped clarify the osteology of Postosuchus. The type specimens represent c. 75% of the skeleton. Together with other referred material, Postosuchus remains one of the most completely known rauisuchids. The paratype skeleton, which is relatively complete, would have been c. 3.5–4 m in length, and the holotype would have been closer to 5–6 m. Analysis of the postcranial skeleton of Postosuchus suggests that it may have been an obligate biped (based in part on limb proportions, which are similar to some theropod dinosaurs, the size of the manus (30% of the size of the pes) and the highly reduced nature of the digits and vertebral measurements). Possible postcranial autapomorphies of Postosuchus include a large, rugose triangular supra-acetabular buttress confluent with the dorsal margin of the iliac blade, and a symmetrical pes with digits two and three being roughly equal in length. During the Triassic Period, archosaurian reptiles diversified into a large variety of carnivorous and herbivorous forms that dominated the global terrestrial landscape (Huene 1938; Bonaparte 1967; Chatterjee 1986; Long & Padian 1986; Parrish & Carpenter 1986; Benton & Clark 1988; Sereno 1991; Lucas 1998; Gower & Sennikov 2000; Langer 2005; Irmis et al. 2007; Sues & Fraser 2010). Of the two main lineages of archosaurs – Avemetatarsalia (bird-line) and Pseudosuchia (crocodile-line) – the pseudosuchians appear to have diversified more rapidly (Gower & Sennikov 2000; Nesbitt 2005a, b, 2011; Butler et al. 2009, 2011). ‘Rauisuchians’, a paraphyletic assemblage of pseudosuchians (alternatively see Brusatte et al. 2010, Butler et al. 2011) including Poposauroidea and Loricata (1⁄4 Paracrocodyliformes of Weinbaum & Hungerbuhler 2007), appear tentatively in Lower Triassic sediments in Asia and Europe and then in North America, with poposauroids having the oldest record (Butler et al. 2011; Nesbitt et al. 2011). Postosuchus kirkpatricki is a rauisuchid, a clade of pseudosuchians within the Paracrocodylomorpha (Fig. 1) and is one of the better-known genera of rauisuchid (Chatterjee 1985; Long & Murry 1995; Gower 2000; Weinbaum 2002; Weinbaum & Hungerbuhler 2007; Nesbitt 2011; Weinbaum 2011). Postosuchus was a relatively large animal estimated to have grown to between 5 and 7 m in length. Postosuchus has been considered by most workers to be closely related to crocodylomorphs (Benton & Clark 1988; Parrish 1993; Wu & Chatterjee 1993; Juul 1994; Long & Murry 1995; Gower 2000; Weinbaum & Hungerbuhler 2007; Nesbitt 2011; alternatively see Gower 2002 and Brusatte et al. 2010). However, the description of Postosuchus and its phylogenetic affiliations has been the subject of considerable debate since Chatterjee’s (1985) paper (Parrish 1993; Long & Murry 1995; Alcober & Parrish 1997; Gower 2000). Gower (2000) questioned the validity of some of Long & Murry’s (1995) identification of material (i.e. Long & Murry 1995, figs 125 & 136) from the Placerias Quarry to Postosuchus, but I agree with Long and Murry in attributing those elements to Postosuchus, as the morphology of those elements shows little variation from the type material. Material re-examined from the collection of the Museum of Texas Tech University, as well as a new, partially articulated skeleton from North Carolina (Peyer et al. 2008), an articulated, headless skeleton from Ghost Ranch, New Mexico, and specimens from other institutions have provided much data and context previously unknown from the Post Quarry ‘rauisuchians’. The specimen from Lithofacies II from the Deerfield Basin of North Carolina is intermediate in size between TTU-P 9000 and TTU-P 9002 (see Appendix I for institutional abbreviations). UNC-15575 was preserved with almost complete left and right pes and tarsus, and a mostly complete right manus and carpus (Peyer et al. 2008). Although the specimen is only From: Nesbitt, S. J., Desojo, J. B. & Irmis, R. B. (eds) Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin. Geological Society, London, Special Publications, 379, http://dx.doi.org/10.1144/SP379.7 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics 10.1144/SP379.7 Geological Society, London, Special Publications published online February 13, 2013 as doi: