Coelophysidae

Abstract: INTRODUCTION Prior to Gauthier’s classic (1986) monograph, our understanding of the interrelationships of theropod dinosaurs could be described as murky at best. Most works still adhered to the old notion of a coelurosaur versus carnosaur dichotomy that separated small gracile forms from the larger, more robust, taxa. Nevertheless many had expressed doubts as to the ‘naturalness’ (monophyly in modern parlance) of these groupings. Gauthier established a basal dichotomy in Theropoda that cut across the big versus small division. The two branches were the Ceratosauria and the Tetanurae. Gauthier’s Ceratosauria included the former carnosaur Ceratosaurus nasicornis and the small, gracile coelophysoids, while the Tetanurae, included the true carnosaurs and the true coelurosaurs. The monophyly of the Tetanurae as constituted by Gauthier has never been seriously questioned since and it is supported by a number of synapomorphies of the cranial and postcranial anatomy, although the exact node at which these synapomorphies fall on varies between analyses, depending on the basal topology of the Tetanurae and which basal tetanuran taxa are included (Holtz 1994; 2000; Sereno et al. 1994, 1996, 1998; Rauhut 2003). The monophyly of Gauthier’s Ceratosauria has not been so widely accepted, with suggestions that the larger C. nasicornis and its close relatives, the newly recognized Abelisauroidea, share a more recent common ancestor with tetanurans than they do with coelophysoids (Bakker 1986; Forster 1999). However, the closer relationship of C. nasicornis to Coelophysoidea than to Tetanurae continued to find support form phylogenetic analyses through the 1990s and into the early years of the new millennium. Several recent analyses that have incorporated substantial amounts of new information have overturned these results (Carrano et al. 2002, Rauhut 2003, Sereno et al. 2004). It is now the majority view amongst theropod systematists that Ceratosauria contains Ceratosaurus spp. and Abelisauroidea and that this clade is more closely related to Tetanurae than it is to Coelophysoidea although Tykoski & Rowe (2004) continue to support the inclusion of Coelophysoidea within Ceratosauria. The break-up of Ceratosauria is carried further in Rauhut’s (2003) comprehensive study of early theropod relationships. He found that, despite a relatively distinctive morphology, the monophyly of the broader coelophysoid assemblage (Dilophosaurus wetherilli, Liliensternus spp. and Coelophysidae) was questionable. In particular Dilophosaurus wetherilli was found to share a number of derived characteristics with Ceratosauria and Tetanurae not present in other coelophysoids. Nevertheless the position of D. wetherilli was unstable and depended upon the inclusion of the enigmatic taxon Shuvosaurus inexpectatus which may or may not be a dinosaur (Long & Murry 1995). When S. inexpectatus was included Dilophosaurus wetherilli formed a clade with Ceratosauria + Tetanurae but when Shuvosaurus inexpectatus was excluded it became equally as parsimonious for Dilophosaurus wetherilli to be included within Coelophysoidea as for it to form a clade with Ceratosauria + Tetanurae. Other analyses have continued to support the inclusion of D. wetherilli within Coelophysoidea (Sereno et al. 2004; M.T. Carrano, pers. comm.) but the support for this clade is very weak (M.T. Carrano, pers. comm.). If D. wetherilli really does share a more recent common ancestor with Ceratosauria + Tetanurae then it would imply that its coelophysoid-like characteristics such as its elongate and acutely pointed premaxilla, subnarial gap, elongate skull, expanded dentary tip and possibly tall, paired naso-lacrimal crests on the snout were symplesiomorphies of basal Theropoda. In other words, theropods might have passed through a ‘coelophysoid’