A revision of Poposaurus gracilis (Archosauria : Suchia) based on two new specimens from the Late Triassic of the southwestern USA

Two associated partial skeletons, one from the McCarty ranch (MOTTU site 0690) near Kalgary, Texas, Tecovas Formation, and the other from the Petrified Forest Member of the Chinle Formation, outside the Petrified Forest, Arizona, represent the most complete specimens of Poposaurus gracilis found to date, and increase our understanding of the osteology of this rare pseudosuchian archosaur. The cervical centrum of TTUP-10419 possesses an accessory dorsal parapophyseal rib facet, as in Lythrosuchus. The ilia of these new fossils are the first of Poposaurus to preserve the complete preacetabular blade, which is long and blade-like rather than paddle-shaped as previously thought. These specimens also confirm that isolated elements previously referred to Poposaurus (fused sacral vertebrae, the pubes and the ischia), but not present in the holotype indeed belong to that taxon. The morphology of the cervical centrum and the preacetabular blade of the ilium are identical to the much larger type of Lythrosuchus langstoni, suggesting that Lythrosuchus is a junior synonym of Poposaurus. However, we retain the species Poposaurus langstoni nov. comb. on the basis of larger size, absence of a distinct ridge on the lateral surface of the ilium, and the lack of a pit on the proximal end of the ischium. A phylogenetic analysis suggests a monophyletic Poposauroidea and a paraphyletic Rauisuchia.