Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution
William G. Parker, Sterling J. Nesbitt, 2013. "Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution", Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin, S. J. Nesbitt, J. B. Desojo, R. B. Irmis
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The partial postcrania of Poposaurus gracilis, a bipedal poposauroid convergent with theropod dinosaurs, has been known for nearly a century, but the skull of P. gracilis has proven elusive. P. gracilis is part of a clade of morphologically divergent pseudosuchians (poposauroids) whose members are sometimes bipedal, lack dentition (i.e. beaks) and some have elongated neural spines (i.e. sails). However, the timing and acquisition of these character states is unknown given the uncertainty of the skull morphology of the ‘mid-grade’ poposauroid P. gracilis. Here, we present the first confirmed skull remains of P. gracilis directly associated with diagnostic pelvic elements that overlap with the holotype. The incomplete skeleton (PEFO 34865) from the Chinle Formation of Petrified Forest National Park (Arizona, USA) includes a left maxilla with a large, mediolaterally compressed tooth, left dentary, right prearticular and a partial postcranium. The character states of P. gracilis (bipedal, ‘sail-less’ and toothed) demonstrate that the evolution of bipedalism, the origin/loss of a dorsal ‘sail’ and the shift to an edentulous beak are complex in poposauroids. P. gracilis is widespread in the Upper Triassic formations in the western USA and is restricted temporally prior to the Adamanian–Revueltian faunal turnover during the Norian.
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Archosaurs, an important reptile group that includes today’s crocodiles and birds, arose during the Triassic in the aftermath of the greatest mass extinction of all time. In the last 20 years, our understanding of the early evolution of the group has improved substantially with the discovery of new fossils and species of early archosaurs and their closest relatives, a better understanding of the relationships of these animals, and new insights into their palaeobiology. In order to synthesize these new data, researchers of early archosaurs from around the world met at the first symposium of early archosaur evolution at the IV Congreso Latinoamericano de Paleontología de Vertebrados (September 2011) in San Juan, Argentina. This symposium facilitated collaboration and strove to paint a better understanding of these extraordinary animals. The resultant body of work is a state-of-the-art examination of early archosaur groups and their close relatives including historical, anatomical, biogeographical, evolutionary and palaeobiological data. This contribution furthers our knowledge of the anatomy, relationships, and palaeobiology of species-level taxa as well as more global patterns of archosaur evolution during the Triassic.