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Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution

By
William G. Parker
William G. Parker
Jackson School of Geosciences, the University of Texas at Austin, 1 University Station, C1100, Austin, TX 78712, USAPetrified Forest National Park, PO Box 2217, Petrified Forest, AZ 86028, USA
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Sterling J. Nesbitt
Sterling J. Nesbitt
Department of Biology, University of Washington, Seattle, WA 98145, USA
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Published:
January 01, 2013

Abstract

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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Geological Society, London, Special Publications

Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin

S. J. Nesbitt
S. J. Nesbitt
University of Washington, USA
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J. B. Desojo
J. B. Desojo
Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’, Argentina
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R. B. Irmis
R. B. Irmis
Natural History Museum of Utah, USA
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Geological Society of London
Volume
379
ISBN electronic:
9781862396395
Publication date:
January 01, 2013

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