2013/01/01 by William G. Parker, Sterling J. Nesbitt · 1 citation
Earth and Planetary Sciences · Environmental Science · #Paleontology and Evolutionary Biology #Ichthyology and Marine Biology #Paleontology and Stratigraphy of Fossils #Geology #Paleontology #Taxon
paper · doi:10.1144/sp379.3
openalex publication_date 2013/01/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/06/11
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.