Monday, July 16, 2012

Phytosaurs - Some Are Real and Some Are Just Dinosaurs

Phytosaurs may not be crown group archosaurs (according to Nesbitt 2011), but they are definitely important in archosaur evolutionary history as well as Triassic ecology. Stocker's new paper increases the known diversity of phytosaurs with the description of the new species Protome batalaria and reinforces the importance of apomorphy-based identifications. Some recent accounts of partial skeletons have suggested that phytosaurs may have survived into the Early Jurassic, but Barrett and Xu re-affirm their end-Triassic demise and urge caution in identifying poorly preserved, partial remains, which tend to belong to dinosaurs.

Stocker, M. R. 2012. "A new phytosaur (Archosauriformes, Phytosauria) from the Lot’s Wife beds (Sonsela Member) within the Chinle Formation (Upper Triassic) of Petrified Forest National Park, Arizona." Journal of Vertebrate Paleontology 32(3): 573-586
A new phytosaur taxon from Petrified Forest National Park, Arizona, is here described based on cranial material from a single individual. This specimen previously was included in an extensive phylogenetic analysis, and it was found to possess a combination of character states that differs from all known phytosaur taxa in addition to two autapomorphies within the braincase and an autapomorphy of the mandible. The new taxon adds to the taxonomic diversity recognized from the Sonsela Member of the Chinle Formation. The continued increase in phytosaur diversity emphasizes the need to more accurately characterize and identify taxa within a phylogenetic systematic context in order to produce a more refined signal for biostratigraphic correlations, biochronologic inferences, and faunal dynamics during the Late Triassic.

Barrett, P. M., and X. Xu. 2012. "The enigmatic reptile Pachysuchus imperfectus Young, 1951 from the lower Lufeng Formation (Lower Jurassic) of Yunnan, China." Vertebrata PalAsiatica 50:151-159. [Free download here]
Phytosaurs are generally considered to have become extinct at the end of the Triassic Period, but several records have suggested that they survived into the basal Jurassic in Europe and Asia. The Asian record consists of Pachysuchus imperfectus from the lower Lufeng Formation (?Hettangian-Sinemurian) of Yunnan, China. However, this specimen differs from phytosaurs in numerous aspects and is more likely a poorly preserved, indeterminate sauropodomorph dinosaur skull. The referred specimens of this species are also regarded as indeterminate, thereby removing the post-Triassic record of phytosaurs from Asia. The European records of Jurassic phytosaurs are also shown to be doubtful, suggesting that this clade was restricted to the Late Triassic. 


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