A PRELIMINARY STUDY OF PHYTOSAUR FEMORA FOR USE IN TAXONOMY AND PALEOECOLOGY

The taxonomy and phylogeny of phytosaurs, non-archosaur archosauriform reptiles found in Middle-Upper Triassic rocks, have proven to be contentious and confusing. Because all current phylogenies only use cranial characters, descriptions of phytosaur postcrania have become increasingly needed. Describing the morphology of postcranial material may also be useful in understanding the ecological roles of phytosaurs. As Kimmig notes, certain morphologies (such as a strongly curved femur) seem to be more suitable for an aquatic lifestyle, while others (a nearly straight shaft) would more likely suit a terrestrial lifestyle.

We visited the collections at the University of California Museum of Paleontology (UCMP) to observe postcranial material from phytosaurs. We photographed thirty phytosaur femora and measured each midshaft width, head width, fourth trochanter, and total femur length. Using methods based on Kimmig 2008, we began preliminary descriptions of the facies articularis antotrochanterica (faa) and the tip of each head as one of three categories: high nearly pointed, short tip; high rounded, short tip; low rounded and long tip. The shaft curvature was also noted as one of three types: gently curved, strongly curved, or nearly straight.

The femora we studied were generally consistent with the characters defined by Kimmig. However, we suggest separating the faa and tip codes into their own categories as there is occasional variation between these traits. When these characters were split up, we found ten different femur morphologies. When looking at the codes for faa, tip, and curve, we found that, in general, a strongly curved shaft is never paired with a low rounded faa or long tip. We found no relationship between the size of the femur and the shape of the faa or tip, which leads us to suspect that variation in these traits is not due to ontogeny.

Further work needs to be done to understand the validity of these characters and the implications for lifestyle and taxonomy of phytosaurs. If morphology of the postcrania are taxonomically significant, then they could help sort out phytosaur relationships and convergences.