TEASING OUT MORPHOLOGICAL VARIATION IN THE ILIA AND FEMORA OF PHYTOSAURS

Phytosaurs are an extinct group of archosauriform reptiles found in Triassic rocks. They have been the focus of many taxonomic and phylogenetic analyses over the years; however, most of the analyses have relied only on craniodental characters, ignoring the postcrania. The purpose of this study is to analyze if there is detectable morphological variation in the femora and ilia of phytosaurs that could be used in taxonomy, phylogeny, and paleoecology. We approached this question with two different methodologies: character identification and geometric morphometric analysis.

ImageJ was used for applying the seven characters developed by Devore, et al. (2020) to analyze twenty-four different ilia specimens and for the seven femur characters developed by Grimes and McLain (2019) to analyze twenty-eight femora.

Utilizing geometric morphometrics, variability of the ilia and femora were measured to determine if this data could ascertain distinct groupings of species. Points and curvature landmarked unique characters of the bones. TPSDig2 measured landmarking and R Studio variability. For twenty-seven ilia specimens, nine points were plotted, such as shape of anterior iliac blade, deflection of the posterior iliac blade, and the presence or absence of the ventral pubic process. The curvature of the iliac blade and posterior iliac notch were also measured. The process was repeated employing ten points for twenty-four femora, including the length of the femur and its fourth trochanter, and width of the femur head and midshaft viewed from a posteromedial angle. The curvature of the shaft and head/neck were also measured.

We did detect differences when comparing ilia and femora from various phytosaur taxa while using both morphological characters and geometric morphometric analysis. For the geometric morphometric analysis, we used two classifiers (reported species identification and whether the specimen was a right or left element). Our preliminary results suggest at least two clusters visible in principal components analysis plots for both ilia and femora. More work needs to be done to tease out how postcranial data can enhance our understanding of phytosaur taxonomy, phylogeny, and paleoecology.