Elliptic Fourier analysis as a tool for the taxonomic identification of isolated theropod pedal phalanges

Studies of Upper Cretaceous deposits in North America have provided invaluable insights into the continental ecosystems of this time. Theropod (Saurischia, Dinosauria) pedal phalanges are commonplace in these deposits but can be difficult to identify at a finer taxonomic resolution. This, in part, results from a lack of data regarding the individual, ontogenetic, and intraspecific variation that exists among theropod pedal phalanges, and potential differences in phalanx shape that distinguish individual families and species. In this study, we use elliptic Fourier analysis to quantify pedal phalanx shape in a sample of individuals of known species from multiple theropod families in multiple views. The resulting variables were analyzed using principal component analysis to identify patterns of (dis)similarity among different theropod taxa and between phalanx positions and, as such, provide insights into the phalanx shape variation that existed among North American theropods during the Late Cretaceous. This study finds that caenagnathids often possess more gracile pedal phalanges relative to other theropod taxa, particularly when viewed dorsally or ventrally (e.g., for digits II, III, and IV). Although tyrannosaurid and ornithomimid pedal phalanges are generally similar to one another, even in similarly sized individuals the former are found to be more robust in shape than the latter for certain phalanges (e.g., II-1, III-2, and III-3) in dorsal and ventral views. In some instances, the shape of the lateral, medial, and distal margins of tyrannosaurid pedal phalanges differ considerably from those of ornithomimids (e.g., IV-1 in dorsal/ventral view; II-1, III-4, IV-5 in lateral/medial view). The use of elliptic Fourier analysis here provides quantitative data on the variation present among Late Cretaceous theropod pedal phalanges and provides evidence that theropod taxa can be identified down to the family level based on pedal phalanx morphology. This greatly increases the potential utility of isolated theropod phalanges in biodiversity assessments of Cretaceous fossil assemblages, particularly those with a lack of well-preserved skeletal samples.