An examination of dental morphology and variation in theropod dinosaurs: Implications for the identification of shed teeth
Although isolated theropod dinosaur teeth occur frequently in Late Mesozoic rocks, an understanding of their taxonomic utility remains elusive. This neglect of theropod tooth data is based on an impression that it is difficult or impossible to discriminate these elements. However, while theropod crowns are simple structures possessing few homologous landmarks from which measurements concordant with modern morphometrics can be made, theropod dental anatomy is largely unexplored and opinions regarding theropod tooth usefulness are premature. An attempt was made to devise a methodology by which isolated teeth could be confidently assigned to or discriminated from a standard dataset obtained from the in situ dentitions of a number of well established taxa (e.g., Tyrannosaurus, Deinonychus, Gorgosaurus). The standard was established using a number of revised and newly derived variables that describe total crown length, base length and width, base shape, “squatness,” and apex location, as well as those that examine serration size. Crown recurvature was described by collecting X, Y data from digitized images of the mesial and distal crown profiles. The square root function Y = a + bX0.5 was fit to the data after they were translated to 0,0 and normalized to 1 (by dividing the data by the range of x values). The b value generated from this nonlinear regression provides a measure of crown face curvature and can be isolated for use in additional analyses. These methods were combined with qualitative description to produce the first detailed study of the dentition of a single theropod, Tyrannosaurus rex, to assess the feasibility of obtaining valid systematic characters from theropod dentitions. The variables developed were then used to analyze a number of isolated theropod crowns. Using discriminant function analysis, isolated tooth test cases of presumed tyrannosaurid and non-tyrannosaurid affinities were compared against the standard. Using these methods, cf. Tyrannosaurus teeth were correlated with Tyrannosaurus and teeth thought not to be Tyrannosaurus were separated from T. rex with statistical confidence. These results suggest that increasing the number of taxa in the standard should permit confident identification of numerous types of isolated crowns.