Morphometric analysis of evolutionary trends in the ceratopsian postcranial skeleton

Abstract

ABSTRACT The cranial anatomy of ceratopsian (“horned”) dinosaurs is well understood, but the postcranial skeleton has been largely ignored in previous studies of phylogeny, evolution, and function. In order to study morphological differences among ceratopsian postcranial elements and to compare evolutionary patterns, multivariate (Principal Components Analysis) and bivariate methods were used to analyze linear measurement data, and the shape methods Resistant-Fit Theta-Rho Analysis (RFTRA), Least-Squares Theta-Rho Analysis (LSTRA), and Euclidean Distance Matrix Analysis (EDMA) were applied to biological landmark data. Results of the analyses show that size is the primary change through ceratopsian skeleton evolution. Elements display positive allometry, and increasing structural support is evident, especially in the radius and fibula. Phylogenetic distribution of ceratopsian postcrania agrees with the skull material included in recent cladistic analyses. Psittacosaurus elements are in many ways derived relative to those of non-ceratopsid neoceratopsians, and evidence suggests that Psittacosaurus was bipedal, while non-ceratopsid neoceratopsians were not, except maybe for Udanoceratops. With increasing body size, neoceratopsian limbs bowed laterally. The variety of methods allows for unbiased interpretation of results, provides more information than any one method, and provides controls for each other. However, sample sizes are not ideal, and all results should be treated with caution at this time.