Evaluation of long bone surface textures as ontogenetic indicators in centrosaurine ceratopsids

The search for criteria for aging non-mammalian fossil vertebrates has preoccupied paleobiologists in recent years. Previous studies of the long bones of pterosaurs and modern and subfossil birds as well as of cranial material of centrosaurine ceratopsid dinosaurs have documented variations in surface textures that seem to be ontogenetically related. In this study, long bones from the centrosaurine ceratopsid genera Centrosaurus, Einiosaurus, and Pachyrhinosaurus are examined to test the hypothesis that changes in bone surface textures and reduction of surface porosity could be correlated with size (and presumably age) classes, as has been previously documented in pterosaurs and birds. The data set includes 141 bones representing all six long bone elements, collected from monodominant centrosaurine bone beds. Bone surface patterns are documented by macroscopic visual examination, and a sequence of five texture classes ordered by decreasing surface porosity is described based on the common distributions of these patterns. Calculations of Spearman's rank correlation coefficients reveal significant correlations between texture class and size. The smallest bones are invariably associated with porous midshaft textures that grade to fibrous and long-grained patterns proximally and distally [Texture Class (TC) 1]. Post-hoc analysis after Kruskal-Wallis ANOVA on ranks confirms that the mean size of TC1 bones is, in most cases, significantly different than the mean size of bones in other texture classes. Results of this study suggest the presence of an ontogenetic surface textural signal in centrosaurine long bones; however, comparison of texture classes with size-independent maturity criteria is needed to clarify further the potential ontogenetic significance of higher texture classes.