HOW WELL DOES A PART REPRESENT THE WHOLE? A COMPARISON OF CRANIDIAL SHAPE EVOLUTION WITH WHOLE-ORGANISM EVOLUTION IN THE TRILOBITE FAMILY PTEROCEPHALIIDAE

For taphonomic and practical reasons, our understanding of morphological evolution within and among species is based primarily on measurements taken from one or a few morphological traits. However, patterns can be highly dependent on trait choice, making it difficult to draw conclusions about evolution of species or clades as a whole. Here I test whether patterns of evolutionary change in the shape of a part are coincident with patterns of evolutionary change based on a more comprehensive description of the organism. The former is based on landmark data of the trilobite cranidium and the latter on discrete characters describing the exoskeleton, collected from species belonging to the Cambrian family Pterocephaliidae. Using these two datasets, I compare the amount of change occurring along phylogenetic branches, as well as changes in morphospace occupation and changes in different measures of disparity through time. Results suggest that cranidial shape is a fairly robust proxy for species-level disparity and evolutionary change in this family of trilobites, but minor discrepancies do lead to a fuller understanding of evolution within the clade. Although the specific result of this study will be directly relevant for other studies of trilobite evolution, the methodological strategy I use for making this determination may be applied to any system.