EVOLUTION OF THE BOVID SKULL: A 3D MORPHOMETRIC APPROACH

Bovidae (antelopes and relatives) comprises the most diverse family of extant terrestrial large mammals. With over 130 living species spread across four continents, bovids rapidly came to dominate terrestrial fossil faunas across Eurasia and Africa after their first appearance in the Early Miocene. Extant bovid dental morphology and taxonomic attribution is strongly associated with particular vegetational habitats, and these relationships have been used extensively for paleoecological reconstruction. However, an evolutionary and phylogenetic analysis of cranial shape in relation to environmental variables has been lacking. Here we performed 3D geometric morphometric analyses on 129 crania representing 86 extant and 10 fossil bovid species in order to quantify and assess the evolutionary development of and potential drivers behind bovid cranial shape. Given the broad ecological range of extant species, we expected that bovid cranial morphology would be widely distributed in morphospace, with low overlap among ecologically-differentiated subclades. Furthermore, we expected that cranial modularity might have played an important role in the generation of the diversity of cranial shapes observed in bovids today.

Our results confirm previous expectations that phylogeny, size allometry, and diet are significantly correlated with cranial morphology. Size and facial elongation dominate PC1, while diet and habitat best explain PC2. Morphological disparity is also correlated with clade age and species richness. Surprisingly, however, we find that bovid crania are highly clustered in morphospace, with a large number of species occupying a very similar range of morphology, and with little unoccupied morphospace. Convergence is quite high, and morphological evolution was largely diffusive (ie Brownian), with the extant mean clade morphology located close to the ancestral mean. The bovid cranium additionally appears to be highly integrated, and effectively non-modular, without significant differences among subclades. These results indicate that bovid cranial evolution was largely gradualistic and that only relatively small modification of the ancestral bovid cranial form were needed to produce large ecological differences. The inclusion of fossils does not significantly alter the total morphospace, but provides increased confidence in the evolutionary reconstructions, including tempo and mode of change, and the placement and reconstruction of hypothetical common ancestors.