Functional and phylogenetic constraints in Rhinocerotinae cranio-dental morphology

Hypotheses: After the effect of phylogeny is statistically removed, cranial structures that are employed solely for mastication should covary the most with hypsodonty (high-crowned cheek teeth are termed ‘hypsodont’). Such structures should also be the least phylogenetically constrained. A corollary: structures that are highly influenced by shared ancestry will exhibit greater morphological integration than those that are affected less. Organisms: All extant rhinoceroses and a number of extinct, European, Plio-Pleistocene species. Analytical methods: Using two-dimensional geometric morphometrics, we studied skull shape in the dorsal and lateral views, mandible shape in the lateral view, and the upper tooth row shape in the occlusal view. To reflect feeding habits, we used a surrogate variable, the hypsodonty index. Using phylogenetically independent contrasts and variation partitioning, we separated shape variation into function, phylogeny, and size components. We tested morphological integration with Escoufier’s RV coefficient. Results: The mandible and the upper tooth row have the highest covariance with hypsodonty and the least with phylogeny. Skull morphology shows the reverse; it has the smallest covariance with hypsodonty and the highest with phylogeny. The degree of morphological integration between the upper tooth row and the other structures is relatively low, indicating that the former component is the least phylogenetically constrained. In keeping with our predictions, the cranial region associated with chewing is constrained by function and not as much by phylogeny, whereas others show stronger phylogenetic constraint.