Patterns of covariation in the masticatory apparatus of hystricognathous rodents: implications for evolution and diversification.

The mammalian masticatory apparatus is a highly plastic region of the skull. In this study, a quantification of shape variation, the separation of phylogeny from ecology in the genesis of shape brings new insights on the relationships between morphological changes in the cranium, mandible, and muscle architecture. Our study focuses on the Ctenohystrica, a clade that is remarkably diverse and exemplifies a rich evolutionary history in the Old and New World. Current and past rodent diversity brings out the limitations of the qualitative descriptive approach and highlights the need for using integrative quantitative methods. We present here the first descriptive comparison of the whole masticatory apparatus within the Ctenohystrica, by combining geometric morphometric approaches with a noninvasive method of dissection in 3D, iodine-enhanced microcomputed tomography. We used these methods to explore the patterns of covariation between the cranium and the mandible, and the interspecific morphological variation of the skull with regard to several factors such as phylogeny, activity period, type of habitat, and diet. Our study revealed strong phylogenetic and ecological imprints on the morphological traits associated with masticatory mechanics. We showed that, despite a high diversification of lineages, the evolutionary history of Ctenohystrica comprises only a small number of morphotypes for the skull and mandible. The position of the eye was suggested as a key factor determining morphological evolution of the masticatory apparatus by limiting the number of possible pathways and promoting convergent evolution toward new habitats and diets between different clades.