Qualitative approaches reveal parallelism of mandibular function in therian carnivores

The evolution of the mandible in mammalian carnivores is influenced by ecological demands that have changed over their phylogenetic history.  We combined geometric morphometrics and biomechanical analysis (including beam analysis and finite element analysis or FEA) to assess the interaction between form and function as the mandible has adapted independently to carnivorous diets in therian clades, including Metatheria, Mesonychia, “Creodonta,” and Carnivoramorpha. Our goal was to determine the relative contributions of mechanical advantage, mandibular force or resistance to bending and torsion, to the evolution of mandibular shape in these groups, and to determine whether they produce differential rates of shape evolution in the horizontal and ascending rami, which respectively are the tooth-bearing and muscle-loading parts of the structure.

We found that the ascending ramus has higher rates of evolution than the horizontal ramus, making it the more rapidly evolving portion of the mandible. Statistical evaluation supports this interpretation as mechanical advantage and resistance to force explain more of the variance in shape than do the beam mechanics estimates that are heavily influenced by the mandibular body.  Regression analysis shows that the evolution of specialized carnivory was associated with stronger mandibles in which mandibular shape changed by shortening and thickening of the mandible, increasing the areas of muscle attachment, and increasing the carnassial blade length. Principal component analysis of mandible shape shows that different clades in Theria have been able to fill out similar specialized carnivorous niches with similar functional metrics despite having different mandibular morphologies.