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.