HOGUE, A.S.*; RAVOSA, M.J.: Mandibular Symphyseal Fusion in Mammals: A Test of Competing Hypotheses
The mammalian lineage is replete with instances of convergent acquisition of mandibular symphyseal fusion. While a variety of models have been proposed to explain this phenomenon, three are currently debated in the literature. Two of these, the dorsoventral (DV) shear and wishboning models, suggest that fusion functions to strengthen the symphysis against DV shear of the symphysis and lateral transverse bending of the mandible, respectively. A third model, referred to here as the transverse stiffness model, argues that fusion serves to stiffen the symphysis to facilitate transverse occlusal movements. ANCOVA and a nonparametric ANOVA were used to test mandibular scaling and occlusal orientation predictions from each of these models in a sample of 44 species of selenodont artiodactyls. Consistent with the wishboning and transverse stiffness models, taxa with fused symphyses (camelids) have more horizontally oriented M2 and M2 occlusal wear facets and expanded corpus and symphysis dimensions in the horizontal plane (p<0.05). Contrary to the DV shear model, camelids do not have relatively deeper corpora to resist greater parasagittal bending of the balancing-side corpus. While taxa with ossified symphyses have relatively larger symphysis cross-sectional areas (consistent with the DV shear model), this appears to be the byproduct of an increase in AP symphysis length which serves to counter increased lateral transverse bending of the mandible. A consideration of the biomechanics of mastication further suggests that strength, not stiffness, is the critical factor necessitating symphyseal ossification. These results provide additional evidence that fusion and transverse masticatory loads are functionally linked, further securing the role of wishboning as the primary determinant of symphyseal fusion in mammals.
