Meeting Abstract
The transition from pre-mammalian cynodonts to true mammals involved the reduction of the post-dentary bones, changes in the site of the jaw articulation, and changes in the orientation and magnitude of the adductor musculature of the jaw. These changes are hypothesized to have occurred under distinct selective pressures. On the one hand, the auditory apparatus is hypothesized to require low joint reaction forces, while on the other hand, the feeding system is hypothesized to favor high bite forces. Crompton and Hylander (1986) employed 2D free body analysis to explore the relationship between joint reaction forces and bite force in the pre-mammalian cynodont, Probainognathus. Here we build on this landmark analysis using 3D free body analysis to evaluate the effect of changes in musculoskeletal configuration of early mammals, in a manner that mimics the broader evolutionary trends in mammalian fossil record. We show that low joint reaction forces and a high bite force can be achieved over a continuum of musculoskeletal configurations. We identify three key variables influencing the relationship between joint reaction forces and bite force: 1) the reorientation of the resultant adductor force in the anterior direction, 2) changes in the ratio of the tooth position to the position of the jaw joint, and 3) the elevation of the jaw articulation above the occlusal plane. This model predicts that the musculoskeletal configuration of the cynodont lower jaw can be evolutionarily labile without negatively impacting the performance of the auditory and feeding system, and it supports the hypothesis that the orientation of the adductor force and the position of the jaw articulation are functionally correlated on a broad evolutionary scale.
