WILLIAMS, SH*; VINYARD, CJ; WALL, CE; HYLANDER, WL; Ohio University; NEOUCOM; Duke University; Duke University: Experimental observations on symphyseal fusion in selenodont artiodactyls

In most mammals, the mandibular symphysis is a mobile joint in which the two dentaries are united by ligaments, fibrocartilage and interdigitating bony rugosities. In some mammalian clades, however, the symphysis ossifies early during ontogeny prior to or around the time of weaning. Because of the timing of symphyseal fusion, fusion is thought to be integral for resisting forces generated during repetitive mastication. Some studies further emphasize the influence that transverse forces, particularly those generated by the jaw muscles, may have on symphyseal morphology and fusion. Here, we test the hypothesis that alpacas (Lama pacos), which fuse their symphyses during early ontogeny, have jaw-muscle activity and symphyseal bone strain patterns indicative of increased transverse masticatory loads. Electromyographic data from the jaw muscles show that alpacas have a balancing-side deep masseter that is recruited late relative to other jaw muscles. Given that this muscle has a large transverse component to its force and it acts partially independent of the other jaw muscles, these data are congruent with the hypothesized link between symphyseal fusion and transverse masticatory force. However, symphyseal strains associated with balancing-side deep masseter activity appear to be small. The laterally-directed force from the delayed activity of the balancing-side deep masseter should result in lateral transverse bending of the symphysis, yet peak symphyseal strains in alpacas are most congruent with twisting of the symphysis about a transverse axis. Thus, there appears to be no evidence for a link between transverse masticatory loads and symphyseal fusion in alpacas.