DOCTORAL DISSERTATION RESEARCH: Symphyseal Fusion in Ungulates and Anthropoids: A Case of Functional Convergence?

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In the last three decades anthropologists have been among the most active researchers on the mammalian masticatory apparatus. In addition to numerous comparative morphological studies, physical anthropologists have contributed significantly to the growing literature on in vivo masticatory function. As a result, much of what is known about the mammalian cranial form is based on primate models. These studies have revealed a number of interesting patterns regarding the evolution of primate craniofacial form. In particular, research suggests that jaw-muscle function has played a significant role in the evolution of primate masticatory form. During the early development of anthropoid primates (i.e., monkeys, apes and humans), the mandibular symphysis (the joint between the two halves of the mandible) fuses or becomes completely ossified. However, like most mammals, the earliest true primates (called strepsirrhines or prosimians) retain the primitive condition of a highly mobile, unfused symphysis. Interestingly, recent work shows that jaw adductor recruitment patterns during mastication differ between anthropoids and strepsirrhines with respect to the timing and magnitude of recruitment of transversely oriented force from the balancing-side (i.e., non-chewing side) muscles. Of particular interest is the activity of the balancing-side deep masseter muscle, which has a large transverse component to its force. Compared to strepsirrhines, in anthropoids this muscle is recruited late relative to the other jaw adductors and the magnitude of force from this muscle is increased. This pattern of deep masseter activity causes the two halves of the mandible to be pulled apart resulting in significant stress and strain in the symphysis. Thus, there is evidence that jaw muscle firing patterns, symphyseal loading and the evolution of symphyseal fusion are linked in anthropoids. The research on goats, alpacas and horses outlined in this study will contribute to the growing literature on the mammalian masticatory apparatus from an in vivo perspective. As little is known about masticatory function in ungulates, particularly in alpacas and horses, the electromyographic, strain and kinematic data collected here will expand our understanding of cranial form and function in two additional mammalian orders. More importantly, as alpacas and horses also have fused symphyses, this study will shed light on the evolution of symphyseal fusion in non-primates. To this end, these data will aid in identifying convergent trends in the evolution of masticatory form in primates and ungulates. Specifically, this research will clarify whether there are similar functional constraints on the symphysis in these animals despite differences in facial morphology. In addition to the scientific contributions outlined above, the broader impacts of this research include the participation and graduate training of a minority and female Co-PI. As much of the data collected in this study will be pertinent to future research, funding for this project will be integral in advancing the scientific career of this Co-PI.