Constraints on masticatory system evolution in anthropoid primates

It is well established that some observed patterns of force production in t he primate masticatory system match those predicted by a simplified lever m odel. This model is also commonly invoked in adaptive explanations of crani odental diversity. However, systematic studies of the predictive power of t his model are missing, leaving open the possibility that factors not tradit ionally included in the model alter the function and evolution of the masti catory system. One such factor was proposed for mammals generally by Greave s ([1978] J. Zool. (Lond.) 184:271-285), who argued that the temporomandibu lar joint (TMJ) was poorly suited to being pulled apart. In this constraine d lever model, the avoidance of joint distraction leads to limitations on m asticatory system form and function. The goal of the present study was to q uantify masticatory system diversity in anthropoid primates for comparison with these predictions. Results indicate that all sampled taxa exhibit a form that is consistent wi th selection against regular distraction of the TMJ. Also apparent from obs erved patterns of scaling is a regular interaction among a limited set of c ranial and dental dimensions, in accordance with the constrained model. How ever, the data indicate that specific positional relationships among the mu scles, joints, and teeth differ from those predicted by Greaves (1978). The pattern of deviation suggests that selection has favored a conservative ma sticatory system configuration that safeguards the TMJ from distraction dur ing the dynamic processing of irregular foods. The resulting buffered model leads to alternative hypotheses regarding the response of the masticatory system to dietary selection pressures, It may, therefore, improve our under standing of the adaptive significance of primate craniofacial form. (C) 199 9 Wiley-Liss, Inc.