Two major goals in physical anthropology are reconstructing phylogenetic relationships among living and extinct taxa and investigating morphological adaptations to diet. The temporomandibular joint (TMJ), as a critical portion of the masticatory apparatus and as the only point at which the mandible directly contacts the cranium, has the potential to provide information regarding dietary adaptations. Furthermore, since characters on and around the TMJ are frequently used in phylogenetic analyses, understanding variation in this morphology is crucial for understanding relationships among taxa. However, few data are currently available regarding how the TMJ varies within primates; furthermore, how variation in this structure reflects differences in evolutionary relationships or masticatory function is unclear. To address this lack of data, this project will assess the phylogenetic and functional significance of the TMJ by testing a series of hypotheses designed to examine a range of factors that may govern TMJ shape, including phylogeny, diet, and size related shape differences among taxa. These hypotheses will be tested using three-dimensional landmark data of the skull and geometric morphometrics, a powerful quantitative method by which differences in shape among individuals and groups can be summarized and compared. This methodology is particularly useful for quantification of the TMJ, the complexity of which has hindered previous analyses. Data will be collected for a wide cross-section of primate species. The congruence between TMJ shape and molecular relationships in primates will be examined, TMJ variation in relation to dietary differences will be assessed via multiple pairwise comparisons between closely related species with distinct diets, and analyses of the influence of body size will be conducted using a phylogenetically broad, interspecific approach.
This study will include the quantification of a poorly understood region of cranial anatomy that is frequently used in studies of fossil and living primates; as a result, there are several benefits to this research. First, this approach can reveal important differences in TMJ shape among taxa that have previously been unidentified. Second, linking this variation to known evolutionary, dietary, or size differences between taxa will provide future researchers with detailed knowledge regarding the functional and evolutionary significance of this region, which is crucial for understanding the evolution of human and primate cranial shape. This research will also have implications for interpreting the evolutionary history and dietary adaptations of extinct primates, because it will contribute to the growing set of three-dimensional data describing primate variation. Finally, the proposed research may have important implications for clinical analyses of the bony response of the TMJ to treatments associated with TMJ dysfunction.
