Ankle Morphology: Interface of Genetics, Ontogeny and Use
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A central concept in Evolutionary theory is the character trait. It provides a context in which to explore differences and similarities among taxa, both extant and extinct. It is expanded in scope in Evolutionary Developmental theory to functional units with a biological role, "evolutionarily stable configurations." The talo-crural joint is such a configuration, a highly canalized structural unit in primates forming the interface between organism, and foot and substrate. It is a microcosm in which to examine the relationship of shape with environment and function and the interplay of genetics, ontogeny, and use. Geometric Morphometric analysis of landmark data was employed in studying the articular surfaces of the talus in a diverse sample of adult specimens in nine catarrhine taxa. The influence of four factors on talar shape was examined: superfamily, a proxy for phylogeny; size and mass, a proxy for physical attributes; and substrate preference, a proxy for behavior. All significantly affected shape, and substrate preference was unrelated to the others. Appositional articular morphology, the shape of the subchondral bone surfaces of the talo-crural joints in an expanded sample of 12 taxa, showed a significant effect of the four proxies on the tibial and talar components, and substrate preference was weakly related to the other proxies in each. Singular Warp analysis of the cross-covariance matrices of the joints demonstrated sorting of taxa by substrate use and signals of convergent and divergent evolution among hominoids and cercopithecoids in joint shape. The ontogeny of the appositional articular shape was examined using adult and subadult specimens grouped by molar eruption. Singular Warp analysis demonstrated a genetic signal in the subadults, strongest in the slowly maturing African hominoids, and an epigenetic signal across taxa to substrate use in the adults. The talo-crural joint, a highly canalized, modular, and integrated "evolutionarily stable configuration," provides a model for the study of the evolution of shape. The epigenetic signal observed is consistent with plasticity or developmental plasticity in response to the interaction of the joint complex with the environment due to a behavioral effect, substrate use. This dissertation included previously unpublished, co-authored material.