1Department of Community and Family Medicine, Duke University, 2Department of Biology, Bronx Community College, City University of New York, 3School of Human Evolution and Social Change, Institute of Human Origins, Arizona State University
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Considerable variation exists in mandibular ascending ramus form among primates, particularly great apes. Recent analyses of adult ramus morphology by Rak and colleagues have suggested that ascending ramus form, especially the coronoid process and sigmoid notch, can be treated as a phylogenetic character that can be used to reconstruct relationships among great ape and fossil hominin taxa. However, it is currently unknown how ontogeny of the ramus contributes to adult variation. Specifically, it is unclear whether differences among adults appear early and are maintained throughout ontogeny, or if these differences appear, or are enhanced, during development.
To address these questions, the present study examined a broad ontogenetic sample of great apes and humans (Gorilla, Pan, Pongo, and Homo sapiens) using two-dimensional sliding semi-landmark analysis. Variation among and within species was summarized using principal component analysis, and Procrustes distances and discriminant function analyses were used to compare species and age classes. Results suggest that morphological differences among species in ramus morphology appear early in ontogeny and persist into adulthood. Morphological differences among adults are particularly pronounced in the height and angulation of the coronoid process, the depth and anteroposterior length of the sigmoid notch, and the inclination of the ramus. In all taxa, the ascending ramus of the youngest specimens is more posteriorly inclined in relation to the occlusal plane, shifting to become more upright in the adult. The implications of these results for analyses of hominin phylogeny and the taxonomic assignment of hominin sub-adult mandibular remains are discussed.