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SEXUAL DIMORPHISM AND PHYLOGENY OF THE MANDIBULAR NOTCH IN HUMANS, APES AND FOSSIL HOMININS.

Abstract

The mandibular notch, located superiorly on the ascending ramus, extends from the coronoid process to the mandibular condyle. Given the dimorphism of the inferior cranial vault and mandible, we investigated the potential to differentiate human females and males (n = 14) using solely the shape of the mandibular notch. To determine if the differences between the human sexes can be generalized to hominids, we also included female and male Gorilla, Pan and Australopithecus. We chose elliptical Fourier analysis to capture the shape of the mandibular notch. Photographic images for each individual were rotated to standardize the position of the ascending ramus. The mandibular notch was then traced and binarized, and input into SHAPE v.2 where differences between the samples and a standard ellipse were recorded as amplitudes of the harmonics and reduced to principal components scores. The first two PC vectors, accounting for 75% of variation, showed the convex hulls surrounding female and male human groupings are nearly separate. The convex hull representing human males overlaps slightly with gorillas while the fossil hominin grouping is completely distinct with the exception of AL 288-1 falling close to chimpanzees. When the first 7 PC scores, representing 95.6% of the variance, are classified using canonical scores analysis, human females and males are entirely separated from one another on CS1, and both are distinct from the apes and fossil hominins, which are polarized on CS2. The apes have different patterns of sexual dimorphism of the mandibular notch than humans, and the sex of fossil hominins appears to correspond to humans in some aspects and apes in others. These results demonstrate the utility of using mandibular notch shape in human sex estimation and suggest the morphology of the superior ramus has the potential to be used to infer sex membership in the hominin fossil record.

Acknowledgements

Department of Anthropology, Georgia State University and Western Carolina Univeristy.

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