Morphometric analysis of the occipital bone in an adult South African cadaveric sample

Forensic sex estimation is a key component in attempts to identify human remains. The skull is often used for this purpose, but is often damaged or recovered in fragments. Studies of individual cranial bones are hampered by subjectivity in visual assessment and difficulties in the ability to capture complex shape variations, as well as accounting for population-specificity in the expression of sexual dimorphism. The present study employed geometric morphometric analysis to assess morphological variation of the occipital bones of 792 adult South African individuals. Though sexual differences in size were detected, shape dimorphism was not detected until ancestry was simultaneously accounted for. Female occipital bones were less robust but more sloped than those of males. Detected variation between ancestry groups was sufficient to produce approximately 80% classification accuracies for Black and White groups, and 74% for the highly heterogeneous Coloured group. When variation was assessed according to sex and ancestry simultaneously, groups could be distinguished with 74.8% accuracy. Shape variation showed no significant age or secular trend changes, though some weak positive allometric influence was detected in the variation between sexes and between the Coloured group and the other two ancestry groups. This study demonstrated that even isolated bones such as the occipital could be used for sex and ancestry estimation, though population differences must be considered. The geometric morphometric analysis was found to allow objective and sufficiently sensitive detection of variation in the complex occipital shape to allow for accurate distinction of even highly heterogeneous and closely related groups, even when using a limited number of cranial landmarks. Thus, similar analyses of the occipital may be employed in forensic cases where the cranium has been damaged or only the occipital bone is recovered and available for analysis.