Vault thickness in two Pleistocene Australian crania

This study examines vault thickness in two Pleistocene crania at the centre of discussions about the evolution of Indigenous Australians and competing scenarios about modern human origins: Willandra Lakes Human (WLH) 3 and WLH 50. We estimated total vault thickness and thickness of the diploë, external and internal table on CT-scans, and undertook a quantitative comparison of them. We found that they differ significantly in absolute thickness, but are similar in terms of the relative contribution to thickness made by the diploic space and total table. They also differ significantly in terms of relative variation in thickness, and in several instances, WLH 50 shows greater variability than WLH 3. This finding undermines pathology as an explanation for thickness in the former cranium. Among later hominins, WLH 50 is highly unusual in possessing an extremely thick vault at bregma. We show, however, that vault thickness at this landmark is positively allometrically scaled in Indigenous Australians, but negatively scaled in Ngandong Homo erectus. Thus, vault thickness is mostly explained by the extremely large size of WLH 50: one of the largest hominin crania ever found. Moreover, its extreme thickness firmly establishes its affinities with modern humans rather than the Ngandong group. Despite the important role played by epigenesis in the development of the cranial vault, similarities between WLH 3 and WLH 50 indicate some canalisation in the extent to which the various vault components contribute to total thickness.