Assessment of variation in symphyseal morphology among Middle Stone Age hominin mandibles from Klasies River Main Site, South Africa, by diffeomorphic surface mapping

The hominin fossils from the Middle Stone Age (MSA) deposits of Klasies River Main Site have played a significant role in discussions over the emergence of modern human morphology in the Late Pleistocene. While some display manifestly modern characteristics, others evince more archaic traits, leading to differing assessments of the assemblage. Four mandibles with varying degrees of tuber symphyseos development have been the focus of considerable debate. Two likely date to 83-108 ka, one to 89-101 ka, and the oldest to ca. >110-123 ka. Thus, the oldest and youngest may be separated by possibly 23,000 years. Symphyseal expression has been assessed qualitatively in several studies; the sole quantitative evaluation of all four employed a contour guide to record linear measurements in the sagittal plane. Here, we employ diffeomorphic surface mapping (DSM) to contextualize the degree of morphological variation these jaws exhibit in relation to a sample of adult mandibles from Holocene Later Stone Age (LSA) archaeological sites in South Africa spanning ca. 8000 years. DSM was applied to segmented surface volume renderings of high-resolution μ-CT scans of the fossils and CT/μ-CT scans of the LSA jaws. Although differences in chin development among the Klasies fossils exceed those among LSA jaws, their symphyseal variation is comfortably accommodated within the envelope of morphological diversity displayed by the LSA sample. Thus, variation exhibited by these MSA mandibles does not necessarily indicate more than a single population lineage through time.