1Evolutionary Anthropology, Duke University, 2Institute for Human Evolution, University of the Witwatersrand
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The acetabulocristal buttress is an indistinct feature in australopiths, but becomes prominent in early and modern Homo. It has been suggested that this thickening is due to increased bending moments in the iliac blade from m. gluteus medius acting in abduction; alternately, it may protect against bending stresses due to increased medial curvature of the anterior iliac blade. Australopithecus sediba has several pelvic features that suggest a modern alternating pelvic tilt mechanism with increased abduction of the gluteal muscles, including more vertically oriented iliac blades and distinct sigmoid curvature. Therefore, one would expect to find a relatively pronounced iliac pillar in Au. sediba.
We examined the differences in the acetabulocristal buttress between hominin groups, measuring the angle, the distance posteriorly from the anterior superior iliac spine, and the thickness of the buttress. Data were collected from original fossils and casts representing Australopithecus afarensis, Au. africanus, Au. robustus, Au. sediba, Homo erectus, H. neanderthalensis, and early Pleistocene African Homo sp. indet.. Measurements were also taken on a sample of 107 modern human skeletons from the Raymond Dart collection at the University of the Witwatersrand.
We found significant differences in measures between australopiths, early Homo, and modern humans; Au. sediba was not significantly different from australopiths. Furthermore, a significant difference was detected between early and modern Homo in both buttress angle and thickness. This suggests that neither increased forces in abduction nor increased medial curvature of the iliac blade are the direct cause of the buttress thickening evident in early Homo.