Acetabular Orientation in Hominins and Other Anthropoids

Abstract

Hominin pelvic form differs dramatically from that of other primates by having more laterally facing iliac blades, a wider sacrum and a larger, transversely broad pelvic inlet. Acetabular orientation may differ as well, plausibly related to differences in load transmission during habitual bipedal posture and locomotion. Here, we test the hypothesis that hominin acetabular orientation differs from that of other anthropoids using two approaches. First, we performed a 3D geometric morphometric analysis of landmarks collected on the acetabula of 220 specimens representing 30 extant taxa, humans, A. afarensis (AL 288-1), and a Neandethal (Kebara 2). Second, we measured the 3D orientation of the acetabulum on in silico models of individual hipbones that were aligned to the median plane by registering models to landmark coordinates that had been collected on articulated pelves. We fit a plane to the acetabular rim and measured its orientation relative to the median plane, a “transverse” plane fit to the lumbosacral joint, and a “coronal” plane orthogonal to the median and lumbosacral planes. Angles between the acetabular plane and reference planes are 3D angles, measured as the angle between the normal vectors of the acetabular and selected reference plane in the plane containing both normal vectors. The sample for this analysis included 77 specimens from 22 extant anthropoid taxa, humans, Kebara 2, A. afarensis (AL 288-1), and A. sediba (MH 2). Results of the geometric morphometric analysis do demonstrate differences between hominins and non-hominins, but results are confounded by variation in acetabular size. Analysis of angular relationships between the acetabulum and reference planes, though, support the hypothesis that hominin acetabula indeed are more anteriorly and inferiorly oriented than those of other anthropoids. Other taxa are generally similar, although African apes exhibit slightly more anteriorly oriented acetabula than other anthropoids. Results suggest that the orientation of the acetabulum is a key component in the suite of pelvic characteristics related to habitual bipedality in hominins and should be considered in future analyses of hominin pelvic morphology.