Femoral head trabecular bone structure in two omomyid primates

The study of the three-dimensional structure of trabecular bone and its relationship to locomotor behavioral differences across different primate taxa provides a potentially useful analytic tool for reconstructing the behavior of extinct taxa. The purpose of the current study is to quantify the three-dimensional architecture of trabecular bone in the femoral head of Omomys carteri and Shoshonius cooperi and to compare this structure to that of several extant strepsirrhine taxa. Bone volume fraction (BV/TV) and fabric anisotropy were quantified in three dimensions using serial high-resolution X-ray computed tomography scan data collected from one femoral head from each fossil taxon. Three cubic volumes of interest (VOI) were identified within the femoral head. The BV/TV was quantified by assessing the percentage of bone voxels within each VOI and the structural anisotropy was quantified using the star volume distribution method. The Omomys femur used here has a high BV/TV with the galagine-like pattern of decreasing BV/TV from the superior to the inferior half of the femoral head. The fabric structure, however, is more lorisine-like in being relatively isotropic throughout the femoral head. The trabecular structure in Omomys is unique in its mix of features and appears to be most similar overall to the lorisines, suggesting that Omomys engaged in a quadrupedal mode of locomotion. By contrast the Shoshonius specimen possesses a relatively uniform BV/TV across the head but displays the distinctly galagine-like pattern of increasing anisotropy moving inferiorly in the femoral head. Taken as a whole, the trabecular structure in Shoshonius appears to be most like that of the galagines and is consistent with that of either an occasional leaper-quadruped or a specialized leaper. Despite the overall similarities in the external postcranial anatomy of Omomys and Shoshonius, the results of this study indicate potentially important differences in the magnitude and orientation of the external loads at the hip joint, suggesting that these animals engaged in divergent locomotor behaviors.