Timothy M. Ryan* Department of Anthropology, The University of Texas at Austin, Austin, Texas 78712, U.S.A. E-mail: tmryan@duke.edu Richard A. Ketcham Department of Geological Sciences, The University of Texas at Austin, Austin, Texas 78712, U.S.A. E-mail: ketcham@mail.utexas.edu Received 12 December 2001 Revision received 14 May 2002 and accepted 15 May 2002 Keywords: trabecular bone, high-resolution X-ray computed tomography, Omomys, Shoshonius. 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 recon- structing 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.  2002 Elsevier Science Ltd. All rights reserved. Journal of Human Evolution (2002) 43, 241–263 doi:10.1006/jhev.2002.0575 Available online at http://www.idealibrary.com on Introduction Reconstruction of the locomotor behavior of extinct taxa is an important endeavor with implications for locomotor evolution, habitat utilization, diet, and phylogenetic relationships. Locomotor behavioral recon- structions, however, are often hampered by scant and fragmentary fossil evidence, inap- propriate modern analogs, and the inability to determine with certainty the significance of unique traits that may hold critical functional importance. In addition, the irre- placeable nature of fossils precludes any destructive investigation of the internal morphology, thereby limiting analyses to external anatomical features alone. It is *Current address: Division of Fossil Primates, Duke University Primate Center, 1013 Broad Street, Durham, North Carolina 27705, U.S.A. 0047–2484/02/080241+23$35.00/0  2002 Elsevier Science Ltd. All rights reserved.