Femoral neck and shaft structure in Homo naledi

The abundant femoral assemblage of Homo naledi provides a unique opportunity to test hypotheses regarding the taxonomy, locomotion, and loading patterns in this species. Here we describe neck and shaft cross-sectional structure of several H. naledi femoral specimens and compare them to a broad sample of fossil hominins, recent humans, and apes. Cross-sectional data from the femoral neck (base of neck and midneck) and diaphysis (midshaft and subtrochanteric) were taken from CT scans and data on comparative samples are either our own previously collected samples or data from the literature. Results show that H. naledi femoral neck midsection shape is superoinferiorly elongated and resembles early hominins, but only marginally overlaps with H. sapiens and apes. Cortical distribution around the superior and inferior margins is asymmetrical and falls within the range of H. sapiens (thin superiorly and thick inferiorly) to the exclusion of early South African hominins, but H. naledi is notable for having absolutely thicker cortex (higher %CA) than modern humans. H. naledi femoral shafts are gracile, falling below the range of majority of Pleistocene Homo specimens and overlapping with Pan and Pongo. Homo naledi midshaft diaphyseal shape is approximately round and resembles most other hominins and apes with the exception of Middle Pleistocene and Upper Paleolithic humans. However, H. naledi shows significantly higher relative AP rigidity at the subtrochanteric level than other hominins. As observed from previous analyses, H. naledi appears unquestionably Homo in its femoral anatomy, but unique among hominins in its gracility and combination of characters.