Femoral neck anteversion in Xenarthra

Abstract

Degree of femoral neck anteversion distinguishes extinct sloth genera from each other, particularly when analyzed in combination with femoral neck angle. These data have been previously analyzed using a comparative sample including only extant sloths, which limits functional interpretations. Here, we analyze both femoral neck anteversion and femoral neck angle broadly across Xenarthrans to reconstruct the locomotor behavior of extinct Caribbean sloths.Femora of four extinct sloth genera (n=49; Acratocnus; Megalocnus; Neocnus; Parocnus) and nine extant Xenarthra genera (n=63; Bradypus; Choloepus; Cyclopes; Dasypus; Euphractus; Myrmecophaga; Pridontes; Tamandua; Tolypeutes) were surface scanned. Morphological axes representing the femoral neck, shaft, geometric axis of the femoral condyles (GA), and maximum dimension of the femoral shaft cross section at 30% of whole bone length were extracted. Degree of femoral neck anteversion was calculated relative to both the GA and cross section axes. Femoral neck angle was additionally calculated as the angle between the femoral neck and shaft axes. Non‐parametric MANOVAs with post hoc multiple comparison tests were used to test how the genera differed when the angle of femoral neck anteversion and femoral neck angle were analyzed in conjunction.Parocnusdoes not significantly differ from any of the armadillo taxa when femoral neck angle is analyzed with either of the two anteversion angles (p>0.171 for all pairwise comparisons), supporting previous suggestions that Parocnuswas largely terrestrial. This finding also indicates that Parocnus may have had similar hindlimb biomechanics to armadillos, and possibly utilized digging behaviors. Megalocnusand Acratocnusdiffer significantly from all extant Xenarthrans (p<0.043 for all pairwise comparisons), but do not exhibit significantly different morphologies from each other (p=0.911). This suggests that the two genera may have utilized similar hip mechanics; however, their locomotor repertoires cannot be accommodated within the range of those exhibited by extant Xenarthrans.