Hind Limb Bone Proportions Reveal Unexpected Morphofunctional Diversification in Xenarthrans

Abstract

Divergence in forelimb morphology is often associated with functional habits exhibited within the Xenarthra, ranging from terrestrial-digging in armadillos to arboreal-suspension in sloths. We hypothesized that quantitative differences in hind limb form also will be predictive of the diverse lifestyles observed in this small clade. A total of 26 morphofunctional indices were calculated from 42 raw measurements of bone length/width/depth in a sample of N = 76 skeletal specimens (18 species). Index data for each species were categorized by substrate preference and use and then evaluated using a combination of stepwise Discriminant Function Analysis (DFA) and Principal Component Analysis (PCA) to determine significant osteological correlates (traits) among extant taxa. Additionally, character states of the morphometric data were inferred using a recent hypothesis of xenarthran phylogeny. DFA determined 14 distinct morphofunctional indices relating to femur robustness, hip/ankle/limb mechanical advantage, and foot and claw length as the most discriminating features. PCA clearly separated armadillos from sloths in morphospace based on overall robustness versus gracility, as well as proximal versus distal lengths of skeletal elements (including the claws), whereas these characteristics were intermediate in the hind limbs of anteaters and selected armadillos having either a larger greater trochanter or modified foot/claw proportions. Two-toed and three-toed sloths showed further separation from each other in morphospace primarily driven by proportions of their tibia and hind feet despite evidence of convergence for numerous functional traits. Moreover, the majority of the traits measured had significant phylogenetic signal and several of these indicated clear patterns of convergent and divergent evolution in xenarthrans by evaluation of their tip states. Our assessments expand functional interpretations of xenarthran limb form and identify potentially conserved and secondarily modified traits related to fossoriality across taxa, including in three-toed sloths, demonstrate possible morphological trade-offs between digging and climbing habits, and suggest derived traits adapted for arboreal lifestyle and suspensory function.