Indicators of locomotor habits in xenarthrans: Evidence for locomotor heterogeneity among fossil sloths

ABSTRACT

A striking diversity in limb morphology among fossil sloths suggests that the traditional distinction between extant “tree” sloths and fossil “ground” sloths merits closer scrutiny. Morphologically, xenarthrans (sloths, anteaters, and armadillos) are divergent from other mammalian orders. However, morphological shape indices of aspects of the elbow, hip, and knee joints that clearly discriminate locomotor modes in primates, carnivores, and bovids also succeed in differentiating the fully arboreal, semiarboreal, and terrestrial genera of anteaters. When compared to this extant data base, fossil sloths from both the Pleistocene radiation of the Antilles and the Santacrucian radiation of Argentina exhibit a range of values for these indices comparable to those of primates and anteaters. These results indicate that the diversity of limb structure in fossil sloths is associated with variation in posture and locomotor habits including arboreality, semiarboreality, and terrestriality.