EXAMINING BEHAVIOR AND LEVELS OF ARBOREALITY IN EXTINCT SLOTHS THROUGH SCAPULA MORPHOLOGY

Extinct sloths have been commonly referred to as “ground sloths,” despite that several groups were much smaller and possibly less terrestrial than the archetypal giant ground sloths of the Miocene, Pliocene, and Pleistocene. Varying levels of arboreality have been demonstrated in these smaller intermediate sloths, many of which were from the Caribbean, by comparing limb measurements which have been shown to distinguish arboreal and terrestrial primates, as well as modern arboreal and terrestrial anteaters, the sister taxon to sloths. A geometric morphometric study was performed to determine if these patterns of arboreality could also be parsed out through scapular morphology. 3D landmarks and sliding semi-landmarks were measured on the scapulae of extant arboreal sloths and extinct sloths, and extant anteaters. A principal component analysis was performed, as well as statistical analyses of each genera’s position in morphospace. To assess the effects of phylogeny, a phylogenetic analysis was performed using a combination of previously published matrices and the result was used to estimate ancestral shapes for the sloths. The three modern anteater genera were easily differentiated by scapula shape. Extinct intermediate sloths are closest in morphospace to modern arboreal sloths, and more distant from terrestrial giant ground sloths than modern sloths are. The three groups of sloths did not have significantly different morphological disparities, possibly due to their low metabolic activity rate. There was no significant phylogenetic signal found in the data, and the estimated ancestral shapes show that the ancestral sloth may have been similar to Bradypus in morphology. These results show that the Caribbean islands hosted a group of sloths that was functionally different from most of the sloths from the American continents, and that better understanding of their morphology is critical to understanding the evolution of modern tree sloths.