ON THE POST-CRANIAL ANATOMY OF MYLODON DARWINII, AND IMPLICATIONS FOR THE FUNCTIONAL MORPHOLOGY OF THE FORELIMB

The South American ground sloth Mylodon darwinii (Mammalia: Pilosa: Mylodontidae) is best known and distinguished from other mylodontid sloths from cranial features such as having a reduced dental formula (4/4), a long and narrow skull, and the very unique retention/ossification of nasal cartilage into a bony arch. This taxon has a long historical record, dating back to the mid-1800s, but for nearly a century it was wrapped up in disagreements over nomenclature with other Pleistocene mylodontids (e.g. Glossotherium, Lestodon, Paramylodon), which along with a collection bias for cranial materials left the post-crania as relatively unknown. Discovery in the collections of the Field Museum of Natural History of a forgotten skull with associated post-crania belonging to a young adult of Mylodon darwinii from Buenos Aires, Argentina provides an opportunity to establish and compare taxon-specific characters for this genus with those of other contemporary mylodontids. While the specimen is far from complete, there is enough material present to put forth novel characterizations for the atlas and for the left ulna, radius, carpals, metacarpals, and astragalus, which successfully distinguishes them from other mylodontids beyond the historical standby of size. The nearly complete left forelimb also permits a preliminary study of its functional morphology and provides a basis for inferring the potential habits of Mylodon during its life. Overall, the successful identification of these elements opens the door for recognizing additional post-cranial elements of Mylodon and will further aid in our understanding of this extinct giant.