Thoracic member (pectoral girdle and forelimb) bones of Mylodon darwinii Owen (Xenarthra, Mylodontidae) from the Late Pleistocene of Central Argentina and their phylogenetic implications

A nearly complete skeleton, including most of the thoracic member bones of the sloth Mylodon darwinii , have been found in Upper Pleistocene strata from Anisacate River, Argentina. The thoracic member bones resemble their homologues in Glossotherium robustum , Paramylodon harlani , and Mylodonopsis ibseni in the following traits: (1) the olecranon is mediolaterally compressed; (2) the radius has an acute styloid process; (3) the radial diaphysis medial border is straight for two thirds of its length; (4) the radial shaft medial border forms an angle with the medial border of the styloid process. The radius presents a distinctive, mostly proximally facing articular circumference. The unfused epiphyses and feeble muscle attachment ridges indicate a sub-adult ontogenetic stage. Deviation of the olecranon and weak M. teres major origin and insertion, suggest a low fossorial specialization. The structure of the thoracic limb bones does not support climbing habits, because pronation-related features are reduced and the humeral head is not prominent. The structure of the radius suggests graviportal adaptations: the proximal head is mediolaterally expanded and the diaphysis straight. A phylogenetic analysis adding thoracic member characters recovers M . darwinii as part of a clade that includes Glossotherium robustum and Paramylodon harlani , but excludes Lestodon armatus . This contrasts with the results of previous analyses focusing on the head skeleton, highlighting the relevance of sampling postcranial characters in phylogenetic analyses of mylodontine sloths.