New osteoderm morphotype (Xenarthra, Mylodontidae) from the middle Pleistocene of Argentina

Associated osteoderms of unique aspect have been found in Ensenadan age deposits near San Pedro, in the Pampean Region of Argentina. Their general shape closely resembles that of mylodontid sloths. They are unique in presenting a homogeneous octahedral shape, contrasting with the heterogeneous shape of other ground sloth osteoderms. The lack of expanded spaces within the bone is shared with some other mylodontid sloths. Histologically, the bone is well vascularized, full of fiber bundles (including Sharpey's fibers), and very similar to the condition in Mylodon darwinii. The lack of individualized fibers near and perpendicular to the external surface suggests a relatively deeper position in the dermis than in the Pleistocene Paramylodon' garbanii. The taxon to which these novel osteoderms belonged is uncertain, but the possible carriers are discussed.