Cranial osteology of the basal megatherioid sloth Schismotherium (Mammalia, Xenarthra) and its taxonomic implications

Abstract

ABSTRACT This study comprises a comparative description of the skull anatomy in the basal megatherioid sloth Schismotherium fractum, based on specimens from the Santa Cruz Formation (late early Miocene; Santacrucian SALMA) in southern Argentine Patagonia. We provide more detailed descriptions and illustrations of the skull bones and their sutural connections, as well as the cranial foramina and their contents, than have previously been available. The anatomy in S. fractum is compared to contemporaneous basal megatherioids from Santa Cruz, Hapalops elongatus and Pelecyodon cristatus. The latter is a close relative of S. fractum in several phylogenetic analyses, but given the high levels of anatomical variation and other taxonomic uncertainties that surround these sloths, previous authors have suggested that the two may not represent distinct taxa. Our observations demonstrate that although the skulls of S. fractum do indeed exhibit variation in a broad variety of features, there are invariant, apomorphic features that consistently distinguish Schismotherium and Pelecyodon from one another, including at least 17 features derived from virtually every region of the skull. While supporting the current taxonomic status of these important, basal-most megatherioids, the present study highlights the need for further anatomical and phylogenetic investigations of Santacrucian sloths.