Running Over the Same Old Ground: Stegomastodon Never Roamed South America

The diversity of South American proboscideans during the Quaternary has been a subject of discussion for decades. The presence of Cuvieronius hyodon in South America is unquestioned and unanimous; however, the taxonomy of the known second South American proboscidean is still a controversy. Some authors argue that the South American species traditionally referred to Stegomastodon should instead be referred to a distinc genus, namely Notiomastodon, endemic to South America. Others authors, however, do not accept this attribution and continue to recognize the genus Stegomastodon as present in South America. In this study, we recognize several differences in the mandible, skull, dentition, and postcranial morphology of North American species of Stegomastodon and Notiomastodon, that further support the validity of Notiomastodon as a taxon separate from Stegomastodon. Indeed, a phylogenetic hypothesis of trilophodont bunodont proboscideans supports the separation between Stegomastodon and Notiomastodon, and the diversification of the common ancestor of these proboscideans may have occurred during the middle to late Miocene. No specimen with Stegomastodon diagnostic features is recognized in South America. The Stegomastodon records are restricted to the Pliocene-middle Pleistocene of North America, while Notiomastodon records are found only from the middle Pleistocene-early Holocene of South America. In this way, we recognize that Stegomastodon records are restricted to North America and that only Cuvieronius and Notiomastodon are recorded in South America.