New Small Bunodont Metatherian from the Late Eocene of the Argentinean Puna

We describe here a new small bunodont metatherian from Priabonian deposits of the Geste Formation exposed in the Puna of Catamarca Province (Argentina). The material, an isolated right M2?, was recovered by screen-washing techniques and allows us to erect the new species Apeirodon sorianoi . The combination of diagnostic characters includes triangular occlusal pattern, bunoid, with an equally developed stylocone and cusp D, absence of cusp C, protoconal base wide, preparacrista and postmetacrista short, well-developed paracingulum, presence of double rank postvallum-prevallid shear, and presence of small paraconule and larger metaconule that is distolingually located. The overall morphology of Apeirodon more closely resembles Glasbius (particularly G. twitchelli ) than any other Cretaceous-Paleogene American metatherian, supporting the largely accepted hypothesis proposing a common origin for some North and South American metatherian lineages. Apeirodon also resembles Reigia punae (another small metatherian recovered from the same stratigraphic levels), Chulpasia , Palangania , and specimens referred to Bobbschaefferia . Tentatively, we propose the inclusion of Apeirodon and Reigia as early divergent members of Polydolopimorphia based on the presence of bunodonty, enlarged stylocone and cusp D, and a distolingually-shifted metaconule. These features, incipiently developed in these late Eocene taxa, are emphasized in later representatives of the order. The presence of a generalized putative polydolopimorphian in Priabonian deposits of northwestern Argentina highlights the key temporal and spatial position of the region in the context of the evolution of Paleogene South American mammals.