Contribution to the knowledge of the Sparassocynidae (Mammalia, Metatheria, Didelphoidea), with comments on the age of the Aisol Formation (Neogene), Mendoza Province, Argentina

FORASIEPI AM; GOIN FJ; MARTINELLI AG

JOURNAL OF VERTEBRATE PALEONTOLOGY

Association of Vertebrate Paleontology

Lugar: Lawrence; Año: 2009 vol. 29 p. 1252 - 1263

0272-4634

A new specimen of a sparassocynid didelphoid, consisting of a single skull, is described and analyzed. The specimen comes from the upper levels of the Aisol Formation outcropping in Mendoza Province (Argentina), and constitutes the first marsupial hitherto described for this unit. The entire Aisol Formation was originally regarded as Middle Miocene (Friasian) in age, based on its fossil content. Nonetheless, the discovery of new specimens suggests that at least the upper stratigraphic level, where the marsupial was found, may have deposited during the late Miocene (not older than Huayquerian times). The specimen studied here is identified as belonging to a new genus, Hesperocynus, of the family Sparassocynidae (Didephoidea). A new combination is proposed for the already known species H. dolgopolae (Reig 1958), previously recognized as a species of Thylatheridium Reig 1952. The new specimen from the Aisol Formation is assigned to the hypodigm of this taxon. The genus Hesperocynus encompasses small-sized carnivorous feeders as evidenced by their dental specializations and palate morphology; however, they are clearly more generalized than Sparassocynus, the other genus of the family. Sparassocynids have traditionally been considered as the close relatives of living didelphids. If this hypothesis is correct, then the sparassocynid clade has a long ghost lineage that al least covers more than 10 million years (as evidenced by the oldest known sparassocynid and didelphid remains). Sparassocynids, as well as all other didelphoids, probably radiated after the global cooling event that occurred after the Eocene-Oligocene boundary.