A NEW SPARASSOCYNID GENUS (MAMMALIA, METATHERIA, DIDELPHIMORPHIA) AND COMMENTS ABOUT THE AGE OF THE AISOL FORMATION (NEOGENE; MENDOZA PROVINCE, ARGENTINA)

FORASIEPI, A.M.; GOIN, F.J.; MARTINELLI, A.G.

Neuquen

Congreso; III Congreso Latinoamericano de Paleontologia de Vertebrados; 2008

Universidad Nacional de Comahue

A new specimen of a sparassocynid didelphimorphian, consisting of a single skull, is analyzed. The material comes from the upper levels of the Aisol Formation outcropping in Mendoza Province (Argentina), and constitutes the first marsupial hitherto recovered from this unit. With regard to the systematics, a new combination is proposed, as the specimen is referred to a new genus of the family Sparassocynidae (Didephoidea) but belonging to the already known species “Thylatheridium” dolgopolae Reig (see also Goin and Montalvo, 1988; Goin, 1995). The new genus 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 closest relatives of living didelphids (Goin, 1995). If this hypothesis is correct, then the sparassocynid clade has a ghost lineage of more than 30 Ma. based on the time of divergence calculated for living didelphids by molecular studies (e.g., Steiner et al., 2005). An alternative hypothesis based on the fossil record is that sparassocynids, as well as all other didelphoids, radiated more recently, after the global cooling event that happened worldwide after the Eocene-Oligocene boundary, during the Neogene, and the calibration of molecular phylogenies should be reconsidered. “Thylatheridium” dolgopolae has been previously recovered from late Miocene (Huayquerian) deposits of Catamarca and La Pampa provinces. Originally, the entire Aisol Formation was regarded as middle Miocene (Friasian) in age, based on its fossil content (Soria, 1983). Nonetheless, the presence of the recently discovered marsupial, as well as other taxa (i.e., caviomorph rodents), suggest that at least the upper stratigraphic level, where the specimens were found, was deposited during the late Miocene (not older than the Huayquerian).