A new Akodontini (Mammalia: Rodentia: Cricetidae) from the Pliocene of Argentina: enlarging the past diversity of the sigmodontines

P. E. ORTIZ; J. P. JAYAT; U. F. J. PARDIÑAS; R. F. GONZÁLEZ

Mendoza

Congreso; 4th International Palaeontological Congress, Mendoza, Argentina; 2014

Despite some significant recent findings, the fossil record of South American cricetids is still scarce and unevenly distributed across the continent. This is particularly noticeable in the case of the Pliocene fossils, most of them recovered from central Argentina deposits. Here we report a new Akodontini coming from late Pliocene deposits of the Middle Unit of the Uquía Formation (Marplatan stage) in San Roque, Jujuy province, northernmost Argentina. These levels can be dated at about 2.5?3 Ma. The new akodontine was found in close association with other several small vertebrates including frogs, lizards, birds, marsupials, and caviomorph and other extinct cricetid rodents. The studied material consists of six mandibles, two fragments of maxillae with molar series, and a few isolated molars, representing at least three individuals. These specimens show morphological affinities with the genera members of the Akodon division. However, the particular combination of characters in skull (e.g. short incisive foramina), mandible (e.g. masseteric crest above the middle height of the mandible, its anterior margin developed as a weak knob and above the level of the mental foramen; capsular projection situated somewhat backward; condyle something broad dorsoventrally; anterior point of the symphysis at the same level or slightly above the alveolar toothrow plane) and molars (e.g. M1 with vestigial anteroloph and mesoloph and no trace of enteroloph in young individuals; m1 with shallow anteromedian flexid and no trace of mesolophid, ectolophid and ectostylid, even the youngest individuals; anterolabial cingulum well developed, arising from the anterolabial conulid) suggest that the material belongs to an undescribed genus of small akodontine. According to the paleoenvironmental signal from the small vertebrate assemblage, the new akodontine would have lived in open arid environments in Puna-like or Monte-like habitats. The discovery of an extinct akodontine related to Akodon division in the Central Andes during the late Pliocene supports the hypothesis that this region represented an area of diversification for the tribe or, at least, a particular episode in the early evolution of the Akodontini. The absence in the Uquía Formation of Akodon, a genus whose species constitute dominant elements of the recent communities in the southern half of South America, suggests that most of the living akodontines, at least the forms with mainly Andean distribution, radiated after the late Pliocene. The presence of the extinct akodontine in the late Pliocene of the Central Andes indicates a greater diversity than previously thought.