A new genus and species, Banderomys leanzai, from the Cerro Bandera Formation (Early Miocene?) of Neuquén province, Argentina, is described. It is known through a mandibular fragment with two molars and several isolated cheek teeth. With wear the upper molars develop a bilobate pattern, whereas the lowers molars attain a trilophodont one. Banderomys is referred to the Cephalomyidae because it has an 'asymmetric' dental pattern as other members of the family, but it is less hypsodont, and therefore it is more primitive than any other contemporary or older cephalomyids so far known. A phylogenetic analysis suggests that the relationships between the Cephalomyidae and the Cavioidea are closer than what was traditionally assumed. The cephalomyids would have radiated in pre-Deseadan times, from an ancestor with a dentition very close to that of Banderomys, and reached their main diversity during the Deseadan-Colhuehuapian lapse, when they constituted the dominant hypsodont rodent group in South America.

Key words: Rodentia, Hystricognathi, Cephalomyidae, Cerro Bandera Formation, Miocene, Neuquén province, Patagonia.

Alejandro Gustavo Kramarz [agkramarz@macn.gov.ar], Consejo Nacional de Investigaciones Científicas y Técnicas, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Sección Paleontología de Vertebrados, Angel Gallardo 470, C1405DJR Buenos Aires, Argentina.