UN NUEVO CARDIOMYINAE (RODENTIA, CAVIIDAE) DE LA FORMACION ARROYO CHASICO (PLIOCENO INFERIOR) DE LA PROVINCIA DE BUENOS AIRES

Abstract

This paper can be divided into two fundamental parts. In the first part an analysis is made of the Arroyo Chasico Formation, denomination that replace that of the? Chasico Formation, and the original Chasicoense. A history of its discovery and the criteria? used for its geo and biochronological position in the scale of the Argentine Cenozoic formations are analyzed. The second part is devoted to the principal purpose, in which a new form of Cardiomyinae (Rodentia, Caviidae), found in the before mentioned geological formation; of the southwestern part of the province of Buenos Aires, is described, under the name Procardiomys martinoi gen. nov. et sp. nov. It is recognized to be ancestral to the genus Cardiomys, from which it differs fundamentally in having the M3 composed of three fundamental cordiform lobes, and an additional posterior one, rounded, which represents a degree of evolution already theoretically infered. The mandibular portion that completes the material dubiously related presents cheek-teeth of the Dolichotinae type, because of which a more direct phylogenetic relation of that subfamily with the Cardiomyinae is recognized. Finally the characters of the subfamily are analyzed, its relation with the Dolichotinae arid Caviidae, and its very possible descent from the Oligocene and Miocene Eocardiidae phyllum.