Se redescribe Theridomysops parvulus (Rovereto, 1914) del Mioceno tardío de Catamarca. Se asigna esta especie a los Echimyidae Eumysopinae, y se la relaciona especialmente al clado Clyomys-Euryzygomatomys como su grupo hermano plesiomórfico. La presencia de un roedor de este grupo en el «Araucanense» indica que la dicotomía entre los clados Clyomys-Euryzygomatomys y Thrichomys ya había ocurrido para el Mioceno tardío.
Theridomysops parvulus (Rovereto, 1914), a primitive Eumysopinae (Rodentia, Caviomorpha) of the Late Miocene of Argentina.Theridomysops parvulus (Rovereto, 1914) from the Late Miocene of Catamarca Province is redescribed. It is a small rodent with a marked unilateral hypsodonty in upper teeth. It is characterized by a long spur in the anterolophid of dp/4, and a large remnant of the protolph in Dp4/-M1/. These characters allow us to assign it to the Echimyidae Eumysopinae, and to relate it to the Clyomys-Euryzygomatomys clade, as its plesiomorphic sister group. The presence of a rodent of this group in the “Araucanense” shows that the dichotomy between the clades Clyomys-Euryzygomatomys and Thrichomys had already occurred before the Late Miocene. This species is part of an important radiation of the Eumysopinae occurred in central Argentine during the Late Miocene, south of their present distribution. This radiation almost vanished during the Pleistocene.
The problem of loph homologies among the upper cheek teeth of the caviomorph rodents and the polarity of some of the observed changes are reanalyzed. This analysis is made upon the evidences provided by both living and fossil representatives. It is concluded that pentalophodonty is an homology for the caviomorphs. There is only one pentalophodont pattern, while there are two different tetralophodont configurations. In all of them, the third loph is a mesoloph. The tetralophodont patterns represent successive steps of a general process of loph reduction. In brachyodont molars, this reduction is part of the adaptation to open environments, such as the development of hypsodonty is. The scarcity of pentalophodont genera in the fossil record and their present restriction to the Brazilian subregion are related to the early development of open environments in the southern part of South America.