Primitive species in the differentiation of the subfamily Ctenomyinae are revisited, and, on molar evidence, a new interpretation of the evolutionary pattern of these earliest members of the group is proposed. The octodontid Phtoramys is rejected as possible ancestor, whereas the genus Chasichimys (including Pattersomys), previously included in the family Echimyidae, is transferred to the Octodontidae as a primitive member of the ctenomyine radiation. During the Chasicoan and Huayquerian Ages [Late Miocene), an anagenetic event represented by an increase of hypsodonty in Chasichimys, may have led to the differentiation of the primitive euhypsodont ctenomyines of the genus Xenodontomys. Coeval cladogenetic processes, occurring during the protohypsodont evolutionary stage of the group, would have resulted in the separation of the genus Palaeoctodon from the lineage Chasichimys-Xenodontomys. These ancient representatives of the subfamily would have differentiated in central Argentine pampas. The recognition of such evolutionary events and concordant evidence from other octodontoid rodents suggest temporal differences among the outcroppings of the Cerro Azul Formation in central Argentina.
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