The record of Neogene faunas from the northeast of Argentina (Mesopotamia) is sporadic, whereas the late Miocene vertebrate-bearing horizons in this area contain a very abundant and diverse fauna, the Pliocene deposits are poorly known and have been unfossiliferous until now. This situation makes the understanding of the biogeographic and evolutionary history of the Mesopotamian taxa difficult. In this contribution, the first mammals are described that were recovered from the most basal levels of the Punta Gorda Group ( = Alvear Formation) at Entre Ríos Province, assigned to the late Pliocene (Lower Matuyama) by previous paleomagnetic studies. These beds are paleomagnetically bracketed between 1.95 and 2.6 Ma (late Pliocene) in the Alvear section and help to refine the biostratigraphy of these species elsewhere in northeastern Argentina. The echimyids Paramyocastor diligens (Ameghino, 1888) and Eumysops sp., and the hegetotheriid Paedotherium cf. typicum herein reported support a Pliocene age for the sediments from which they were recovered. These mammals suggest a paleoenvironment characterized by semiarid and open lowlands, markedly different than that inferred for the Late Miocene in this area.