Abstract This paper deals with the new mammal remains from the Rio Quinto Formation, found at the Arroyo La Petra locality (San Luis, Argentina). They consist of an incomplete skull and two mandibular fragments of a macraucheniid, both with milk dentition and M1/m1 erupting, corresponding to a very young individual. Morfometrical characteristics of the specimens allow their identification as Oxyodontherium zeballosi and provide the first cranial data for this species. The holotype of this species is considered a juvenile individual. We maintain a conservative position in maintaining O. zeballosi as a valid species though based on juvenile specimens. However, a full systematic revision of the Family Macraucheniidae is needed to verify the validity and phylogenetic relationships of the taxa included in it. The identification of O. zeballosi indicates a late Miocene age for the fossiliferous level, which implies a cronoestratigraphic extension of the Rio Quinto Formation, thus far considered as Pliocene in age. In addition, the presence of Oxyodontherium in San Luis Province increases the geographical distribution of this macrauchenid, whose record previously was limited to Miocene outcrops of the Parana River (Entre Rios Province). Its presence in the Mio–Pliocene beds of Uruguay is very doubtful because it is based on a metapodial.