The Paleogene faunal assemblage from Antofagasta de la Sierra (Catamarca, Argentina), is here presented, both in its geological and systematic aspects. The fossil bearing levels are referred to the Geste Formation (Pastos Grandes "Group"). The described specimens belong to the Classes Reptilia (Orders Crocodylia, Serpentes and Chelonii) and Mammalia (three taxa from the Superorder Marsupialia, representatives of the Orders Edentata, Condylarthra, Pyrotheria and Astrapotheria, and six families of the Order Notoungulata). This fauna is referred to the Mustersan Age, which in Patagonia represents the Middle Eocene. Such chronologic assignment is based on the presence of characteristic taxa, their evolutionary stage and on stratigraphic evidence. Finally, a brief comparison with other faunal assemblages from the Early Tertiary of Argentina and Chile, is presented. 

As a result of the enrichment of eotheriodont material by one of us (P.K.T.), these specimens (essentially Biarmosuchur and Eotitanosuchur) are reexamined and refigured. A reevaluation of their particularities supports the distinction of two families, for which new diagnoses are proposed. This leads us to discuss the affinities of these families, with respect to the sphenacodonts on one hand, and to the South African primitive theriodonts on the other (gorgonopsids and ictidorhinids). This study contains inherent paleogeographic consequences which are considered in conclusion.