Eocardiid rodents from the late early Miocene Pinturas Formation (Ameghino's “Piso Astrapothericu-lense”) of Patagonia are here studied. Three species are recognized: Luantus propheticus Ameghino and Luantus toldensis sp. nov. are recorded exclusively from the Pinturas Formation; Phanomys mixtus Ameghino is also known from the Santa Cruz Formation (Santacrucian SALMA). The dentition of Luantus propheticus has rudiments of accessory prisms in M3 and p4, and the cheek teeth exhibit a bilobated occlusal pattern with interlobular cement and discontinuous enamel covering in senile stages. Luantus toldensis has more hypsodont cheek teeth than L. propheticus, even more developed M3 and p4 accessory prisms, and a bilobated pattern that is attained in earlier stages of wear. Luantus propheticus, L. toldensis, and Phanomys mixtus represent an evolutionary series that leads to the hypselodont pattern of Eocardia; thus, the traditional dichotomy between “Luantinae” and “Eocardiinae” is not supported. Luantus propheticus occurs in the lower and middle sequences of the Pinturas Formation, whereas the more derived species are recorded in localities interpreted as corresponding to the upper sequence. This distribution agrees with that of other faunal components recorded at the Pinturas Formation.