Caviidae and Hydrochoeridae (extant Cavioidea s.s.), probably the most peculiar among rodents, are characterized by their evergrowing double-heart-shaped cheekteeth. They have classically been rooted in Eocardiidae, which ranges from Deseadan to ‘Colloncuran’ (late Oligocene-middle Miocene) in Patagonia, although in the Deseadan and Colhuehuapian (early Miocene) remains are very scanty. For the Colhuehuapian, only one species coming from the southern cliff of the Colhué Huapi Lake (Gran Barranca, Chubut, Argentina) was known so far. In this paper, the first Colhuehuapian eocardiids from outside Gran Barranca are reported: Luantus minor, sp. nov., and Chubutomys leucoreios, sp. nov. They are represented by two fragmentary mandibles and a few isolated cheek teeth, from the Trelew Member of the Sarmiento Formation at Bryn Gwyn, lower valley of the Chubut River, Chubut. The new species enlarge the knowledge of eocardiid diversity, and reinforce the hypothesis of a basal dichotomy for the group. L. minor represents one of the smallest species of the series Asteromys Luantus Phanomys Eocardia, which likely gave rise to modern Cavioidea s.s. C. leucoreios pertains to a group of species with precocious hypsodonty and apparently low diversity, but without modern descendant.