Mandibular remains of Procardiomys martinoi Pascual, 1961 (Hystricognathi, Cavioidea) from the Arroyo Chasicó Formation (early late Miocene) of Argentina, and the phylogenetic position of the genus within Caviidae.
Hydrochoerinae is a clade of caviomorph rodents broadly distributed in South America, which includes the maximum body size recorded among extant rodent taxa. The most basal forms of this group are an assemblage of small to medium body size extinct taxa with a plesiomorphic dentition, traditionally clustered in the group cardiomyines. One of the oldest known cardiomyine isProcardiomys martinoi(Chasicoan South American Land Mammal Age SALMA; early late Miocene), which was known only from the holotype, a fragmentary palate with the left and right molar series. New mandibular remains from the Arroyo Chasicó Formation (Chasicoan SALMA) are described and identified here as belonging toP. martinoibecause they share a unique combination of characters (as well matching in size) with the upper dentition of the holotype. These materials help in critically reviewing the taxonomic identification of the mandibular remains previously assigned toProcardiomysand allow testing the phylogenetic affinities of this taxon within Caviidae.P. martinoiis depicted as one of the most basal forms of Hydrochoerinae, placed basally on the lineage leading to extant capybaras after the split between the common ancestor ofKerodonandHydrochoerus.