The giant rodent Chapalmatherium from the Pliocene of Argentina: New remains and taxonomic remarks on the family Hydrochoeridae

Two nearly complete skulls and incomplete postcranial remains of a giant ro dent of the family Hydrochoeridae are described and identified as Chapalmat herium novum. They come from the Calera Avellaneda locality, near Olavarria , Buenos Aires Province, Argentina, considered to be of Montehermosan age ( early Pliocene). Diagnostic characters of the genera of Protohydrochoerinae are discussed, and synonymy between Chapalmatherium and Protohydrochoerus is proposed: the species P. perturbidus becomes Chapalmatherium perturbidum . The known data concerning Chapalmatherium species and this new material l ead us to question die taxonomic validity of C. irenense and C. sanvedrai; we consider them to be junior synonyms of C, novum. A cladistic analysis of die family Hydrochoeridae, with the caviid Dolichotts as the outgroup, yie lded three most parsimonious cladograms. The results show that Cardiatherii nae is a paraphyletic group, Anatochoerinae is not a monophyletic subfamily , and Hydrochoeropsis must be included in Hydrochoerinae. The Protohydrocho erinae appear to be a monophyletic group, constituting the sister group of the Hydrochoerinae.