A new species ofHolochilus(Rodentia: Sigmodontinae) from the middle Pleistocene of Bolivia and its phylogenetic significance

A series of mandibles and maxillae from the Ensenadan (middle Pleistocene) sediments of the Tarija Basin of Bolivia are assigned to Holochilus primigenus, sp. nov. This marsh rat of the subfamily Sigmodontinae (= “South American cricetid”) is one of the few extinct muroids known from South America. A cladistic analysis indicates that this new species is the sister taxon to living species of Holochilus and possesses a morphology transitional between extant Holochilus and their closest living relative, Lundomys. The occlusal molar morphology of H. primigenus is virtually indistinguishable from that in extant Lundomys, but synapomorphic characters of the mandible and palate unite M. primigenus with the extant Holochilus. Holochilus primigenus is a potential ancestor to its living congeners because it possesses no known autapomorphies. The explicit hypothesis of phylogenetic position for this new species combined with its presumed geologic age of 0.7–1.0 Ma may provide a calibration point to estimate ages of divergence among lineages of sigmodontines.