The fossil record of the desert-adapted South American rodent Tympanoctomys (Rodentia, Octodontidae). Paleoenvironmental and biogeographic significance

New mandibular remains of one specimen of the desert-adapted South American rodent Tympanoctomys from the Pleistocene of central Argentina are described. These materials, together with a hemimandible previously analyzed by Ameghino, represent the limited paleontological record of the genus, and the first undoubted record of a living genus of the Octodontinae subfamily. The taxonomic, paleoenvironmental and biogeographic significance of these materials is discussed. The sediments yielding the new remains, at the coastal region of Mar del Plata, represent one of the glacial events that deposited the Pichileufu Drift in the Cordillera region of Argentina (between isotopic stages 23 and 25). We propose that, as a consequence of the glaciation, favorable habitats for populations of Tympanoctomys would have extended eastwards. Consequently, the present distributional pattern of the genus may be considered a relict.