A diverse mammalian fauna has been recovered from the central Chilean Andes at El Volcan (33° 49' S, 70° 08' W) providing new chronologically important material for the poorly dated host unit, the Abanico (=Coya-Machali) Formation. Only four radiometric dates of questionable reliability have been published for the section from which these fossils were recovered (Munoz et al., 2006). This thesis represents the initial effort to identify and describe elements of the El Volcan fauna. Four caviomorph dentitions were selected for study based on the quality of informative morphology preserved. These specimens were described, compared to closely related Oligocene-early Miocene genera, and identified to the lowest taxonomic rank possible. Three new caviomorph taxa have been identified (Cephalomys sp. nov., Erethizontidae Gen. et. sp. nov., Echimyidae Gen. et. sp. nov.), all more closely related to Colhuehuapian than to Deseadan forms. The fossil bearing horizons at El Volcan likely fall between Deseadan age and Colhuehuapian age between ~25 and 21 Ma. This paleontologically-based age estimate provides an independent test of the radiometric ages produced by Munoz et al. (2006).