Mammalian Biochronology of Blancan and Irvingtonian

Significant mammalian faunas of Pliocene (Blancan) and early Pleistocene (early and medial Irvingtonian) age are known from the Rio Grande and Gila River valleys of New Mexico. Fossiliferous exposures of the Santa Fe Group in the Rio Grande Valley, extending from the Española basin in northern New Mexico to the Mesilla basin in southernmost New Mexico, have produced 21 Blancan and 6 Irvingtonian vertebrate assemblages; three Blancan faunas occur in the Gila River Valley in the Mangas and Duncan basins in southwestern New Mexico. More than half of these faunas contain five or more species of mammals, and many have associated radioisotopic dates and/or magnetostratigraphy, allowing for correlation with the North American land-mammal biochronology. Two diverse early Blancan (4.5–3.6 Ma) faunas are known from New Mexico, the Truth or Consequences Local Fauna (LF) from the Palomas basin and the Buckhorn LF from the Mangas basin. The former contains five species of mammals indicative of the early Blancan: Borophagus cf. B. hilli, Notolagus lepusculus, Neo-toma quadriplicata, Jacobsomys sp., and Odocoileus brachyodontus. Associated magnetostra-tigraphic data suggest correlation with either the Nunivak or Cochiti Subchrons of the Gilber