Pliocene (Blancan) vertebrate fossils from the Camp Rice Formation near Tonuco Mountain, Dona Ana County, southern New Mexico
— Gary S. Morgan, Spencer G. Lucas, and John W. Estep

The Tonuco Mountain local fauna is proposed as a new name for a middle Blancan (late Pliocene) vertebrate assemblage from the Cedar Hill area southeast of Tonuco Mountain in Data Ana County, southern New Mexico. The fossils are derived from the Camp Rice Formation in the western Jornada Basin. The stratigraphic section of the Camp Rice Formation at Cedar Hill consists of about 50 m of sandstone and conglomerate, with a minor component of sandy mudstone. The Tonuco Mountain fauna is composed of 16 species, including: the mud turtle Kinosternon; the land tortoises Gopherus and Hesperotestudo; a duck; a rabbit; the badger Taxidea; the coyote-like canid Canis lepophagus; the bone-eating dog Borophagus; the horses Nannippus cf. N. peninsulatus, Equus (Dolichohippus) simphcidens, and E. scotti; the peccary Platygonus cf. P bicalcaratus; the camels Camelops, Hemiauchenia blancoensis, and a small undescribed species of Hemiauchenia; and the gomphotheriid proboscidean Cuvieronius. Among these taxa, C. lepophagus, Borophagus, Nannippus cf. N. peninsulatus, E. simplicidens, Platygonus cf. P bicalcaratus, and H. blancoensis are indicative of the Blancan land mammal age (between 4.5 and 2.0 Ma). Several taxa help to further limit the age of this fauna within the Blancan. E. simplicidens is absent from very early Blancan faunas, Platygonus and Camelops do not appear until the beginning of the middle Blancan (about 3.7 Ma), and most Blancan records of Nannippus in the southwestern United States predate the Gauss-Matuyama magnetic reversal at about 2.6 Ma. The absence of South American immigrants suggests the fauna is older than 2.8 Ma, the earliest date for the onset of the Great American Faunal Interchange. These biostratigraphic data restrict the age of the Tonuco Mountain local fauna to the middle Blancan (between 3.7 and 2.8 Ma). Previous magnetostratigraphic studies of the Camp Rice Formation at Cedar Hill help to further constrain the age of this fauna. The entire section is within the Gauss chron (younger than 3.6 Ma), and the fossiliferous interval is below the top of the Kaena subchron (older than 3.0 Ma). The presence of a carpal bone of the rhinoceros Teleoceras from the Rincon Valley Formation in the Cedar Hill area suggests a late Miocene (Hemphillian) age, which is in accordance with a radioisotopic date of 9.6 Ma on the Selden Basalt Member of the Rincon Valley Formation.