New Mexico Geological Society Annual Spring Meeting — Abstracts

The Tonuco Mountain local fauna (If) is a middle Blancan (late Pliocene) vertebrate assemblage from the Cedar Hill area southeast of Tonuco Mountain in Dona County, southern New Mexico. The fossils were derived from the Camp Rice Formation in the western part of the 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 If 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) simplicidens, and Equus 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 the mammals in the Tonuco Mountain If, Canis lepophagus, Borophagus, Nannippus cf N. peninsulatus, Equus simplicidens, Platygonus cf P. bicalcaratus, and Hemiauchenia 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 2.5 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 If 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 3.4 Ma) and the fossiliferous interval is below the top of the Kaena subchron (older than 2.9 Ma). A carpal of the rhinoceros Teleoceras was collected from the Rincon Valley Formation, which unconformably underlies the Camp Rice Formation in the Cedar Hill area. The presence of Teleoceras suggests a late Miocene (Hemphillian) age, which is in accordance with a radioisotopic date of 9 Ma on a basalt from the lower part of the Rincon Valley Formation.