Pleistocene vertebrates from Rio Arriba and Taos counties, northernmost New Mexico
— Gary S. Morgan and Spencer G. Lucas

Nine Pleistocene vertebrate fossil sites are known from Rio Arriba and Taos counties in northernmost New Mexico: Gobernador, Youngsville, Abiquiu, Santa Cruz, Ojo Caliente, and San Antonio Mountain (SAM) Cave in Rio Arriba County and Mesa Vibora, Picuris, and Vadito in Taos County. The Gobernador site in northwestern Rio Arriba County is in the Colorado Plateau physiographic province, and the Santa Cruz site in southeastern Rio Arriba County is in the Basin and Range province; the other seven sites are located in the Southern Rocky Mountains province. All nine Pleistocene sites from these two counties occur above 6000 ft (1830 m) in elevation, and SAM Cave is the highest Pleistocene site in New Mexico at 8980 ft (2737 m). Six of the nine sites from Rio Arriba and Taos counties contain a single species of large mammal: Gobernador (Equus niobrarensis), Youngsville (Mammuthus columbi), Ojo Caliente (proboscidean, probably mammoth), Mesa Vibora (Bison sp.), Picuris (Mammuthus sp.), and Vadito (Mammuthus columbi). The Abiquiu site has three large mammals (Euceratherium collinum, Bison antiquus, and Mammuthus sp.), and Santa Cruz has five large mammals (Canis dirus, Equus sp., Camelops hesternus, Bison sp., and Mammuthus sp.). SAM Cave has the most diverse Pleistocene vertebrate fauna in this region (41 species), almost all of which are small species. SAM Cave is the oldest Pleistocene vertebrate fauna in these two counties, dating to the late early Pleistocene (medial Irvingtonian, ~0.74 0.99 Ma). The Youngsville, Abiquiu, Santa Cruz, Mesa Vibora, and Vadito sites are medial or late Pleistocene (Rancholabrean) in age based on the presence of Bison or Mammuthus columbi, both of which are typical of Rancholabrean faunas. The only Pleistocene site from this region with an absolute date is Mesa Vibora; an AMS radiocarbon date of 24,740 ± 140 years Before Present, on bone collagen from a Bison humerus.