Cenozoic vertebrates from Sierra County, southwestern New Mexico
— Gary S. Morgan and Spencer G. Lucas

Fossil vertebrates of Eocene, Oligocene, Pliocene and Pleistocene age are known from Sierra County in southwestern New Mexico. These fossils provide important biochronological control of the ages of the sedimentary rocks that yielded them. The oldest mammal fossil known from Sierra County is a lower jaw of the brontothere Duchesneodus uintensis from the Rubio Peak Formation in the northern Black Range. Duchesneodus indicates a late Eocene age (~37-40 Ma; Duchesnean North American land-mammal “age”—NALMA). Slightly younger fossil mammals from the Rubio Peak Formation in the northern Black Range, including the oromerycid artiodactyl Montanatylopus matthewi and the rodent Jaywilsonomys ojinagaensis, are of latest Eocene age (~ 36-37 Ma; early Chadronian NALMA). Fossil mammals from the upper part of the Palm Park Formation in the southern Caballo Mountains, including the hyaenodontid creodont Hyaenodon horridus, the horse Mesohippus cf. M. texanus, and the oreodont Merycoidodon presidioensis, are also of early Chadronian age. Two species of large oreodonts from the Seventyfour Draw Local Fauna (LF) in the Black Range, Desmatochoerus cf. D. megalodon and Megoreodon cf. M. grandis, are late Oligocene (early Arikareean NALMA) taxa derived from volcaniclastic strata bracketed by tuffs dated at ~ 26 Ma and ~ 28 Ma. No Miocene mammals are known from Sierra County. The most extensive fossil mammal assemblages from Sierra County are of late Pliocene to early Pleistocene age (~ 2.0-3.6 Ma; Blancan NALMA) and come from the Palomas Formation in the Palomas and Engle basins of the Rio Grande rift. The Cuchillo Negro Creek LF is in close stratigraphic association with the 3.1 Ma Mud Springs pumice and documents the association of the gomphotheriid proboscidean Stegomastodon primitivus, the borophagine canid Borophagus hilli, and the horse Equus scotti, suggesting an age of between 3.1 and 3.3 Ma (late early Blancan). Based on the presence of S. primitivus and E. scotti, the Elephant Butte Lake fauna is similar in age. The Truth or Consequences LF is about 40 m lower in the Palomas Formation than the Cuchillo Negro Creek LF and the 3.1 Ma Mud Springs pumice, which together with associated magnetostratigraphic data and the presence of the lagomorph Notolagus lepusculus and the woodrat Neotoma cf. N. fossilis, suggests a somewhat older age within the late early Blancan (~ 3.3-3.6 Ma). The Williamsburg LF documents the co-occurrence of the mylodontid ground sloth Paramylodon cf. P. garbanii, a South American immigrant and participant in the Great American Interchange that did not appear in temperate North America until after 3.0 Ma, and the dwarf three-toed horse Nannippus peninsulatus, which disappeared from New Mexico before 2.6 Ma, constraining the age of this fauna to the early late Blancan (~ 2.6-3.0 Ma). The Palomas Creek fauna is similar in age based on the presence of Nannippus peninsulatus and the horse Equus simplicidens and a magnetostratigraphic section indicating referral to the uppermost Gauss Chron (2.58-3.04 Ma). The Caballo LF is the youngest Blancan fauna in Sierra County (latest Blancan; ~ 2.0-2.6 Ma) based on the presence of the glyptodont Glyptotherium arizonae, a South American Interchange mammal, absence of Nannippus (disappeared about 2.6 Ma) and Mammuthus (first appeared about 1.6 Ma), and magnetostratigraphic data indicating referral to the lower Matuyama Chron, above the Gauss/Matuyama boundary (2.58 Ma) and below the Olduvai Subchron (1.95 Ma). Several late Pleistocene (Rancholabrean NALMA) sites, including Alkali Flat, Alkali Spring, and Salt Creek, occur on the White Sands Missile Range in the Tularosa basin in easternmost Sierra County. The Alkali Flat and Alkali Spring sites occur in lacustrine and fluvio-deltaic deposits of the Otero Formation, and include bones and teeth of the Columbian mammoth Mammuthus columbi, the horse Equus occidentalis, and the giant llama Camelops hesternus, as well as trackways of a proboscidean, probably mammoth. Radiocarbon dates bracket the age of the Alkali Flat and Alkali Spring sites between 19 and 31 ka. The Salt Creek sites also contain fossils of Mammuthus columbi, Equus, and Camelops hesternus, as well as proboscidean tracks, and occur below a gypsum marsh deposit with a radiocarbon date of about 11 ka. Palomas Creek Cave in the Black Range in western Sierra County contains a late Pleistocene fauna consisting primarily of small mammals.