New Mexico Geology

A mandible of a tapir (Tapirus sp.) from the late Pliocene (early Blancan North American land mammal age–NALMA), Tonuco Mountain Local Fauna (LF), Doña Ana County, southern New Mexico, is a significant addition to the small sample of fossil tapirs known from the late Cenozoic of New Mexico. The Tonuco Mountain tapir mandible is not identified to the species level because the diagnostic characters in the genus Tapirus are primarily found in the skull. It is most similar in size and morphological features to the mandible of the late Blancan species Tapirus lundeliusi from Florida. The Tonuco Mountain LF consists of 17 species of vertebrates, including a mud turtle, two tortoises, a duck, and 13 species of mammals. Among mammals in this fauna, the camel Camelops, the peccary Platygonus, and the horse Equus scotti first appeared in North American early Blancan faunas at about 3.6 Ma, whereas the horses Nannippus peninsulatus and Equus simplicidens became extinct in New Mexico in the late Blancan at about 2.6 Ma. The association of these mammals, together with the absence of mammals of South American origin that first appeared in the American Southwest at about 2.7 Ma, restricts the age of the Tonuco Mountain LF to the late early Blancan, between 2.7 and 3.6 Ma. The fossils from the Tonuco Mountain LF are derived from sediments of the axial-fluvial lithofacies of the ancestral Rio Grande, referred to the Camp Rice Formation. The sediments in the lower 30 m of the Camp Rice Formation section containing the Tonuco Mountain LF, including the Tapirus mandible, are normally magnetized and correspond to the lowermost portion of the Gauss Chron (C2An.3n), above the Gilbert/Gauss boundary (younger than 3.58 Ma) and below the base of the Mammoth Subchron (C2An.2r; older than 3.33 Ma). The mammalian biochronology and magnetostratigraphy restrict the age of the Tonuco Mountain LF to between 3.3 and 3.6 Ma (early late Pliocene, late early Blancan). The tapir mandible from the Tonuco Mountain LF is the first record of Tapirus from the early Blancan (2.7–4.9 Ma) of North America. Tapirus had a restricted geographic distribution in the late Pliocene and earliest Pleistocene (late early and late Blancan; ~1.6–3.6 Ma) of temperate North America, occurring primarily in the southern United States from Florida to California, including New Mexico.