New Mexico Geological Society Annual Spring Meeting — Abstracts

Three stratigraphically superposed Plio-Pleistocene vertebrate faunas are known from the Camp Rice Fm in the Mesilla Basin, Doña Ana County: Mesilla Basin Faunas A, B, and C, from oldest to youngest (hereafter Mesilla A, B, and C). Mesilla A and B are from the late Blancan land-mammal “age” (late Pliocene) and Mesilla C is from the early Irvingtonian land-mammal “age” (early Pleistocene). The Mesilla A fauna is composed of only six species, including the glyptodont Glyptotherium and the three-toed horse Nannippus peninsulatus. The presence of Glyptotherium establishes a late Blancan or younger age for this fauna because the first appearance of South American immigrants in North America, including Glyptotherium, defines the beginning of the late Blancan at about 2.7 Ma. The presence of Nannippus peninsulatus is indicative of Blancan faunas older than 2.2 Ma. Nannippus and several other typical North American Blancan mammal genera (Borophagus, Hypolagus, Rhynchotherium) went extinct at ~2.2 Ma (Nannippus extinction datum). Mesilla A occurs in normally magnetized strata in the upper Gauss Chron above the Kaena Subchron (3.04 Ma) and below the Gauss/Matuyama boundary (2.58 Ma). Mesilla A is one of only six late Blancan faunas in the Southwest that document the presence of South American immigrant mammals in the upper Gauss Chron (~2.6-2.8 Ma). Mesilla B is the most diverse late Blancan fauna in New Mexico, with at least 35 species of vertebrates: 3 bony fish, including gar; 2 amphibians (frog and salamander); 7 reptiles (4 turtles, 2 snakes, lizard); 2 birds; and 21 mammals. The most age-diagnostic mammals in the Mesilla B fauna are Glyptotherium arizonae; the mylodont ground sloth Paramylodon harlani; the mole Scalopus; the rabbit Aluralagus virginiae; the small sabercat Smilodon gracilis; the large tapir Tapirus haysii; and the giant camelids Gigantocamelus spatula and Blancocamelus meadei. Gigantocamelus and Blancocamelus confirm a Blancan age for Mesilla B as these two genera are not known from the Irvingtonian. Glyptotherium arizonae, Paramylodon harlani, and Smilodon gracilis are unknown in southwestern faunas before the latest Blancan, an age supported by the absence of Blancan genera that disappear by 2.2 Ma (e.g., Nannippus) as well as Irvingtonian mammals that appear after 1.8 Ma (e.g., Mammuthus). Aluralagus virginiae is restricted to latest Blancan faunas. Biostratigraphy and magnetostratigraphy constrain the age of Mesilla B to the latest Blancan (1.8-2.2 Ma). The Mesilla C fauna contains Mammuthus, the defining genus for the Irvingtonian. In addition to the primitive mammoth Mammuthus meridionalis, other mammals from Mesilla C indicative of the Irvingtonian are the megalonychid ground sloth Megalonyx wheatleyi, the wolf Canis armbrusteri, the beaver Castor canadensis, and the lamine camelid Palaeolama mirifica. Magnetostratigraphy places Mesilla C in the upper Matuyama Chron between 1.07 and 1.81 Ma and two pumice beds dated at 1.32 Ma and 1.59 Ma occur in the stratigraphic interval containing early Irvingtonian mammals. Biostratigraphy, radioisotopic dates, and magnetostratigraphy constrain the age of Mesilla C between 1.1 and 1.6 Ma. Mesilla C thus correlates with the Tijeras Arroyo local fauna from the Albuquerque Basin, which also contains Mammuthus meridionalis and is bracketed by dates of 1.26 and 1.61 Ma.