LATE PLIOCENE (BLANCAN) VERTEBRATE FAUNAS FROM PEARSON MESA, HIDALGO COUNTY, NEW MEXICO AND GREENLEE COUNTY, ARIZONA

Abstract

Exposures at Pearson Mesa in the Duncan basin along the New Mexico-Arizona border have produced a diverse assemblage of late Pliocene (Blancan) vertebrates. The stratigraphic section at Pearson Mesa consists of more than 60 m of sandstones, mudstones, and sedimentary breccias of the Gila Group. Two distinct vertebrate faunas occur at Pearson Mesa: the early late Blancan Pearson Mesa Local Fauna (LF) is derived from the lower 15 m of the stratigraphic section and the latest Blancan Virden LF occurs in the upper 20 m of the section. The Pearson Mesa fauna consists of 25 species: 3 land tortoises ( Gopherus and two species of Hesperotestudo ); box turtle ( Terrapene ); colubrid snake; bird; and 19 mammals. Age-diagnostic mammals from the Pearson Mesa LF include: the ground sloth Paramylodon cf. P. garbanii ; the pocket gopher Geomys persimilis ; the cotton rat Sigmodon medius ; the three-toed horse Nannippus peninsulatus ; the one-toed horses Equus cf. E. cumminsii , and E. simplicidens ; and the peccary Platygonus bicalcaratus . The association of Paramylodon and Nannippus defines a restricted interval of time in the Blancan between the first appearance of South American immigrants (including Paramylodon ) in the southwestern US at ~3.0 Ma and the extinction of Nannippus at ~2.2 Ma. Magnetostratigraphy further constrains the age of the Pearson Mesa LF, with five normally-magnetized samples from the lower part of the section referred to the uppermost Gauss Chron (Chron 2An.1n; 2.58-3.04 Ma). Southwestern early late Blancan faunas (~2.6-3.0 Ma) correlative with Pearson Mesa include: Anapra, NM; Wolf Ranch and 111 Ranch, AZ; and Cita Canyon and Hudspeth, TX. A 10-m-thick sedimentary breccia overlying the Pearson Mesa LF lacks fossils and may represent a hiatus. The Virden LF consists of 22 species: toad; large Hesperotestudo ; Terrapene ; colubrid snake; lizard; two birds; and 15 mammals. A latest Blancan age (~1.8-2.2 Ma) for the Virden LF is indicated by the presence of the dwarf cotton rat Sigmodon minor and the small camelid Hemiauchenia gracilis , both restricted to latest Blancan faunas, and the association of the glyptodont Glyptotherium arizonae with the coyotelike canid Canis lepophagus . Southwestern latest Blancan faunas correlative with Virden include: La Union, NM and Curtis Ranch and San Simon, AZ.