Biostratigraphic significance of late Neogene vertebrate fossils from the Gila Group, Pearson Mesa, southwestern New mexico and southeastern Arizona
— Gary S. Morgan and Spencer G. Lucas

Gila Group exposures on Pearson Mesa in the Duncan basin along the New Mexico-Arizona border have produced a diverse assemblage of late Pliocene (Blancan), and possibly early lrvingtonian (early Pleistocene), vertebrate fossils. The Pearson Mesa fossils consist of 20 species: four turtles, including two species of the land tortoise Hesperouissudo, the tortoise Gopherus sp., and the box turtle Terrapene sp.; a heron; and 15 mammals, including the mylodont ground sloth Glossotherium cf. G. chapadmalense; the glyptodont Glyptotherium arizonae; a wolf-sized canid, Canis sp.; a small fetid; a large machairodontine felid; the geomyid Geomys (Nertemgeomys) cf. G. (N.) persimilis; the horses Nannippus peninsulatus, Equus cf. E. cumminsii, E. scotti, and E. simplicidens; the peccary Platygonus bicalcaratus; the camels Camelops sp., Hemiauchenia cf. H, blancoensis, and a small undescribed species of Hemiauchenia; and the proboscidean Stegomastodon rexroadensis. The stratigraphic section at Pearson Mesa consists of an interval more than 60 m thick of sandstones, mudstones, and sedimentary breccias referred to the Gila Group. The lower 15 m of this section contains abundant specimens of Hesperotestudo, the hipparionine horse Nannippus peninsulatus, and the equine horse Equus plicidens, together with rarer specimens of many other taxa, the most significant of which is Glossosherium cf. G. chapadmalense. Nannippus and Glossotherium co-occur in southwestern Blancan faunas during a restricted interval of time between the first appearance of South American immigrants at about 2.7 Ma and the extinction of Nannippus at about 2.2 Ma. A previously published paleomagnetic section for Pearson Mesa further restricts the age of these strata, placing them in the upper portion of the Gauss Chron (2.6-3.0 Ma). A 9-m-thick breccia overlying these strata lacks fossils and may include a hiatus. Just above this breccia is the lowest occurrence in the local section of Glyptotherium. The absence of diagnostic Blancan indicators (e.g., Nannippus) and the presence of Glyptotherium arizonae, a species restricted to early lrvingtonian sites elsewhere in the southwestern United States, suggests that the vertebrate fauna from the upper portion of the Pearson Mesa section is Irvingtonian in age.