A new Miocene local fauna from the Sierra Madre Oriental at San Luis Potosí, Central-East Mexico,and its paleontologic significance

Mexico's Late Neogene mammal faunas are largely known from localities in the Trans-Mexican Volcanic Belt; those from other morphotectonic provinces are few and far apart. Thus, the discovery of Late Miocene vertebrates in western Sierra Madre Oriental at San Luis Potosí, the Paso del Águila local fauna, significantly adds to this meager record. The assemblage was collected from the floodplain facies of the San Nicolás Formation, a ∼1100-m thick, dominantly fluviolacustrine and calcilithitic, 15°–20° NE dipping sequence preserved in the Peotillos-Tolentino Graben, between 22°11’–22°19’ N and 100°30’–100°39° W. It includes remains of cf. Trachemys, a small to medium-sized emydid chelonian, a large camelid, a small cervid and a new species of the equini Pliohippus s.s., comparable in size, cranial morphology and odontographic characters to the Clarendonian-Early Hemphillian horses of the Pliohippus clade. Ar-Ar dates from ash-fall tuffs seemingly above and below the fossiliferous strata, bracket the age between 12.33 and 7.41 Ma (i.e., late Middle to Late Miocene), that is, within the Late Clarendonian-Early Hemphillian NALMA interval, making this fauna the first in Mexico from this age. The Paso del Águila local fauna is at least partly correlative with the Hemphillian local faunas from the TMVB and adjacent areas (e.g., Rancho El Ocote, Guanajuato and Tecolotlán, Jalisco), the Central Plateau (e.g., Arroyo Los Fragmentos, Zacatecas), and the Sierra Madre Occidental (e.g., Yepómera). Elsewhere, it is broadly correlative with the Late Clarendonian-Early Hemphillian faunas from the California Coast Ranges (e.g., North Tejon Hills, Ricardo and Dove Springs in the Mohave Desert), and the Gulf Coast Plain, Florida (McGehee Farm and Mixon). The Paso del Águila local fauna was part of a subtropical savannah and pine-oak forest (with a well-developed understory) biome that thrived on a climate regime much more humid than today.