Abstract
Equids are among the most common mammals found in faunal assemblages of Late Pleistocene age in Mexico. Much of what is known about the Equus species is the result of studies conducted in central and northern Mexico; much less is known about species in lower latitudes of Mexico. Here we describe three species that inhabited Oaxaca and Chiapas states. The fossil localities are in northwestern and central Oaxaca, as well as the central part of Chiapas. In Oaxaca, the largest species, Equus mexicanus, and the medium-sized Equus conversidens are represented by mandibles, skulls, diverse isolated teeth and some postcranial bones, while the smallest species, Haringtonhippus francisci is represented by a skull fragment and few isolated teeth. In Chiapas, E. mexicanus is represented by a mandible and several isolated teeth, E. conversidens by several mandibles and diverse isolated teeth, and H. francisci by isolated teeth and two mandibles. AMS radiocarbon and uranium dating of some of the equid localities in Oaxaca and Chiapas indicate that they were at least present since approximately 44,000 Cal BP years, they were common around 30,000 Cal BP years, and were still present at the end of the Pleistocene, around 12,000 years ago. The record of H. francisci from Chiapas is the youngest in North America. A cluster analysis of extended mesowear data and a discriminant analysis showed that Equus conversidens from Chiapas was obligate grazer, whilst the rest of the equids were variable grazers. Geographic distribution of localities in southern Mexico indicates that during the Pleistocene the equid species moved across the Transvolcanic Belt-Sierra Madre del Sur temperate biogeographic corridor and the Tamaulipas-Central America Gulf Lowlands tropical corridor.
Highlights
Equids are among the most common mammals in the Pleistocene Mexican faunal assemblages
Since we evaluated morphological characters of teeth and bones of fossil horses from southern Mexico and to avoid the unnecessary proliferation of names for the Mexican equid species, we follow the taxonomic scheme of Priego-Vargas et al (2017, table 10) that recognized four Pleistocene Equus species for Mexico: Equus conversidens, E. mexicanus, E. francisci, and E. cedralensis
Three Pleistocene equid species were identified in southern Mexico, the largest is Equus mexicanus, the medium-sized is E. conversidens and the smallest is H. francisci
Summary
Equids are among the most common mammals in the Pleistocene Mexican faunal assemblages. Barrón-Ortiz et al (2017), based on linear and geometric morphometric analyses of Equus specimens from the western interior of North America recognized three morphological groups of Equus in northeastern Mexico (San Josecito Cave, Nuevo León, and Cedral localities of San Luis Potosí) and they taxonomically identify such groups as E. ferus, E. conversidens and E. cedralensis; based on mitochondrial DNA analyses they recognized two equid clades [Caballine and New World Stilt-legged horses (NWSL)] and they referred the NWSL clade to E. conversidens. By means of paleogenomic and morphometric analyses, a new genus of NWSL horse for the Pleistocene of North America was erected: Haringtonhippus francisci This species was reported (as Equus conversidens) from San Josecito Cave, Nuevo Léon, based upon a short mitochondrial DNA sequence (BarrónOrtiz et al, 2017); this record was subsumed into H. francisci by Heintzman et al (2017). Barrón-Ortiz et al (2019) based on a morphology-based tree, determined that Equus comprises eight species, including H. francisci, and suggested that Haringtonhippus should be considered a synonym of Equus, but the authors do not formally synonymize both taxa
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