Interest in the origin and evolution of Equus dates back to over a century, but there is still no consensus on the definition of the genus or its phylogenetic position. We review the placement of Equus within several phylogenetic frameworks and present a phylogenetic analysis of derived Equini, including taxa referred to Equus, Haringtonhippus, Dinohippus, Astrohippus, Hippidion, and Boreohippidion. A new, morphology-based phylogenetic tree was used as an initial hypothesis for discussing what taxa Equus encompasses, using four criteria previously used to define the genus category in mammals: phylogenetic gaps, uniqueness of adaptive zone, crown group definition, and divergence time. According to the phylogenetic gaps criterion, Equus encompasses clade 6 (Ha. francisci = E. francisci, E. conversidens, E. quagga, E. hemionus, E. mexicanus, E. ferus, E. occidentalis, and E. neogeus) based on morphological synapomorphies. Equus is assigned to clade 6, or possibly clade 7, according to the uniqueness of adaptive zone criterion. The crown group criterion places Equus at clade 6. Based on the time-calibrated phylogeny of Equini, the divergence time criterion suggests that Equus encompasses clade 9. This clade comprises all taxa traditionally assigned to Equus analyzed in our study, including the eight taxa listed above as well as E. stenonis, E. idahoensis, and E. simplicidens; the latter two are sometimes referred to the subgenus Plesippus and the former to the subgenus Allohippus. With the exception of the divergence time criterion, the results of our evaluation are congruent in identifying clade 6 as the most suitable position for Equus. The taxonomic implications of delimiting Equus to clade 6 in our phylogenetic tree include elevation of Allohippus and Plesippus to generic rank, assignment of a new genus to "Dinohippus" mexicanus, and synonymy of Haringtonhippus with Equus.

The Muhi quarry of Zimapán Hidalgo, Central México, is one of the most important Cretaceous fish localities of the country because of the great diversity of fishes, and the significance of their record in America. Here we present some advances of the stratigraphy, including a litostratigraphic description, a generalized report of the marine fauna recovered, and the depositional environment of the outcrop. The sequence consists of a light gray mudstone/biomicrite, with chert forming nodules and/or thin layers, and additional marls are intercalated; the strata are assigned to La Negra Facies of the Mid-Cretaceous (Albian-Cenomanian) El Doctor Formation from northeastern Querétaro, south-central and northwestern Hidalgo (this report). The lithology and fossils are indicative of open sea shelf to deep shelf margin Standard Facies Zones, comparable to the Standard Microfacies Types SMF 2 (microbioclastic limestone) and SMF 3 (pelagic mudstone).

-TAXONOMIC REVIEW OF THE PLEISTOCENE EQUIDS FROM MEXICO BASED ON DENTAL MORPHOLOGY.A quantitative and qualitative evaluation of the dental traits of five horse species from 15 fossiliferous areas from the Pleistocene of Mexico was performed. The purpose of this study was to evaluate whether Equus mexicanus, E. conversidens, E. excelsus, E. francisci (= E. tau) and E. cedralensis show particular dental morphologies (measurements and occlusal morphology of the molariform) that provide information of systematic significance. To this end, premolars and molars of adult individuals were used to evaluate a set of ten quantitative variables that were analyzed by bivariate and multivariate methods; also, 13 qualitative variables were used to estimate the frequency in the configuration and development degree of the enamel pattern on the occlusal surface for each species. Our results indicate that 60% of the quantitative variables and 40% of the qualitative provide information to distinguish dental morphologies. Based on measures and the enamel pattern the following specific characteristics were recognized: E. mexicanus: large-size occlusal area and relatively complex enamel pattern, E. conversidens: medium-size and moderately complex occlusal enamel and E. francisci: small-size and relatively simple enamel pattern on the occlusal surface. Concerning E. excelsus and E. cedralensis, their dental morphology is closely similar to those of E. conversidens and E. francisci respectively. The observed dental morphology of the Mexican specimens of E. excelsus indicates that part of those should be reconsidered as E. conversidens and part as E. mexicanus. Nevertheless, E. francisci and E. cedralensis are different species that can be recognized by limb characters rather than dental traits. Furthermore, the developing of particular dental patterns in these species is a factor that allowed their coexistence in Mexico during the Pleistocene.Key words: dental morphology, Equidae, Equus, taxonomy, Pleistocene, Mexico.RESUMO -No presente estudo foi realizada uma avaliação quantitativa e qualitativa das características dentárias de cinco espécies de cavalos de 15 áreas fossilíferas do Pleistoceno mexicano. Os objetivos do estudo foram de avaliar se Equus mexicanus, E. conversidens, E. excelsus, E. francisci (= E. tau) e E. cedralensis mostram morfologias dentárias particulares (medidas e morfologia oclusal de molariformes) que forneçam informações de importância sistemática. Para tal, pré-molares e molares de indivíduos adultos foram utilizados para avaliar um conjunto de dez variáveis quantitativas que foram analisadas por métodos bivariantes e multivariantes. Também foram utilizadas 13 variáveis qualitativas para estimar a frequência no grau de configuração e desenvolvimento do padrão de esmalte na superfície oclusal para cada espécie. Os resultados indicaram que 60% das variáveis quantitativas e 40% das qualitativas proveram informações para distinguir morfologias dentárias. De acordo com as medidas e o padrão do esmalte foram...

Pleistocene camels from Mexico include representatives of llamas and camels. Their record spans from the Early Blancan to the

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Proyecto académico sin fines de lucro, desarrollado bajo la iniciativa de acceso abierto Gliptodontes pleistocénicos del Centro de México 267 ResumenTrabajo paleontológico realizado en la región centro-meridional de Hidalgo, permitió recuperar una muestra importante de restos pertenecientes a Glyptodontidae. El material se recuperó de una secuencia sedimentaria innominada que consiste en arcillas, limos y gravas de pobre a moderadamente consolidadas de origen fluviolacustre; los ejemplares están asociados a restos de bisonte (Bison), que sugiere una antigüedad asignable al Rancholabreano. La muestra incluye tres fragmentos de coraza y 10 osteodermos aislados. Los osteodermos son hexagonales con dos a cinco folículos pilosos, la figura central es poligonal, cóncava y ligeramente sobreelevada; alrededor de la figura central existen de ocho a nueve figuritas periféricas dispuestas de manera simétrica y configuración burdamente trapezoidal. El patrón de la roseta corresponde al observado en Glyptotherium floridanum y, por lo tanto, es que los ejemplares hidalguenses se asignan a esta especie. En México,