Horses were a dominant component of North American Pleistocene land mammal communities and their remains are well represented in the fossil record. Despite the abundant material available for study, there is still considerable disagreement over the number of species of Equus that inhabited the different regions of the continent and on their taxonomic nomenclature. In this study, we investigated cheek tooth morphology and ancient mtDNA of late Pleistocene Equus specimens from the Western Interior of North America, with the objective of clarifying the species that lived in this region prior to the end-Pleistocene extinction. Based on the morphological and molecular data analyzed, a caballine (Equus ferus) and a non-caballine (E. conversidens) species were identified from different localities across most of the Western Interior. A second non-caballine species (E. cedralensis) was recognized from southern localities based exclusively on the morphological analyses of the cheek teeth. Notably the separation into caballine and non-caballine species was observed in the Bayesian phylogenetic analysis of ancient mtDNA as well as in the geometric morphometric analyses of the upper and lower premolars. Teeth morphologically identified as E. conversidens that yielded ancient mtDNA fall within the New World stilt-legged clade recognized in previous studies and this is the name we apply to this group. Geographic variation in morphology in the caballine species is indicated by statistically different occlusal enamel patterns in the specimens from Bluefish Caves, Yukon Territory, relative to the specimens from the other geographic regions. Whether this represents ecomorphological variation and/or a certain degree of geographic and genetic isolation of these Arctic populations requires further study.

The horse is central to many Indigenous cultures across the American Southwest and the Great Plains. However, when and how horses were first integrated into Indigenous lifeways remain contentious, with extant models derived largely from colonial records. We conducted an interdisciplinary study of an assemblage of historic archaeological horse remains, integrating genomic, isotopic, radiocarbon, and paleopathological evidence. Archaeological and modern North American horses show strong Iberian genetic affinities, with later influx from British sources, but no Viking proximity. Horses rapidly spread from the south into the northern Rockies and central plains by the first half of the 17th century CE, likely through Indigenous exchange networks. They were deeply integrated into Indigenous societies before the arrival of 18th-century European observers, as reflected in herd management, ceremonial practices, and culture.

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.

Studies of horse evolution arose during the middle of the 19th century, and several hypotheses have been proposed for their taxonomy, paleobiogeography, paleoecology and evolution. The present contribution represents a collaboration of 19 multinational experts with the goal of providing an updated summary of Pliocene and Pleistocene North, Central and South American, Eurasian and African horses. At the present time, we recognize 114 valid species across these continents, plus 4 North African species in need of further investigation. Our biochronology and biogeography sections integrate Equinae taxonomic records with their chronologic and geographic ranges recognizing regional biochronologic frameworks. The paleoecology section provides insights into paleobotany and diet utilizing both the mesowear and light microscopic methods, along with calculation of body masses. We provide a temporal sequence of maps that render paleoclimatic conditions across these continents integrated with Equinae occurrences. These records reveal a succession of extinctions of primitive lineages and the rise and diversification of more modern taxa. Two recent morphological-based cladistic analyses are presented here as competing hypotheses, with reference to molecular-based phylogenies. Our contribution represents a state-of-the art understanding of Plio-Pleistocene Equus evolution, their biochronologic and biogeographic background and paleoecological and paleoclimatic contexts.

Approximately 50,000–11,000 years ago many species around the world became extinct or were extirpated at a continental scale. The causes of the late Pleistocene extinctions have been extensively debated and continue to be poorly understood. Several extinction models have been proposed, including two nutritionally based extinction models: the coevolutionary disequilibrium and mosaic-nutrient models. These models draw upon the individualistic response of plant species to climate change to present a plausible scenario in which nutritional stress is considered one of the primary causes for the late Pleistocene extinctions. In this study, we tested predictions of the coevolutionary disequilibrium and mosaic-nutrient extinction models through the study of dental wear and enamel hypoplasia of Equus and Bison from various North American localities. The analysis of the dental wear (microwear and mesowear) of the samples yielded results that are consistent with predictions established for the coevolutionary disequilibrium model, but not for the mosaic-nutrient model. These ungulate species show statistically different dental wear patterns (suggesting dietary resource partitioning) during preglacial and full-glacial time intervals, but not during the postglacial in accordance with predictions of the coevolutionary disequilibrium model. In addition to changes in diet, these ungulates, specifically the equid species, show increased levels of enamel hypoplasia during the postglacial, indicating higher levels of systemic stress, a result that is consistent with the models tested and with other climate-based extinction models. The extent to which the increase in systemic stress was detrimental to equid populations remains to be further investigated, but suggests that environmental changes during the late Pleistocene significantly impacted North American equids.

Climate affects habitat, food availability, and the movement and sustainability of all life. In this work, we apply Indigenous and Western scientific methods, including genomics and isotope profiling, on fossils from across Beringia to explore the effect of climate change on horses. We find that Late Pleistocene horses from Alaska and northern Yukon are related to populations from Eurasia and crossed the Bering land bridge multiple times during the last glacial interval. We also find deeply divergent lineages north and south of the American ice sheets that genetically influenced populations across Beringia and into Eurasia. As climate warmed and horses entered the ice-free corridor connecting Beringia and midcontinental America, restricted mobility and food availability impeded population growth. Our combined Western and Indigenous framework offers critical guidance for wildlife conservation amid ongoing climate change.