The Equidae family, encompassing modern horses, asses, and zebras, serves as a cornerstone for understanding macroevolution, boasting a rich fossil record spanning approximately 55 million years. This lineage traces its origins from small, dog-sized creatures known as hyracotheres through numerous intermediate forms and extinct branches, culminating in the Equus genus that includes all living equid species. However, the abundance of fossils has also led to the description of many questionable equid taxa, particularly within the Pleistocene Equus of North America. While numerous species have been identified from fossil evidence, molecular data suggests that many of these may have belonged to, or were closely related to, a single, highly variable species of stout-legged caballine horse, similar to the domestic horse, E. caballus.
A particularly puzzling group within this fossil record is the extinct “New World stilt-legged” (NWSL) equids. These animals exhibit a perplexing combination of physical traits. Their slender, stilt-like lower limb bones and narrow hooves bear a resemblance to extant Eurasian hemionines, such as the Asiatic wild asses (E. hemionus, E. kiang). Conversely, their dentition has been interpreted by some researchers as more consistent with caballine horses, while others suggest it aligns better with hemionines. This dichotomy has traditionally led to the classification of NWSL equids as being allied with hemionines due to their limb bone morphology.
Shifting Phylogenetic Perspectives
Historically, the slender distal limb bones of NWSL equids have led researchers to group them with hemionines. However, palaeogenetic analyses, particularly those using mitochondrial DNA (mtDNA), have consistently placed these NWSL equids closer to caballine horses. This has led to the prevailing phylogenetic model suggesting that the stilt-legged morphology evolved independently in both the New and Old Worlds, potentially as a result of convergent adaptation to arid climates and habitats.
These mtDNA-based models, however, have relied on data that raise concerns. Some studies utilized short control region sequences, which can be unreliable for resolving the placement of major equid groups. Other analyses were based on incomplete or otherwise problematic mitochondrial genome sequences. Given this ongoing uncertainty, which hinders a comprehensive understanding of Pleistocene equid evolution, a new study aimed to clarify the phylogenetic position of NWSL equids using a more robust dataset, including multiple mitochondrial and partial nuclear genomes from specimens across late Pleistocene North America.
Unraveling the Taxonomy and Evolutionary History
The earliest known fossils of NWSL equids date back to the late Pliocene or early Pleistocene, approximately 2 to 3 million years ago, in New Mexico. Over the middle and late Pleistocene, these animals tended to decrease in size and ranged across southern and extreme northwestern North America, including eastern Beringia (Alaska and Yukon Territory). Despite the assignment of several named species, such as E. conversidens and E. tau, there has been considerable confusion and disagreement regarding their taxonomy. Consequently, many researchers have opted to refer to them collectively as Equus (Hemionus) spp. or have avoided formal taxonomic designation altogether.
Radiocarbon dating has provided crucial insights into the extinction chronology of NWSL equids. In areas south of the continental ice sheets, these equids persisted during the last glacial maximum (approximately 26,000 to 19,000 years ago) until around 13,000 radiocarbon years before present. This was shortly before their extinction, which coincided with the demise of their caballine counterparts and most other large Pleistocene fauna. This timeline contrasts with findings from unglaciated eastern Beringia, where NWSL equids appear to have been locally extirpated earlier, during a milder interstadial period around 31,000 radiocarbon years before present, predating the last glacial maximum, the final extinction of caballine horses, and the arrival of humans in the region. The apparently divergent extirpation timelines north and south of the ice sheets suggest that NWSL equid populations responded variably to environmental pressures across their range, a pattern observed in other Pleistocene megafauna.
A New Genus Recognized: Haringtonhippus
The comprehensive analysis of 26 full mitochondrial genomes and 17 partial nuclear genomes from late Pleistocene NWSL equids has yielded a significant breakthrough. The study revealed that individuals from both eastern Beringia and southern North America form a single, well-supported clade. This clade falls outside the known diversity of Equus and diverged from the lineage leading to Equus during the latest Miocene or early Pliocene. This novel and robust phylogenetic placement warrants the recognition of NWSL equids as a distinct genus, which has been formally named Haringtonhippus.
Following a review of potential species names and conducting detailed morphometric and anatomical comparisons, the species name francisci Hay has been determined to be the most well-supported, based on the earliest-described specimen possessing diagnosable features. Therefore, the analyzed NWSL equid specimens are now referred to as H. francisci. New radiocarbon dates indicate that H. francisci was extirpated in eastern Beringia around 14,000 radiocarbon years before present. These findings have significant implications for understanding the Plio-Pleistocene evolutionary history of equids and contribute to the ongoing systematic revisions of equids and other Pleistocene megafauna.
The emergence of Haringtonhippus as a distinct genus provides a clearer framework for studying the evolutionary adaptations and extinction patterns of these unique North American equids. Further research into their paleoecology and interactions with their environment and other species will continue to illuminate this fascinating chapter of evolutionary history.
