The Equidae family, encompassing modern horses, asses, and zebras, serves as a quintessential example of macroevolutionary processes. Its rich fossil record beautifully illustrates a ~55 million-year evolutionary journey, tracing back from diminutive, dog-sized ancestors like Hyracotherium through numerous transitional forms and extinct lineages to the extant genus Equus. However, this abundance of fossil evidence has also led to the proliferation of many dubious equid taxa, particularly within the Pleistocene epoch of North America. While numerous species have been identified from fossils, molecular data suggest that many of these likely belonged to, or were closely related to, a single, highly variable species of stout-legged caballine horse, which includes the domestic horse, Equus caballus.
A particularly enigmatic group within this fossil record is the ‘New World stilt-legged’ (NWSL) equids. These extinct animals present a perplexing combination of morphological traits. Their slender, stilt-like distal limb bones and narrow hooves bear a resemblance to extant Eurasian hemionines, such as the Asiatic wild asses (Equus hemionus, Equus kiang). Conversely, their dentition has been interpreted by some researchers as more consistent with caballine horses, while others suggest it aligns more closely with hemionines. This duality has historically led to significant debate regarding their precise evolutionary placement.
Taxonomic Quandaries and Phylogenetic Insights
On the basis of their slender distal limb bones, NWSL equids were traditionally classified as being closely related to hemionines. However, palaeogenetic analyses, particularly those utilizing mitochondrial DNA (mtDNA), have consistently indicated a closer relationship between NWSL equids and caballine horses. These mtDNA-based phylogenetic models propose that the stilt-legged morphology evolved independently in both the New and Old Worlds, potentially representing convergent adaptations to arid environments.
Despite these molecular findings, the current understanding of NWSL equid phylogeny has been based on limited and sometimes questionable data. Early studies relied on short mtDNA control region sequences, a data type known for its potential unreliability in resolving major equid groups. More recent analyses have utilized mitochondrial genome sequences that were either incomplete or presented other issues. This ongoing uncertainty about the phylogenetic position of NWSL equids has hampered a comprehensive understanding of Pleistocene equid evolution. To address this, a new study aimed to clarify their position by analyzing multiple mitochondrial and partial nuclear genomes from specimens across late Pleistocene North America.
Evolution and Extinction of NWSL Equids
The earliest recognized fossils of NWSL equids date back to the late Pliocene or early Pleistocene, approximately 2 to 3 million years ago, with finds in New Mexico. Middle and late Pleistocene NWSL equids were generally smaller than their early Pleistocene predecessors and inhabited regions across southern and extreme northwestern North America, including eastern Beringia (Alaska and Yukon Territory). Several species have been named, including E. conversidens, E. tau, E. francisci, and E. calobatus, but taxonomic classification remains a source of considerable confusion and disagreement. This ambiguity has led some researchers to group them collectively as Equus (Hemionus) spp. or to avoid formal taxonomic designations altogether.
Radiocarbon dating from Gypsum Cave, Nevada, indicates that NWSL equids survived in areas south of the continental ice sheets during the Last Glacial Maximum (LGM), approximately 26,000 to 19,000 years ago, persisting until around 13,000 radiocarbon years before present. They became extinct shortly thereafter, along with their caballine counterparts and most other Pleistocene megafauna. This timeline contrasts with data from unglaciated eastern Beringia, where NWSL equids appear to have been locally extirpated much earlier, during a milder interstadial interval around 31,000 radiocarbon years before present, predating the LGM, the final extinction of caballine horses, and the arrival of humans in the region. The differing extirpation chronologies north and south of the ice sheets suggest that NWSL equid populations responded variably to demographic pressures across their range, mirroring patterns observed in other megafauna.
Establishing a New Genus: Haringtonhippus
The recent comprehensive analysis of 26 full mitochondrial genomes and 17 partial nuclear genomes from late Pleistocene NWSL equids has provided significant new insights. 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 robust phylogenetic placement warrants the recognition of NWSL equids as a distinct genus, which has now been formally named Haringtonhippus. Based on a review of potential species names and detailed morphometric and anatomical comparisons, the species francisci Hay, described from the earliest known specimen exhibiting diagnosable features, has been identified as the most well-supported designation. Therefore, the analyzed NWSL equid specimens are now referred to as Haringtonhippus francisci. Furthermore, new radiocarbon dates indicate that H. francisci was extirpated in eastern Beringia around 14,000 radiocarbon years before present. These findings contribute significantly to our understanding of Plio-Pleistocene equid evolution and the systematics of equids and other Pleistocene megafauna.
