Abstract
The extinct 'New World stilt-legged', or NWSL, equids constitute a perplexing group of Pleistocene horses endemic to North America. Their slender distal limb bones resemble those of Asiatic asses, such as the Persian onager. Previous palaeogenetic studies, however, have suggested a closer relationship to caballine horses than to Asiatic asses. Here, we report complete mitochondrial and partial nuclear genomes from NWSL equids from across their geographic range. Although multiple NWSL equid species have been named, our palaeogenomic and morphometric analyses support the idea that there was only a single species of middle to late Pleistocene NWSL equid, and demonstrate that it falls outside of crown group Equus. We therefore propose a new genus, Haringtonhippus, for the sole species H. francisci. Our combined genomic and phenomic approach to resolving the systematics of extinct megafauna will allow for an improved understanding of the full extent of the terminal Pleistocene extinction event.
Highlights
The family that includes modern horses, asses, and zebras, the Equidae, is a classic model of macroevolution
According to broadly accepted palaeontological interpretations, the earliest equids exhibiting morphologies consistent with New World stilt-legged’ (NWSL) and caballine attribution appear in the fossil record only ~2–3 and ~1.9–0.7 Ma ago (Eisenmann et al, 2008; Forsten, 1992), respectively, whereas our divergence estimates suggest that these lineages to have diverged between 4.1–5.8 and 3.8–4.5 Ma, most likely in North America
Dating incongruence might be attributed to an incomplete fossil record, but this seems unlikely given the density of the record for late Neogene and Pleistocene horses
Summary
The family that includes modern horses, asses, and zebras, the Equidae, is a classic model of macroevolution. The excellent fossil record of this family clearly documents its ~55 million year evolution. Genomics and Evolutionary Biology eLife digest The horse family – which includes zebras, donkeys and asses – is often featured on the pages of textbooks about evolution. All living horses belong to a group, or genus, called Equus. The fossil record shows how the ancestors of these animals evolved from dog-sized, three-toed browsers to larger, one-toed grazers. This process took around 55 million years, and many members of the horse family tree went extinct along the way
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