What's old is new: A new horse genus emerges through DNA analysis of North American fossils

A family of stilt-legged horses (Haringtonhippus francisci) in Yukon, Canada, during the last ice age is depicted in this illustration. (Illustration courtesy of Jorge Blanco, UC Santa Cruz.)

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Growing up, I read Marguerite Henry's All About Horses and Millicent Selsam's Questions and Answers about Horses with the laser focus of a horse-crazy preteen, memorizing the minutiae about horse breeds and colors and the evolutionary development from Eohippus to the modern-day horse. But it turns out that the equine family tree delineated in these cherished books has been revamped. An international research team has shaken up the equid family tree in a recently published study.1

In North America, horses from the Pleistocene (the geological epoch from 2.6 million to 11,700 years ago) have been classified into two major groups-the stout-legged horses and stilt-legged horses. Both groups became extinct near the end of the Pleistocene. Until now, the stilt-legged horses had been thought to be related to the Asiatic wild ass or a separate species within the genus Equus, which includes today's horses, zebras and asses. As a result of this new study, it is now thought that the New World stilt-legged horses are not closely related to any living population of horses.

Peter Heintzman's research team set out to resolve where the stilt-legged horses sit within the horse family tree by analyzing more ancient DNA than in previous studies. Their analyses showed that the stilt-legged horses were much more distinct than previously thought. In fact, they found that these animals actually belonged outside the genus Equus.

Heintzman and his fellow researchers named the new genus Haringtonhippus and showed that all stilt-legged horses belonged to a single species within this genus, Haringtonhippus francisci. (The new genus is named in honor of Canadian zoologist Charles Richard Harington. And it already has its own Wikipedia page. Check it out.) The extinct species appears to have diverged from the family tree that led to Equus between 4 and 6 million years ago. H. francisci was a widespread and successful species throughout North America, living alongside Equus.

Fossils of New World stilt-legged equids have been studied for more than a century, but it wasn't clear where they should be positioned within the family tree. Anatomical studies by earlier workers ended up being perplexing. According to the study, "That the cues of morphology have turned out to be misleading in this case underlines a recurrent problem in systematic biology, which is how best to discriminate authentic relationships within groups. … The solution we adopted here was to utilize both palaeogenomic and morphometric information in reframing the position of Haringtonhippus, which now clearly emerges as the closest known outgroup to all living Equus."

Reference

1. Heintzman PD, Zazula GD, MacPhee RDE, et al. A new genus of horse from Pleistocene North America. eLife 2017 (6): e29944; doi: 10.7554/eLife.29944.