An analysis of mitochondrial and nuclear DNA from fossils of extinct New World stilt-legged horses reveals that, contrary to previous findings, these enigmatic animals actually belonged outside of the horse genus Equus.

“The horse family, thanks to its rich and deep fossil record, has been a model system for understanding and teaching evolution. Now ancient DNA has rewritten the evolutionary history of this iconic group,” said Dr. Peter Heintzman, a postdoctoral researcher at the University of California, Santa Cruz, and lead author of the study, published in the journal eLife.

Prior to this study, New World stilt-legged horses were thought to be related to the Asiatic wild ass or onager, or simply a separate species within the genus Equus, which includes living horses, asses, and zebras.

The new results, however, reveal that these horses were not closely related to any living population of horses.

Now named Haringtonhippus francisci, this extinct species appears to have diverged from the main trunk of the family tree leading to Equus some 4 to 6 million years ago.

“The evolutionary distance between the extinct stilt-legged horses and all living horses took us by surprise, but it presented us with an exciting opportunity to name a new genus of horse,” said study senior author Professor Beth Shapiro, also from the University of California, Santa Cruz.

According to the study, Haringtonhippus francisci was a widespread and successful species throughout much of North America, living alongside populations of Equus but not interbreeding with them.

In Canada’s North, the species survived until roughly 17,000 years ago, more than 19,000 years later than previously known from this region.

At the end of the last Ice Age, both horse groups became extinct in North America, along with other large animals like woolly mammoths and saber-toothed cats.

“Morphologically, the fossils of Haringtonhippus francisci are not all that different from those of Equus. But DNA tells a fascinatingly different story altogether,” said co-author Dr. Eric Scott, from California State University San Bernardino.

_____

Peter D. Heintzman et al. A new genus of horse from Pleistocene North America. eLife 2017 (6): e29944; doi: 10.7554/eLife.29944