A horse is a horse, of course — except when it isn’t

A depiction of a family of stilt-legged horses (Haringtonhippus francisci) in Yukon, Canada, during the last ice age. Image: Credit: Jorge Blanco
A depiction of a family of stilt-legged horses, Haringtonhippus francisci in Yukon, Canada, during the last ice age. Image: Credit: Jorge Blanco

A previously unrecognized genus of extinct horses that roamed North America during the last ice age has been discovered by an international team of researchers.

The new findings, published this week in the journal eLife, are based on an analysis of ancient DNA from fossils of the puzzling “New World stilt-legged horse” excavated from sites such as Natural Trap Cave in Wyoming, Gypsum Cave in Nevada, and the Klondike goldfields of Canada’s Yukon Territory.

Before this study, these thin-limbed, lightly built 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.

Two skulls of the new genus Haringtonhippus from Nevada (upper) and Texas (lower). Credit: University of California Berkeley
Two skulls of the new genus Haringtonhippus from Nevada (upper) and Texas (lower). Photo University of California Berkeley

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

“The horse family, thanks to its rich and deep fossil record, has been a model system for understanding and teaching evolution,” said first author Peter Heintzman, who led the study as a postdoctoral researcher at the University of California, Santa Cruz.

“Now ancient DNA has rewritten the evolutionary history of this iconic group.”

Senior author Beth Shapiro, professor of ecology and evolutionary biology the university, said the evolutionary distance between the extinct stilt-legged horses and all living horses took the researchers by surprise.

“But it presented us with an exciting opportunity to name a new genus of horse.”

The team named the new horse after Richard Harington, emeritus curator of quaternary paleontology at the Canadian Museum of Nature in Ottawa.

Harington, who was not involved in the study, spent his career studying the ice age fossils of Canada’s North and first described the stilt-legged horses in the early 1970s.

“I had been curious for many years concerning the identity of two horse metatarsal bones I collected, one from Klondike, Yukon, and the other from Lost Chicken Creek, Alaska,” he recalls.

The long, thin, stilt-like leg bone of Haringtonhippus on the right compares to that from a regular horse (Equus). Credit: Grant Zazula
The long, thin, stilt-like leg bone of Haringtonhippus on the right compares to that from a regular horse (Equus). Photo: Grant Zazula

“They looked like those of modern Asiatic kiangs, but thanks to the research of my esteemed colleagues they are now known to belong to a new genus.”

He says he is delighted to have the new genus named after him.

The new findings show that 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, Haringtonhippus 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.

However, Equus survived in Eurasia beyond the last ice age, eventually leading to domestic horses. In contrast, the stilt-legged Haringtonhippus was an evolutionary dead end.

Study co-author Grant Zazula, a Government of Yukon paleontologist, said the researchers were pleased to name this new horse genus after their friend and colleague Dick Harington.

“There is no other scientist who has had greater impact in the field of ice age paleontology in Canada than Dick,” he says.

“Our research on fossils such as these horses would not be possible without Dick’s life-long dedication to working closely with the Klondike gold miners and local First Nations communities in Canada’s North.”

Another co-author, Eric Scott, a paleontologist at California State University San Bernardino, said that morphologically, the fossils of Haringtonhippus are not all that different from those of Equus.

“But the DNA tells a fascinatingly different story altogether,” he said. “That’s what is so impressive about these findings. It took getting down to the molecular level to discern this new genus.”

In addition to Heintzman, Shapiro, Zazula, and Scott, the co-authors include Ross MacPhee at the American Museum of Natural History; James Cahill, Joshua Kapp, and Mathias Stiller at UC Santa Cruz; Brianna McHorse at Harvard University; Matthew Wooller at the University of Alaska Fairbanks; Ludovic Orlando at the Natural History Museum of Denmark; John Southon at UC Irvine; and Duane Froese at the University of Alberta, Edmonton. This work was funded by the National Science Foundation and the Gordon and Betty Moore Foundation.

A new genus of horse from Pleistocene North America
Peter D Heintzman, Grant D Zazula, Ross DE MacPhee, Eric Scott, James A Cahill, Brianna K McHorse, Joshua D Kapp, Mathias Stiller, Matthew J Wooller, Ludovic Orlando, John Southon, Duane G Froese, and Beth Shapiro.
eLife 2017;6:e29944 DOI: 10.7554/eLife.29944

The study, published under a Creative Commons License, can be read here.


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