DNA analysis has shown that the enigmatic species known as "stilt-legged horse" that roamed North America during the last ice age was not closely related to any living population of horses.For the study, published in the journal eLife, the researchers analysed DNA from fossils of the animal excavated from the US and Canada.Prior to 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.Now named Haringtonhippus francisci, this extinct species of North American horse probably diverged from the main trunk of the family tree leading to Equus some four to six million years ago, the study said."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 first author Peter Heintzman, who led the study as a postdoctoral researcher at the University of California, Santa Cruz."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 senior author Beth Shapiro, Professor at UC Santa Cruz.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.The new findings showed that Haringtonhippus francisci was a widespread and successful species throughout much of North America, living alongside populations of Equus but not inter-breeding with them. In Canada's northern regions, 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 sabre-toothed cats. Although Equus survived in Eurasia after the last ice age, eventually leading to domestic horses, the stilt-legged Haringtonhippus was an evolutionary dead end, the study said.