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article imageGenus of extinct North American horse was 'evolutionary dead end'

By Karen Graham     Nov 29, 2017 in Science
A new branch of the horse family, called the stilt-legged horse, has been named after the Canadian who first studied its fossil remains in the Yukon, where it lived until the end of the last ice age.
In a research article published in eLife Science on November 28, the study, conducted by an international team of researchers, reveals the ice age-era mammal was an “evolutionary dead end” in the horse family, which developed through the Equus genus that resulted in our modern-day horses, zebras and asses.
The fossils in the study were found in the Natural Trap Cave in Wyoming, Gypsum Cave in Nevada, and the Klondike goldfields of Canada's Yukon territory. The study authors say the "New World stilt-legged" (NWSL) equines were a "perplexing group of Pleistocene horses endemic to North America."
Two skulls of the new genus Haringtonhippus from Nevada (upper) and Texas (lower). Coauthor Eric Sco...
Two skulls of the new genus Haringtonhippus from Nevada (upper) and Texas (lower). Coauthor 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.
Co-author Eric Scott/eLife Science
The NWSL equines were taller, lighter, thinner stilt-legged horses with narrow hooves. They lived up until approximately 17,000 years ago and were endemic to North America. While the NWSI, along with other large mammals in North America did not survive the last Ice-Age, Equus survived in Eurasia which led to domestic horses.
The new research indicates the extinct NWSL diverged from the main trunk of the family tree leading to the Equus genus some 4 to 6 million years ago. This theory will end up changing the long-held belief that horse evolution was fairly straightforward.
The geographic distribution of Haringtonhippus francisci. Blue circles are east Beringian localities...
The geographic distribution of Haringtonhippus francisci. Blue circles are east Beringian localities. Red circles are contiguous USA localities. Orange circles are localities with tentatively assigned Haringtonhippus specimens only. The green-star-labeled HT is the locality of the francisci holotype - a single specimen upon which the description of a species is based - in Wharton County, Texas, USA.
Eric Scott/eLife Science
The new genus gets its own name
The new genus was named Haringtonhippus francisci, which is derived from the name of Canadian paleontologist Richard Harington, who first studied the fossils in the early 1970s.
At the time, the stilt-legged horse was thought to be related to the Asiatic wild ass, or another separate species belonging to the same genus as horses.
"I am delighted to have this new genus named after me,” Harington, emeritus curator of quaternary paleontology at the Canadian Museum of Nature, said in a news release from the study authors.
Researchers thought the stilt-legged horse was related to the Asiatic wild ass  or onager. This onag...
Researchers thought the stilt-legged horse was related to the Asiatic wild ass, or onager. This onager (Asiatischer Wildesel), Equus hemionus onager, is in the Zoo Augsburg.
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“The horse family, thanks to its rich and deep fossil record, has been a model system for understanding and teaching evolution,” first study author Peter Heintzman, of UC Santa Cruz, said in the news release. “Now, ancient DNA has rewritten the evolutionary history of this iconic group.”
"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 of ecology and evolutionary biology at UC Santa Cruz, according to the UK's Daily Mail.
The researchers analyzed 26 full mitochondrial genomes and 17 partial nuclear genomes from late Pleistocene NWSL equids, which revealed that individuals from both eastern Beringia and southern North America form a single well-supported clade that falls outside the diversity of Equus and diverged from the lineage leading to Equus, showing the new genus is separate.
More about Horses, North America, Canada, evolutionary dead end, DNA level
 
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