About 23 million years ago, a species of rhinoceros - similar in size to the modern Indian rhino but lacking a horn - made its home in the challenging environment of the Canadian High Arctic, which at the time was warmer than it is now but still experienced snow and months of wintertime darkness.
Fossils of the polar rhino, named Epiatheracerium itjilik, were found on Devon Island, a landscape underlain by permafrost, in Canada's Arctic archipelago. With about 75% of its skeleton intact, scientists gained a good understanding of the animal. Its remains were discovered in Haughton Crater, one of Earth's northernmost impact craters, about 14 miles (23 km) wide.
The polar rhino lived early in the Miocene epoch, a time of diversification of many mammalian groups. Until this discovery, no rhinoceros was known to have lived in such a high latitude. The fossil site is in Nunavut, Canada's northernmost territory.
About three feet (one meter) tall at the shoulder, this species approximated the size of the modern Indian rhinoceros, and was smaller than modern African rhinos.
"Devon Island during the Miocene was much more temperate and forested, quite unlike the polar desert that is there today," said Danielle Fraser, head of palaeobiology at the Canadian Museum of Nature in Ottawa and lead author of the study published on Tuesday in the journal Nature Ecology and Evolution.
Summers would have been warm but winters cold enough for snowfall. Fraser compared the climate to that of southern Ontario or northern New York state in modern times, though there would have been months of winter darkness and months of summer daylight.
"So, it remains a mystery as to how animals like a rhino survived, though we know modern mammals dig through snow using hooves and antlers to access plants," Fraser said.
"Our study highlights the importance of the Arctic in mammal evolution," Fraser said. "We often think about the tropics as centers for biodiversity - and they are. But the more fossil discoveries we make in the Arctic, the more it is becoming clear that it was an essential region in the evolution of mammals."
The polar rhino fed on the leaves of trees and shrubs as it roamed a forest that, based on fossilized pollen at the site, featured pines, larch, alder, spruce and birch. Its fossils indicate it had a narrow muzzle, like browsing animals today.
The polar rhino may have had a coat of fur given the freezing winter temperatures, Fraser said. Large horned rhinos with extensive fur coats called woolly rhinoceroses lived during the last Ice Age, but they were not closely related to this species.
Other fossils from the site include those of the early seal Puijila darwini, which had feet rather than flippers.
Rhinos first appeared roughly 48 million years ago and spread to every continent but South America and Antarctica. Five species live today, whereas more than 50 are known from the fossil record.
While horns already had evolved in some rhinos, this species did not have one. It also was quite different from other Miocene North American rhinos such as Teleoceras, which was big, short-legged and barrel-chested like a hippo, with a small horn.
As detailed in a study published in July in which Fraser was a co-author, scientists were able to extract and sequence ancient proteins from the tooth enamel of the polar rhino. Proteins offer valuable information about an organism and survive much longer than DNA. That discovery helped the researchers better understand the rhinoceros family tree.
The polar rhino's closest relatives lived in Europe and in the Middle East and southwestern Asia. This indicates that its ancestors crossed from Europe into North America across a land bridge that previously was thought to have disappeared about 50 million years ago.
"Our study says rhinos were crossing for at least 20 million years longer than we thought. This is, in fact, supported by newer geological studies that show that the two North Atlantic routes - one from the UK over Iceland to Greenland and the other from Finland over Svalbard to Greenland - were potentially crossable into the Miocene," Fraser said.
