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By Claire Eamer
Alberta may have dinosaurs and British Columbia the Burgess Shale, but those aren’t the only treasure troves of ancient life in Canada. The Yukon has its own spectacular collection of long-lost animals.
The Yukon, says Yukon government palaeontologist Grant Zazula, is extraordinarily rich in fossils of animals from the Pleistocene. That’s the period from just over 2.5 million years ago until the end of the last major glaciation, less than 12,000 years ago. It was a period of dramatic climate shifts that saw ice sheets advance and retreat across most of Canada (and the rest of the world).
But not the Yukon.
Significant areas of the Yukon were ice-free through the Pleistocene, says Zazula, so the fossil record wasn’t ground to dust by continental glaciers as in some areas of Canada.
“Also, the permafrost has led to some incredible preservation of ice age fossils,” he says. “I also suspect the rolling, rugged terrain has something to do with it too – lots of slopes to create rapid, mass movement of sediment leading to rapid burial of dead animals.”
Some of those dead animals have made the news lately.
The giant horse in the ice
Zazula was one of the long list of co-authors on a June paper in the scientific journal Nature about sequencing the full genome of a horse that died about 700,000 years ago. It was a giant of a horse, 20 to 30 per cent bigger than its distant relatives, modern horses, and it roamed what were then dry grasslands along with other giants, such as woolly mammoths.
The horse’s foot bone was found at a placer gold mine on Thistle Creek, near the location where the Klondike Gold Rush took place more than a century ago. Like so many Yukon fossils, the bone was frozen in permafrost, and the permafrost is layered with lines of tephra, or volcanic ash, which have been dated. The foot bone was found just below a layer called the Gold Run tephra.
“It is likely that it has remained frozen for the last 700,000 years or so,” says Zazula. “There is good stratigraphic evidence from that site and others that also have the 700,000-year-old Gold
Run tephra that permafrost never completely melted out after that period. Even though it got warm during later interglacials, it was never warm enough to melt out the permafrost in the central Yukon discontinuous permafrost zone.”
Pushing back the DNA horizon
Even in frozen bones, genetic material deteriorates over that great expanse of time. Tyler Kuhn, program coordinator at the Yukon Beringia Interpretive Centre, has worked with ancient DNA. He says this was the best possible situation to get a genome that old. Nevertheless, just a few years ago, it would have been impossible.
“The technology has advanced so that you can piece it together from fragments as small as 25 base pairs,” he says. That’s a miniscule part of a genome made up of roughly 2.7 billion base pairs. The technique, Kuhn says, is called shotgun sequencing. “You sequence everything and then throw away everything not horse.”
That got rid of the microbes, soil bacteria, and bits of plants and animals that mixed with the horse DNA over the millennia. Then, with the help of a great deal of computing power, the remaining horse material was assembled into a genome that has shed new light on the evolution of horses.
One hump, two humps, no humps
Giant horses weren’t the only unexpected residents of the ancient Yukon. So far, the bones of two very different kinds of camel have been found in the territory.
One is the Yukon giant camel, known from a few bones found in the Old Crow region of northern Yukon. The bones date from about one million years ago, and they seem to come from a camel very similar to the giant camel recently discovered on Ellesmere Island in Nunavut, although that specimen is 3.5 million years old.
Farther south, in the goldfield region where the frozen horse bone was discovered, the remains of a much smaller camel have been found. This one is Camelops hesternus or the Western camel, formerly known by the delightful name of Yesterday’s camel. And it’s a whole different beast.
The Yukon giant camel and the Ellesmere Island camel belong to the true camel lineage that led to the modern dromedary and Bactrian camel. The Western camel belongs to another lineage entirely. Its closest living relatives are the llamas and alpacas of South America.
Drawings of the Western camel usually show it with a single dromedary-like hump, but that’s very much open to debate, says Kuhn. Its modern relatives have no humps, and it’s possible that the Western camel looked more like them than a dromedary. Since the hump contains no bones, we won’t know for sure unless a body with remaining soft tissue is found frozen beneath the northern soil.
Rethinking the timelines
In the summer of 2010, a graduate student in geology, Derek Turner, delivered a box of bones to Grant Zazula. He’d found them while studying exposed sediment layers along the White River in the southwest Yukon and had carefully recorded the details of their location.
Most of the bones belonged to the usual Yukon suspects, such as bison and horse. However, there was one oddity that puzzled Zazula at first.
Then he thought, “Could it be a camel? Really? No… couldn’t be. We never find camels.” Just in case, he pulled out a guide to the bones of ancient camels, and there it was. “The bone was bang on with the drawing of a proximal phalanx.”
Bones from ancient camels were not new in the Yukon, but previous finds had been washed out in streams or otherwise separated from the geological context that could date them. These could be dated. According to the stratigraphy that Turner had mapped so carefully during his field work, the bone was deposited during an interglacial period between 115 and 87 thousand years ago.
Fossil pollen and other, larger plant fossils found with the bone reveal a cold world—not cold enough to bring the glaciers marching down to the White River, but too cold for the boreal forest. Fossil remains of algae and pondweed suggest that the camel’s body came to rest in a pond.
“I suspect that Camelops didn’t live here long past about 90,000 years ago,” Zazula says, adding that camels were probably wiped out by the next glacial cold period. “However, I am in the process of radiocarbon dating all the Yukon camel specimens to test my idea.”
Zazula hopes to have those rethought camel timelines complete and published sometime in the next few months. Meanwhile, Tyler Kuhn says the technological advances in recovering and analyzing ancient DNA means other Yukon fossils—including the camel bones—might have a story to tell as detailed as the one revealed by the Thistle Creek horse bone.