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Mammoth mystery at Snowmass

Excavation uncovers the bones of at least 22 different mastodons near Colorado ski resort
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ANGRY COUGAR CREEK No homes were lost due to flooding but Canmore second home owners don't qualify for flood relief funds.

A real whodunnit? Several dozen mastodons were found near Snowmass Village in Colorado, a quarter-mile from the ski slopes. How they got there is anybody's guess.

Did they die in repeated earthquakes? Probably not, paleontologists now say, but they haven't totally discarded the idea.

And what about that adult, male mammoth? Circumstantial evidence suggests it died 50,000 years ago, just as the climate began cooling again, signaling a return of the glaciers. But something about the rocks found with the bones didn't seem right. Could the rocks have been placed there by humans?

The evidence is tantalizing but inconclusive. The reality is that this mystery may never be unravelled.

Discouraged? Hardly. Scientists still think the trove of bones, leaves and other organic matter, retrieved in two brief bursts of intense digging in 2010 and 2011, ranks among the best time-capsules ever discovered from 50,000 to 130,000 years ago, the last time that glaciers receded.

"It's one of the premier finds of the last decade, and arguably — because of the high elevation and the quality of preservation — I think it is one of the five top Pleistocene sites in North America," says Ian Miller, curator of paleontology at the Denver Museum of Nature and Science. "It's right up there with the mammoth site at Hot Springs (South Dakota), La Brea (tar pits of Los Angeles) and the other top finds."

A bulldozer operator made the first discovery. On Oct. 14, 2010, Jesse Steele was enlarging the ancient lake to create a reservoir for Snowmass Village when he noticed something unusual. Bones of cattle are not uncommon in such places but an Internet search he and other employees of Gould Construction conducted that night suggested a far older animal: a mammoth. The species disappeared from North America about 8,000 years ago.

Two days later, at the invitation of the local water district, scientists from the Denver museum arrived to assess the bones. Reservoir excavation was halted and paleontologists were given authority to excavate the sites for about a month that fall, until snowfall and cold halted further work. In May 2011, wallowing in mud and assisted by dozens of volunteers, they resumed working in a sort of inverted anthill of methodical activity, wrapping up on July 4 to ride gleefully in Aspen's Fourth of July parade.

A layer of clay had created an exceptionally well-preserved archive of life at 2,700 metres during the last interglacial period. Spades upturned leaves still green and insects iridescent after 50,000 years. Logs that had gathered along the ancient shoreline were so well preserved that chainsaws were used to slice them.

"It's really a crystal clear picture into the ecosystem archives between 50,000 and 130,000 years ago," says Miller.

Occasionally there were shouts of glee as Miller and other scientists, plus other specialists, triumphantly hoisted bones. They found the remains of 40 different animals, from mice to a bison now extinct, and a ground sloth many times the size of what currently exists. Also, a camel.

Mostly they found elephants: the grass-eating mammoths, three metres tall at the shoulder but with long, curving tusks and trunks shorter than those found in elephants today in Asia and Africa. Deeper in the sediment and more plentiful were mastodons, which are slightly shorter than mammoths and have teeth better adapted to eating leaves and twigs.

As the bones of at least 34 individuals were pulled out, it's among the biggest mastodon discoveries in North America.

Just how so many mastodon bones came to exist in one place has perplexed scientists. One early hypothesis was that an earthquake, or perhaps several at different times, had occurred while the mastodons were in the lake. Earthquakes can have the effect of liquefying lake sediments, anchoring whatever animals are stuck in them like quicksand and preventing escape.

If not totally discarded, scientists now favour a simpler explanation of individual deaths from less exceptional causes, the bodies then scavenged by various opportunists. "Think of a watering hole in Africa," says Miller. "That seems to be the best analogy."

But a mammoth found near the surface poses a mystery. Rocks were intermixed among the bones in ways that didn't seem natural. Quietly, the scientists hypothesized among themselves, wondering if the rocks had been used by people to cache the meat for later use.

If that were the case, this would be one of the top scientific discoveries of the decade. Archaeologists have only been able to confirm human existence in the Western Hemisphere to about 14,000 to 15,000 years ago, near the end of the last ice age. This would, if proven, extend human habitation to 50,000 years ago.

With the need to get out of the lake in 2011 while it was converted into a regulated reservoir, museum personnel wrapped this ensemble of mammoth bones, peat and rocks into a protective cast of plaster for transport to Denver.

Once in Denver, they took three-dimensional photographs, so that scientists in the future can see everything just as they found it.

Top archaeologists from around the country were invited to examine the remains. "Everybody was super intrigued," says Miller. "Nobody was like, 'You guys are out to lunch.' But at the same time everybody said, 'you need to find extraordinary evidence to indeed prove that it was a human cache."

So far, that extraordinary evidence is wanting — and may remain so. "It will likely remain a mystery, but sometimes that's science!" says Miller.

Instead, the greater significance of the Snowmass find is likely to be its ability to document the changing climate of that last interglacial period, when humans had little or no influence on the planetary environment. That record, in turn, may help us better evaluate our own era, with humans more clearly driving changes.

Important to the value of the Snowmass site is its elevation, nearly 2,700 metres — high elevations are more sensitive to changes than lower elevations.

Miller is waiting with "bated breath" as some 47 to 50 scientists of various disciplines continue their laboratory tests and author papers. The 18 ensuing papers will be subjected to peer review and then published early in 2014.

"I really think it will give us some new insights into the penultimate interglacial period, and it might give us new understandings about what is happening today in terms of global warming," says Miller. "That's the million dollar question, and after spending $1 million (on the dig), we would like to have a very good answer to that."