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.