Locations that are rich in fossils often have intriguing stories behind them. Some are the result of landslides that killed and covered animals long ago. Others were once tar pools that became covered in water, snaring unsuspecting animals that waded in for a drink.

A site discovered last year in the Colorado mountains hints at another type of grisly demise. Researchers told the Society of Vertebrate Paleontology’s annual conference in Las Vegas last week that the unfortunate beasts found there may have become trapped not in tar but quicksand, formed in a deadly earthquake.

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The site was uncovered on 14 October 2010, when a bulldozer ran into a number of mammoth bones at Ziegler Reservoir near Snowmass, Colorado. Palaeontologists were called out, including Kirk Johnson of the Denver Museum of Nature and Science, and found the nearly complete skeleton of a Pleistocene mammoth (Mammuthus columbi).

The team went on to find more than 600 other bones, also in near perfect shape, most of them mammoths and mastodons (Mammut americanum) from the same Ice Age time period. A few other species, such as bison (Bison latifrons), were mixed in among them.

The discovery is rare because it is 2,671 metres above sea level, and very few high-elevation sites from the Pleistocene epoch are known. But perhaps the most intriguing aspect of the site is how it became home to so many perfectly preserved bones.

“The animals seem to have died and then their skeletons disarticulated, scattering a bit but otherwise in pristine condition,” Johnson told a packed conference hall in Las Vegas on 4 November. “We were baffled.”

Sticky problem

An explanation was suggested by Daniel Fisher of the University of Michigan in Ann Arbor, another palaeontologist working at the site. Fisher knew that mastodon tusks laid down daily growth rings, which can be used to pin down to the day when an animal died. He and his team compared the tusks of the mastodons at the site to work out if they all died at the same moment, as would be expected if they perished in a catastrophic event.

Fisher found that this wasn’t the case. A youngster at the site seems to have died several months before most of the adults, with the largest males expiring last.

So the team wondered whether the animals could have starved to death after becoming trapped, something that would kill small animals first. There is no tar at the site but when they looked at the geology of the region, another idea struck them: quicksand.

The fossil site — in a region prone to earthquakes — was a shallow lake with very fine sediment at its bottom that the mastodons had been wandering through. “We think the animals were standing in the lake when an earthquake hit and that this caused the water-logged fine sediments of the lake to suddenly become a quicksand-like substance,” says Johnson. “It solidified again when the earthquake ended, snagging them in place.”

This effect, called liquefaction, is well known in geology. The researchers propose that once trapped in the sediment, the mastodons and mammoths slowly starved to death. Youngsters with few fat reserves died first, followed by smaller adults and finally the largest males.

Johnson and Fisher are finding other, similar layers of fossils at the site, suggesting that liquefaction may have taken place multiple times over thousands of years as the lake slowly filled up with sediment, presumably caused by an earthquake and each time catching out animals eager for fresh water at altitude.

More work is needed to test the idea — including analysis of more fossils and a detailed survey of the geology of the area. But if the suggestion does prove to be correct, it will place the Colorado site among the most insidious of fossil traps known.