An ice age graveyard where dozens of huge animals including mammoths, mastodons and a giant ground sloth died up to 150,000 years ago has been unearthed near a popular mountain ski resort.
The fossilised remains, which were discovered in sediment at the bottom of a drained reservoir in the Rocky Mountains, Colorado, are thought to be one of the largest collection of animals from the last ice age to be found in one place and it is already providing scientists with new insights into the prehistoric environment.
Contractors preparing the ground for the construction of a new dam at the reservoir near Snowmass Village, which is part of the Aspen ski resort, uncovered the bones of a mammoth and now more than 600 bones have been recovered from beneath the lake bed before heavy snow halted the excavation.
Palaeontologists leading the dig found the remains of four Columbian mammoths; 10 American mastodons, a distant relation of the mammoth and elephant; four ice age bison, which were twice the size of modern bison; a species of ice age deer; and a Jefferson's ground sloth and a tiger salamander.
They expect to find more fossils when the return to the site when the snow melts in the spring.
Researchers, including experts at the Royal Holloway University of London, are now attempting to piece together how the animals came to be buried in one place and what the ice age landscape would have looked like at the time.
"It is an amazing site and is very unusual," said Dr Kirk Johnson, chief curator and vice president of research and collections at the Denver Museum of Nature and Science who led the excavation. "It is a true treasure trove of ice age fossils.
"Many of the fossils are pristine as they have been very well preserved. Some of the bones we recovered are still white while we are finding leaves that are still green and tree branches with the bark still on.
The first mammoth was discovered at the end of October when a bulldozer struck some of the ice age mammals' bones during work to expand the Ziegler Reservoir, which sits on a plateau at 8,870 feet in the Colorado Rocky Mountains.
Palaeontologists from Denver Museum of Nature and Science spent two weeks excavating a one acre wide area, unearthing more than 25 different animals from seven different species within a two-week period.
Among the most dramatic fossils to be found was the skull of an ice age bison, Bison latifrons, with three foot long horns. Weighing nearly 18 stone, the skull measures almost eight feet across from the tip of each horn when it was pieced together.
They also found more than 15 tusks including massive seven foot long mastodon tusks, bones of an as yet unknown species of ice age deer, the remains of a Jefferson's ground sloth, an extinct plant eater that grew to be the size of an ox.
They also discovered the remains of trees that bore teeth marks from an ice age beaver. Other bones bore teeth marks, suggesting some of the animals may have been killed by predators, although the palaeontologists have yet to find any remains of predators.
The animals are thought to have lived between 150,000 and 50,000 years ago when much of northern Europe and North America was covered in glaciers from the last ice age.
Scientists attempting to unravel what befell the creatures believe many of the animals may have gone to the area where their fossils were found to find drinking water as it is the site of an ice age lake.
This could explain the concentration of so many different creatures in one place as animals would have been drawn to the lake over tens of thousands of years and some will have died along its edge.
Experts think some of the animals may have been walking across the frozen surface of the lake and fallen through, causing them to be preserved in the sediment at the bottom of the lake.
Dr Johnson said: "We have a large variety of fossils through a long period of time through the ice age. Mammoths and mastodons are hardly ever found together on a single site as they lived in very different environments, so here we must have seen a change in the ecosystem around the lake.
"We are seeing two distinct ice age environments – the first was when the lake was fairly deep and had a lot of open water. We are finding mastodons, Jefferson's ground sloths and ice age deer.
"There is five feet of peat which when you tear open there are still green sedge leaves inside.
From what we have found it is clear the area was covered by a forest, but then later it seems to have become a marsh and it is in the sediment from this period that we are finding the mammoths.
"It shows this was far from being a frozen ice covered wasteland."
Unlike other large ice age fossil sites, which are often on the sites of former tar pits where the animals become trapped and died, these fossils were found beneath the lake.
It has meant that large quantities of plant matter, fossilised insects and other small invertebrates have also been preserved.
Professor Scott Elias, an expert in fossilised insects at Royal Holloway University of London, is helping palaeontologists involved in the dig to identify the insects and so learn more about the environment at the time.
He said: "There are some really large mammals that have come out of the sediment, but in many ways the more interesting story is coming from the insects fossilised inside the peat.
"They can tell us a lot about the environment as we can find similar species around today and draw conclusions about what it was like.
"I am seeing a couple of species of bark beetles that would have lived on juniper and Douglas firs at this altitude during the colder periods when the tree line came down to that level."