Uncovering the secrets of North America's Ice Age giants

Eighty thousand years ago the Earth began to cool, marking the start of the last Ice Age. Experts are still discovering how the big freeze affected the giant mammals that prowled its dramatically changing landscape.

Scientists are helping to uncover the secrets of giant Ice Age beasts like the sabretooth cat - by foraging in crates of dirt that were collected during the building of a car park.

Wooden boxes were built around deposits of earth in 2006 after construction workers discovered the near-complete skeleton of a woolly mammoth while digging underground parking spaces at the Los Angeles County Museum of Art.

Laura Tewksbury has worked in one crate for a year

The crates were designed to salvage the remains for scientists at nearby Page Museum while also making way for the bulldozers.

More than three million prehistoric fossils from 600 species have been excavated from California's tar pits, which trapped and preserved Ice Age creatures as they roamed the territory tens of thousands of years ago.

The 23 crates, which each have a resident palaeontologist, and 327 buckets of fossil material are yielding vital discoveries and are expected to take several years to examine in full.

Project 23 is just one of a series of explorations documented by a new BBC series, Ice Age Giants, which has combined cutting edge fossil studies and CGI to create lifelike footage of the prehistoric beasts.

Here are some of the Ice Age giants of North America. The continent was half-covered by an enormous ice sheet at the peak of the last Ice Age, but land south of the ice suddenly became richer than ever before.

Sabretooth cat

Despite the notoriety of the sabretooth cat's seven-inch fangs, it may actually have been its muscular forelimbs and large paws that made it so deadly.

The infamous predator's long, thin teeth were surprisingly vulnerable to snapping after becoming stuck in the sinews or bones of a violently struggling victim. It forced the cat to develop a unique killing method.

Modern day African big cats such as lions usually kill their prey by suffocation - smothering or crushing the windpipe. Their teeth barely break the skin.

But their Ice Age predecessor killed by stabbing and biting with its teeth, having first held the prey steady using its powerful paws and limbs.

Dr Blaire Van Valkenburgh, of the University of California, found the sabretooth had a huge temporal bone linking its jaw and skull - allowing it to open its mouth twice as wide as a lion and deliver huge blows to the throat.

"With those canines they could drive these two things together, then pull backwards and take out a large amount of flesh," she said.

"The animal would probably bleed to death within minutes," she added.

The sabretooth thrived across North America in the Ice Age, with many specimens found in modern day Los Angeles.

Shasta ground sloth

Scientists have used the perfectly-preserved dung of the giant ground sloth to trace its movements across the desert land of the Grand Canyon.

Huge piles of the sloth's dried waste still lie in its cavernous lairs, high in the canyon walls, where it is too dry for the deposits to decompose.

They reveal how the 500lb (226kg) sloth survived in barren conditions, chomping through tough vegetation which other creatures would have struggled to digest.

The shasta ground sloth roamed the Grand Canyon during the Ice Age

Its surviving cousin, the South American tree sloth, lives similarly. It dines on tough, toxic leaves that take weeks to digest and provide little energy - leading to its infamously languid lifestyle.

As well as being as big as a grizzly bear, the ground sloth had long claws to swipe away predators as formidable as the sabretooth cat.

Scientists have analysed layers of the dung to explain the ground sloth's demise.

There are no traces of the dung beyond the peak of the last Ice Age, 16,000 years ago. Conditions became too cold and too dry for its favourite plants to grow and its slow metabolism meant it struggled to stay warm.

Glyptodont

The glyptodont was capable of swimming and ate water-based plants

The size of a small car and equipped with a huge bony shell, an armoured tail and a trunk, the glyptodont is described by Professor Alice Roberts as "by far the oddest mammal I've ever seen."

But as well as being a spectacle for researchers, it provides insights into life in those parts of the world that were not dry and cold during the Ice Age.

Large parts of Arizona, where the glyptodont roamed, were covered in swamps and rivers.

There have been about 20 Ice Ages over the last 2.5 million years, and the proliferation of the glyptodont reflected the impact of the advancing ice sheet on the rest of the world.

The marshes expanded every time the ice sheet grew, leading to a surge in the number of glyptodonts, while they began to die out each time the ice retreated.

Scientists believe a two-mile high ice sheet in North America was acting like a mountain range; pushing moisture-laden winds across the desert and creating a fertile marshland.

Columbian mammoth

The Columbian mammoth was the greatest giant of them all.

It would have towered above the modern elephant and, with a shoulder height of 14ft (4.27m), it stood several feet taller than the woolly mammoth.

Consuming up to two tonnes of grass a week, the Columbian mammoth roamed in search of vegetation as expanding ice replaced sea water. The Ice Age global sea level was about 120 metres lower than today.

Mammoths left their traces on what is now San Francisco

Large coastal rocks just north of San Francisco have provided scientists with vital clues about the mammoth's movements.

The rocks have been worn down to a smooth, polished finish in patches that reach several metres in height.

"You could have a horse sitting on the shoulder of a cow and still not do that. It's too high for domestic livestock," said archaeologist Breck Parkman.

Scientists believe the mammoths used the rocks as scratching posts for exfoliating their skin and grinding away parasites - leaving breadcrumb clues about their journey across the Ice Age world.

Watch episode one of Ice Age Giants on BBC Two at 20:00 BST on Sunday 19 May and afterwards on the BBC iPlayer.