A giant sloth graveyard shows how these enormous animals died — and lived

An enormous pile of enormous bones excavated in Ecuador has given new insight into the behaviour of extinct giant ground sloths.  

Giant ground sloths lived in North and South America for millions of years until their extinction around the end of the last ice age — about 11,000 years ago. The largest species of giant ground sloths rivalled African elephants in size, and this new research suggests that their behaviour might have been very elephant-like as well.

"Between 10,000 and 50,000 years ago, depending on which continent you're on, basically all ice-free continents had ecosystems that looked a lot more like a modern African ecosystem," said researcher Emily Lindsey in an interview with Bob McDonald on Quirks & Quarks. 

"There were a lot of very large animals in that ecosystem and these sloths … were some of the largest."

Seeping tar was the key to fossil preservation

Giant ground sloth remains have been found in many locations in the Americas, but the find at a site called Tanque Loma in Ecuador in the early 2000s was unique. First uncovered by an oil company during exploration, the site was a "tar seep" in which plant and animal remains had been sealed in naturally occurring tar.

The most famous similar site is Lindsey's research home — she's assistant curator and excavation site director at the La Brea Tar Pits in Los Angeles. But she said the preservation here is a little different.

"It's a site that probably formed not through large animals getting actually trapped in the tar, but from an asphalt seep that arose after a bone bed was deposited, and just fortuitously preserved the bone bed in situ."

Excavations over a period of nearly two decades revealed that the tar seep had preserved various plant and animal remains. The most notable were hundreds of bones from one of the largest species of giant sloth that ever lived, known scientifically as Eremotherium laurillardi, which survived as a species for about five million years.

Adults of this species weighed upward of three tonnes and could reach a length of up to six metres. They were ground dwelling herbivores who could feed on foliage high in tree tops by rearing up on their powerful hind legs.

Possibly a family group 

The excavated bones turned out to be from at least 22 individual animals, from small, dog-sized infants to full-sized adults. The context in which the bones were found suggested the animals died at roughly the same time, and also revealed the nature of the location.

"We believe that the site was a spring-fed marsh," said Lindsey. "[It] probably had a lot of vegetation and it was probably an attractive area for large animals to come and spend time in [an] otherwise still somewhat arid tropical environment."

This seemed like a benign environment for giant sloths, so finding the remains of so many animals that had died at the same time was a mystery.

Fortunately the tar seep had preserved more evidence than bones. In and around the remains was a large amount of plant material — small branches, stems and twigs, all sharply cut into roughly 2.5-centimetre lengths.

At first Lindsey didn't know what to make of this material. 

"It wasn't until later when I was talking with a friend who is a retired professor of geography, and he told me a story that he had been monitoring a hippo wallow in Africa back in the '70s," she said.

Her colleague described how a drought had concentrated hippos around the small pond, and as a matter of course, the hippos defecated into it. A large amount of feces concentrated in the water. 

"It created this very unsanitary environment and thus this contributed to this rapid development of this disease that ended up wiping out their population," she said.

Lindsey then realized that the characteristically shaped plant material she'd found associated with the sloth bones had probably been cut by their teeth, and was either gut contents or feces — a large amount of it.

Contaminated water might have led to catastrophe

"At least a plausible explanation for how these animals died was something similar to what happened with this hippopotamus population that my co-author had observed in Africa," Lindsey said.

Animals trapped by drought conditions near the shrinking wallow contaminated it, and this led to the mass death.

That meant they had a likely explanation for how the animals died. But more interesting was what the remains and the site said about how giant sloths lived.

"It's not implausible to think, based on group situations that we see in other very large animals today, like elephants or other large savanna vertebrates, that there's some sort of familial connection," said Lindsey.

"These animals all died at a very similar time and were probably part of one large group. So they probably were social or gregarious animals."

Written and produced by Jim Lebans