Ancient Fossil Footprints Show Humans May Have Hunted Giant Sloths in New Mexico

Friday, April 27, 2018

A dramatic rendering of a giant sloth being confronted by human hunters  Alex McClelland, Bournemouth University

The White Sands National Monument in New Mexico is a vast, barren landscape of sparklingly bright sand twisted by the wind into slowly morphing dunes. Between about 10,000 and 15,000 thousand years ago, it was a very different place, with large animals like mammoths, giant ground sloths and humans leaving ghostly footprints etched in the crusty ground.

If scientists can catch sight of the delicate tracks, they can make incredible discoveries about the past. Scientists now believe humans stalked and then confronted a giant ground sloth—with that story repeating itself several times over with different sloths—thanks to a striking set of prints at White Sands. That finding was reported in a paper published Wednesday in the journal Science Advances.

It took some time for the team to recognize what they had found—at first it seemed like just one species had been present. “It was when we suddenly realized that the two things were interacting that the story got really exciting,” co-author Matthew Bennett, a scientist at Bournemouth University, in the U.K., who focuses on fossilized human footprints, told Newsweek. He was called in to analyze the traces after the discoverers of the new prints realized that humans had also been wandering across the site.

Human footprint inside giant sloth track at White Sands National Monument. Image credit: National Park Service.

“We did have some really nice sloth prints,” co-author David Bustos, chief of resources at the White Sands National Monument and the person who first discovered the tracks, told Newsweek. “You could see the claws and all the different features.”

These prints weren’t left by anything like the chill tree hangers alive today: Extinct giant ground sloths could reach 10 feet tall and weigh more than a ton. Scientists found the traces of bare human feet nestled inside some of those giant sloth paw prints. The scientists didn't see any evidence that the larger prints had been filled with water or dirt before the humans came through. That made them think the humans were purposely following in the footsteps of the sloth, perhaps stalking it as prey or even playing with it, although other experts say it's impossible to assign motives.

As the scientists followed the paths of the tracks across the landscape, they saw that the sloths usually ambled along in more-or-less straight lines. But where human tracks lined up with the sloths, the animals started moving differently, stopping and veering abruptly, as if turning to face a threat.

“The sloth appears to have reared up on its hind legs and swung its arms behind it at whatever was following it in some sort of defensive gesture,” Bennett said.

How closely that story matches what really happened is difficult to know all these years later. “It’s impossible to say with any absolute certainty what’s going on,” said ReBecca Hunt-Foster, a paleontologist with the Bureau of Land Management who wasn’t involved with the new research, adding that she thinks the authors’ interpretation overall makes sense. “Sometimes we see animals making sharp turns that aren’t associated with any type of prey.”

The difference in behavior between sloth tracks on their own and sloth tracks near humans suggests the animals were there at the same time, at least, said Spencer Lucas, a paleontologist at the New Mexico Museum of Natural History and Science who wasn’t involved in the new research. He said he’s hesitant, however, to agree with all of the details of the interpretation of the prints. The two species could have been responding to each other without explicit hunting having been involved, he said.

“These sloths were wild animals, so they’re probably not exactly excited to see people,” Lucas said.

And vice versa, he notes, given the huge size and dangerous claws of the sloths. “The sloths were not the only animals that were concerned about the situation,” he said. “Those people would have been vigilant at the very least.”

Top: mosaic of part of the study site showing two ‘flailing circles’ as well as sloth and human composite tracks. Bottom: interpretation of trackway trajectories. Image credit: Bustos et al, doi: 10.1126/sciadv.aar7621.

Lucas adds that even though footprints are often overlooked, they can reveal the sort of action-packed scenario that scientists typically can’t glean from skeletons. Barring rare exceptions like a fossilized bison with a spearpoint still sticking out from its ribs, typically remains just show that certain species lived in the same general region during a certain period—they can't speak to actual interactions like hunting. 

“The footprint is fossilized behavior,” Lucas said. “Footprints are going to give you a lot more than the bones usually do.”

Although fossilized interactions are rare, this isn’t the only example of its type—Hunt-Foster notes that there are dinosaur tracks near Moab, Utah, that show first a large meat-eating dinosaur passing through, then a large vegetarian dinosaur following precisely in its footsteps, suggesting some connection between the animals.

If the scientists are right about their interpretation of the sloth and human prints, they reveal an incredibly vivid scene of these prehistoric interactions. “It’s close-quarter combat,” Bennett said. “There’s a dance, and it’s probably done to life and death, and it’s written in the mud—so it’s a lovely story.”