A fossilized human footprint inside a giant sloth track was found at White Sands National Monument/NPS

On a remote salt flat with nearby towering, snow white gypsum dunes, a team of scientists is following a string of fossilized footprints back in time to the end of the Ice Age, when humans – adults and children – were likely hunting a giant, razor-clawed ground sloth.

The trackway – a series of tracks and footprints is called a trackway – at White Sands National Monument shows that someone followed a sloth, purposely stepping in its tracks as they did so, said David Bustos, the park naturalist who discovered the trackway 10 years ago.

Team member Matthew Bennett, a professor of environmental and geographical sciences at Bournemouth University in England, believes the ancient humans stalked the sloth.

“So we ask, why? Adolescent exuberance? Possible but unlikely,” Bennett said. “We see interesting circles of sloth tracks in these stalked trackways, which we call flailing circles. These record the rise of the sloth on its hind legs and the swing of its forelegs, presumably in a defensive motion.”

At 7- to 8-feet tall, tightly muscled, and swinging forelegs tipped with wolverine-like claws, the sloth would tear apart any hunter on direct approach. But in addition to tracks of humans following the sloth, there are more human tracks a safe distance away, telling scientists that this was a community action making use of distraction and misdirection to gain the upper hand in deadly, close-quarter combat.

Bennett believes the tracks show the sloth was turning and swinging at the stalker.

“We also see human tracks on tiptoes approach these circles; was this someone approaching with stealth to deliver a killer blow while the sloth was being distracted? We believe so,” he said. “It was also a family affair, as we see lots of evidence of children’s tracks and assembled crowds along the edge of the flat playa. Piecing the puzzle, we can see how sloths were kept on the flat playa by a horde of people and distracted by a hunter stalking the sloth from behind, while another crept forward and tried to strike the killing blow as the animal turned.”

Bustos said, “The thing that is special about these prints and sets them apart from any other fossil trackways in the world is that this discovery records the interaction between humans and Ice Age giant megafauna, and White Sands National Monument has the largest concentration of human and Ice Age giant megafauna prints in the Americas.”

There is a great deal more to learn in the years to come, said team member Vince Santucci, the National Park Service’s senior paleontologist.

“We’re pretty motivated to keep looking and studying,” he said. “These fossils are called ghost fossils for a couple of reasons. One, they’re difficult to see at times because of different atmospheric and moisture conditions – and the light has to be just right. And second, the fossil footprints are ephemeral. Once the overlying sand dunes move and reveal the tracks, they start to weather away and can be gone in months.”

High on the list of things to be learned is the question of just when this episode of hunters and hunted took place. The Ice Age ended about 11,700 years ago, and the fossil record of ground sloths indicates they were extinct by this time. At White Sands, the scientists used an approach called relative dating to estimate a minimum age for the fossils.

“Since the footprints are contemporaneous with animals that died out by the end of the Pleistocene, relative dating tells us those footprints are at least 11,700 years old, or older,” Santucci said.

The team studying and writing about the fossil prints detailed their findings last week in the journal Science Advances. The multi-disciplinary team of federal, state, and academic scientists, technicians, and naturalists includes Bustos, the paper’s lead author, Bennett, Santucci, and National Park Service archaeologist Dan Odess.

Team members visit the remote site under a research permit, but it is a dangerous area of the park’s backcountry that will not be open to the public. Park Superintendent Marie Sauter said the team’s documentation – photographs, video, plastic casts of the footprints, and other materials – will be helpful for park visitors.

“We will benefit from their work by using all of this to develop interpretive materials that people can see, touch, and experience at the park visitor center or online at National Park Service web sites,” she said.