Fossil footprints tell story of giant sloth hunt during Ice Age

Researchers studying a trail of fossilized footprints on a remote New Mexico salt flat have determined the tracks tell the story of a group of Ice Age hunters stalking a giant sloth.

Park naturalist David Bustos says the series of tracks and adult and children's footprints found at White Sands National Monument shows someone followed a now-extinct giant ground sloth, purposely stepping in their tracks as they did so.

The team studying the fossil prints detailed its findings in the latest edition of the journal Science Advances , the Las Cruces Sun-News reports . The publication has drawn attention to White Sands home to the world's largest field of white gypsum sand dunes as members of New Mexico's congressional delegation push to raise the monument's profile by turning it into a national park.

White Sands contains a sizeable collection of fossilized tracks, including saber-toothed cats and wooly mammoths.

It's unclear why ancient humans would have stalked the sloth, said team member Matthew Bennett, a professor of environmental and geographical sciences at Bournemouth University in England. The creature 2 metres (7 to 8 feet)tall with long arms and sharp claws had a distinct advantage in close-quarter encounters.

"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."

Community hunt

But scientists said there are more human tracks a safe distance away, telling them this was a community action.

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," Bennett said.

There is a great deal more to learn in the years to come, such as when this episode of hunters and hunted took place, said team member Vince Santucci, the National Park Service's senior paleontologist.

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.

• Reseacher Sally Reynolds talks to CBC's Bob McDonald about the study on Quirks & Quarks