More than 10,000 years ago, a group of hunter-gatherers chased a group of giant ground sloths along the shores of an ancient lake, and their footprints still preserve the story. West of the sparkling white gypsum dunes of New Mexico’s White Sands National Monument, at a place called Alkali Flat, layers of mud and sand left behind by a long-vanished lake contain more than 100 human and giant ground sloth tracks — rare evidence of Pleistocene hunter stalking, and sometimes curves, giant sloths.
Most are so-called ‘ghost trails’, which are only visible under very specific circumstances.
“Usually they are invisible. There is a lot of salt in the ground and when it rains the salt dissolves. And, crucially, as it dries out, the fill dries at a different rate and the difference between the fill (footprint) and the surrounding sediment makes the trail visible for a short time as it dries,” explains paleoecologist Sally Reynolds of Bournemouth University, UK. , a co-author of the paper. The team, led by David Bustos of the US National Parks Service, used aerial photography to spot the tracks and then selected a few groups for careful excavation and study.
The tracks reveal that ancient humans once hunted a pack of giant sloths along the shores of Lake Otero. Several human footprints are clearly superimposed in the long, kidney-shaped footprints of sloth feet, pointing in the same direction, as if humans were deliberately stepping into sloth tracks. That would take real effort; the average human stride, according to Reynolds and her colleagues, is about 0.6 m, but a giant sloth’s stride was somewhere between 0.8 and 1.1 m, so the human tracker would have to take larger strides.
Based on the way the sediment beneath the imprints was deformed and the lack of even a thin layer of sediment between the surfaces of the imprints, the sloth tracks were still relatively fresh when the hunter followed.
Faster than they look
It seems the sloths weren’t so excited to meet. Sloth tracks bend sharply in several places where they meet groups of human tracks, as if the sloths were trying to avoid the humans. It’s not hard to imagine why; the people would probably have been well armed. Reynolds and her colleagues say the tracks are likely between 15,560 and 10,000 years old (the way the wind mixes organic soils from different layers here makes it hard to be much more accurate), meaning Alkali Flat’s hunters likely spears. that were tipped with the large fluted stone points of the Clovis culture.
Those tracks tell us something about how Clovis people hunted; based on the mix of sizes, the party seems to have included both adults and children.
In some places, groups of prints preserve old confrontations between humans and sloths. Several sets of prints mark what Reynolds and her colleagues call “waving circles,” or places where a giant sloth appears to have reared up on its hind legs to its full height and lashed out with its long, clawed front legs in an attempt to defend itself. These spots are marked by slightly deformed tracks of the hind legs, surrounded by a circle of irregular prints and long claw marks or abrasions in the sand.
“In one case, a row of human toe impressions leads to the center of the circle, suggesting that someone was approaching on raised toes,” Reynolds and her colleagues wrote. In another, a single trail of an adult human appears in the center of a swinging circle.
Approaching a giant sloth in that situation would have been a dangerous undertaking, even for a hunter armed with a stone-tipped spear. We think of sloths as slow, quiet, almost comical animals, but Pleistocene giant ground sloths were nothing like the slow DMV worker in zootopia. Imagine an animal weighing about three tons, a length of 6 meters from head to tail, with large, sharp claws – then imagine looking down with a group.
“Sloths would have been formidable prey,” Reynolds and her colleagues wrote. “Their strong arms and sharp claws gave them lethal range and distinct advantage in close encounters.”
The one that got away?
What we don’t know is how the clashes captured in the dried mud of Alkali Flat unfolded. The cycles of wet and dry conditions on the playa, plus the high pH of the water and sediment (it’s called Alkali Flat for a reason) would have quickly eroded the bones left behind, whether lazy or human. Modern hunter-gatherers are more likely to go home empty-handed; the Hadza people of Tanzania fail to capture their prey about 94 percent of the time, according to Reynolds and her colleagues. But Pleistocene hunters in North America must have been successful often enough, as they are currently the prime suspects in the extinction of most of the continent’s megafauna, including giant sloths.
Based on the different sizes of the sloth tracks — anywhere from 12 to 12 inches long and 10 to 35 inches wide — multiple giant sloths moved along the shores of Lake Otero, trying to evade their likely well-armed human neighbors. It’s impossible to say what species of giant sloth left the tracks; fossils of two different genera have been found in the area, but their sizes were so similar that it is difficult to match the prints to one or the other.
Their presence, however, could tell paleontologists more about how giant ground sloths behaved and socialized. Today’s sloths tend to be loners, but Reynolds says these living species probably aren’t a good guide to the behavior of their much larger, long-extinct cousins.
“We know very little about the behavior of extinct sloths, but the footprints at White Sands can help us understand the group size, and possibly even other dimensions of the behavior of this extinct species, which is exciting!” she said to Ars.
A wealth of other tracks are waiting to be studied at White Sands, including mammoths; old canines and cats; extinct cattle related to modern antelope, buffalo, cow and goat; and ancient relatives of today’s camels.
“The site continues to provide new and fascinating insights into human and animal life at the end of the Pleistocene,” Reynolds said.
Scientific progress2018. DOI: 10.1126/sciaadv.aar7621 (About DOIs).