Last April, Matthew Bennett lay in a white salt desert in New Mexico, discovering petrified footprints left in the white stone. The pressure belonged to a ground sloth – a bulky animal whose large feet and arched claws left behind apostrophic impressions. There were many such traces, but Bennett found one that was very different.
Inside the outline of the 20-inch foot of the sloths was a human footprint.
He looked at the next track in the series and found the same – a human footprint perfectly embedded in a sloth. There were at least 10 of them, all in a row. "It was dawning on me what was going on," he says. Thousands of years ago, a down-to-earth sloth had run across this place, and one person had followed it, carefully matching each step of the way. "There was a lot of profanity [from me]," Bennett adds. "Geologists do that when we discover something."
If the person's sloth had followed, his larger footprint would have destroyed the smaller one. If the person had followed in the footsteps of a long-gone pawn, their feet would have crushed themselves into any water or sediment that had accumulated in the old tracks and created a distinctive pattern. Bennett and his colleagues did not find such a pattern. All the evidence was consistent with someone keeping up with an animal that was ahead of them.
"It really looks like they're the same time," says Anthony Martin of Emory University, who specializes in footprints and other so-called trace fossils. "This is a common problem we have with dinosaur tracks: we have something that looks like the following behavior, but could have been delayed by days or weeks, here people might have had the sloth in sight."
Ground sloths were no slow idlers like the sloths we know today. They were well-armed and potentially dangerous animals ranging in size from bear-sized to elephant-sized. Those living in New Mexico were at the smaller end, but they were still substantial animals with feet of footsteps. A human would have had to stretch to follow in his footsteps. What did she own?
Bennett believes the pursuer has been trying to provoke the sloth – and if he's right, it definitely worked. At the end of the overlapping tracks, the team found a very different set of sloth marks pointing to spinning feet and scratching claws. The animal rose on its hind legs and swung its claws around.
Meanwhile, another set of human footprints is approaching from the opposite direction. These are daintier, with prints of raised toes. It seems that while the sloth was waving, someone else typed from behind. This is a hunt, says Bennett. "The strategy was just stalking to distract and irritate the animal, and to turn it away from the blind side."
This is just one possible interpretation, but it is consistent with other tracks from the region. Sloth railroads usually go in a straight or gently curved line. But if there are human traces, the sloth paths change sharply. The sloths appeared to perform evasive maneuvers, and Bennett's team also found many other circular orbits that indicated upright, spinning animals. They call these "wandering circles" – a term that makes me feel more connected to floor sloths than ever before.
However, this scenario does not explain why the haunting person stepped into the footsteps of the sloth. There's something almost playful about it, and I ask Bennett if the tracks could have come from a bunch of teenagers bullying the sloths for kicks.
"It's really hard to deny that," he says. "But I think that's very unlikely, they were terrifying animals, they had claws like Wolverine, I did not want to go with a head, it would be a very stupid risk." I underestimated the willingness of teenagers to take stupid risks, but I also take his point.
Ricardo Melchor of the National University of La Pampa is not entirely convinced either. Hunting is the simplest explanation, but "the human footprints are not well preserved," he says. And why would humans hunt a sloth near a lake – a flat landscape "where the animals can easily escape?"
The only other idea that makes sense, Martin says, "is that they hunt an animal as an exercise for a future, deliberately step on the tracks to get a feel for its movements, the only alternative I see may, is that they were scientists! "
Bennett's team found its footsteps in New Mexico's White Sands National Monument. After the last ice age, the lake was dry. The wind swept away his bed, creating the largest expanse of white gypsum sand dunes in the world, leaving huge salt pans. They are hot and dazzling white. Bennett is there when I call him. "I have my hat on and I have sunscreen," he says. "I can see the shadowy outlines of the tracks that are about 20 meters away – humans, mammoths, sloths – there are more tracks here than on any other side – it's the largest concentration of tracks in America – and maybe in the world. "
Despite their large number, the tracks are difficult to recognize. They can be shallow and their visibility depends on subtle humidity conditions; David Bustos, the resource manager for White Sands, calls her Ghost Traces. On several occasions he brought visitors to see her, and nobody could. When he invited Bennett from the University of Bournemouth, Bennett was skeptical. Only when he saw the tracks himself did he realize that "White Sands is hiding its secrets well".
Bennett, Bustos, and their colleagues identified the traces using geophysical techniques that sought subtle changes in magnetic properties from the ground. They also used a much lower-tech method: they took pictures from the air by clamping cameras to large poles. In this way, the team has discovered hundreds of thousands of traces of mammoths, mastodons, ground sloths, camels, bisons, terrible wolves, humans and more.
"This is just the starting point to unlock an archive that will tell us something really interesting about how early human pioneers interacted with this glacial fauna about 11,000 years ago," says Bennett.