Last April, Matthew Bennett was lying on a white salt flat in New Mexico, uncovering fossilized footprints that had been preserved in the white rock. The print belonged to a ground sloth—a bulky animal, whose large feet and curved claws left apostrophe-shaped impressions wherever it walked. There were many such tracks around, but Bennett found one that was very different.
Inside the outline of the sloth’s 20-inch-long foot was a human footprint.
He looked at the next track in the series and found the same thing—a human footprint, perfectly nestled inside a sloth one. There were at least 10 of these, all in a row. “It slowly dawned on me what was happening,” he says. Thousands of years ago, a ground sloth had walked along this site, and a person had followed it, carefully matching its every step. “There was a lot of profanity [from me],” Bennett adds. “That’s what geologists do when we discover something.”
If the sloth had been following the person, its larger footprint would have annihilated the smaller one. If the person had been walking in the footsteps of a sloth that had long passed, their feet would have squished into any water or sediment that had collected in the old tracks, creating a distinctive pattern. Bennett and his colleagues found no such pattern. All the evidence was consistent with someone keeping pace with an animal that was ahead of them.
“It really does look like they were contemporaneous,” says Anthony Martin from Emory University, who specializes in tracks and other so-called trace fossils. “This is a common problem we have with dinosaur tracks: We have something that looks like following behavior, but could have been offset by days or weeks. Here, the humans maybe had the sloth in sight.”
Ground sloths were not slow-moving slackers like the sloths we know today. They were well-armed and potentially dangerous animals, which ranged from bear-sized to elephant-sized. Those that lived in New Mexico were on the smaller end, but they were still substantial beasts with meter-long strides. A human would have had to stretch to walk in its footsteps. What possessed them?
Bennett thinks the pursuer was trying to provoke the sloth—and if he’s right, it clearly worked. At the end of the overlapping tracks, the team found a very different series of sloth prints, indicative of pivoting feet and scraping claws.