Tens of thousands of years ago, a human walked north at what is now White Sands National Monument in New Mexico. A large proboscidean, possibly a Columbian mammoth, later walked west, stepping onto one of the footprints left by the human. Soon after, a person—perhaps the same individual who had gone north—walked south, parallel to the earlier tracks. And in walking south, they stepped directly into one of the mammoth’s footprints.
Whether it was the same human walking both directions can’t be determined, nor can the exact time frame in which these events occurred. But the fact that these species walked over each other’s prints indicates that this all happened in close succession, according to the researchers who studied the site.
These footprints are the subject of a paper published today in Scientific Reports. Following previous work in which the authors used magnetometers to study these footprints, they outline their improved efforts using ground-penetrating radar. What they have found is potentially groundbreaking for those studying fossils and archaeological sites.
Archaeologists, who study ancient human life and artifacts, have been using ground-penetrating radar for some time. But except for recent work in the Connecticut River Valley and at a NASA construction site, ground-penetrating radar hasn’t been used in paleontology to study ichnofossils, or fossilized traces of life like footprints. Today’s paper offers an innovative and nonintrusive way to discover what lies beneath the surface in ever-improving detail. This is particularly important in a site like White Sands, in which tracks often disappear from view depending upon environmental conditions—a phenomenon called “ghost tracks.”
“We began with magnetic sensors (magnetometers) largely because they do not require contact with the ground, and we were concerned about protecting the tracks,” Tommy Urban, research scientist at Cornell University and lead author of the paper, told Gizmodo in an email. “We didn’t want to walk over and drag anything over the tracks in order to collect the data.”
Using ground-penetrating radar unfortunately means pulling the device over the surface, which is why this was not their first option when they published earlier this year. The team took pains to cover the tracks with foam pads before beginning their scan. Doing so let them capture smaller tracks they didn’t know existed (such as human and sloth tracks) and saved them time when it came to downloading and digitizing the information. It also let them preserve the tracks in 3D imagery rather than excavating them, where they tend to quickly disintegrate unless cast. They could also see details of the layers underneath the tracks that can help them understand the weight, size, and stride of the track maker. All together, this information can give us a better picture of extinct life. In short, it’s a highly specialized method of finding buried treasure.
“We get two things [from ground-penetrating radar]: (1) tracks’ presence and volume; and (2) we can see how the sediment below the track has been compressed. This is like a stored pressure-record of the foot; variation in pressure across the foot tell us about the way the foot moved,” ichnologist Matthew Bennett of Bournemouth University, co-author of the new paper, told Gizmodo by email. “It is this last bit which is perhaps the most important aspect. It is equivalent to taking an extinct animal, bringing it back to life, taking it to the biochemics lab and getting it to walk on a pressure plate. That is what is so special and why we are so excited.”
Or, as Ashley Leger, paleontologist and proboscidean specialist at Cogstone Resource Management, told Gizmodo in an email: “We cannot ever watch a mammoth walk through the mud or run from a predator, but if it leaves its footprints behind, that’s the only way we can learn about the behaviors, having no way to directly observe the animal.”
White Sands National Monument is known for its wealth of fossil tracks. Not only does it contain the tracks of ancient humans, giant ground sloths, camels, dire wolves, sabre-toothed cats, and mammoths, but the length of these trackways is considerable. Some have been found to stretch continuously up to 2 miles. In 2016, research into these tracks accelerated, but scientists face significant challenges when working here. The size of the Monument, at 275 square miles, is a major one; the fact that tracks cannot be seen by the human eye in some conditions is another. These so-called ghost tracks indicate that so much more is yet to be found, and they are one of the most compelling reasons to preserve this National Monument for future generations.
Tommy Urban told Gizmodo by phone how his team had already written the original draft of today’s paper when they were going over the site again, only to discover sloth tracks they hadn’t known existed in that same area.
“And then the next day, they were gone again. Completely invisible! They’d disappeared,” Urban said. “That’s why this technique is so useful. You don’t know when you’re going to be able to see these footprints, because they disappear and reappear based on environmental conditions. It had rained, it had moistened the surface, so the sloth tracks just popped out. And then overnight, the wind had changed direction and blew a veneer of gypsum and you couldn’t see anything anymore. So that was it.”
The story this particular set of tracks tell is significant. While we may often imagine humans staying together for protection against predators, here is proof that at least one human, if not two, was walking by themselves in a presumably open landscape. We know that large animals like mammoths and giant ground sloths also frequented the area. So what does this tell us about everyday life in the Pleistocene in that environment?
“I hope that people take away from this research, and about ichnology in general, is that ichnology is preserved behavior of a living, breathing animal,” wrote paleontologist Lisa Buckley in an email to Gizmodo. “That animal was going about its daily routine, and even though we may never find the fossilized remains of the animal that specifically made those tracks, we know how it moved, how large it was, how fast it was going, and what it was associated with just by looking at the tracks. One animal will only ever leave one skeleton, but it has the potential to leave countless tracks in its lifetime.”
Buckley wrote in her email:
This is where track site protection becomes important. GPR [ground-penetrating radar] can tell us the subsurface extent of these track sites. This is crucial for palaeontology impact assessments and conservation management decisions. If the track sites are not protected—even if they are not visible on the surface at this time—then there won’t be any tracks to study. Technologies like GPR are only going to improve in their resolution and sensitivity, but the tracks need to be there to be studied. I hope that there is an increase in GPR use in track site conservation assessments so we can extend protections and conservation efforts to more subsurface track sites. If efforts aren’t made to conserve track surfaces for future work, then the only technology that we will be able to use to study ichnology is a time machine.
Efforts to further preserve these trackways and the Monument itself are underway. There is currently a bill (S. 1582) to make White Sands National Monument a national park.
“All of us that have been working there for a while are slowly developing this kind of imaginative picture of what [life at that time in the Pleistocene] must have been like. All of these different possibilities unfold,” said Urban. “And there are tens of thousands of tracks that recorded different scenarios! We’re really just scratching the surface.”