Archaeologists apply radar tech to study ‘ghost footprints’ of ancient humans and animals

Unbeknownst to most of us, there are ancient footprints right under our feet beneath the ground we continue to walk on today, and now scientists are using ground-penetrating radar (GPR) to turn these invisible “ghost” footprints visible again.

When we think of old footprints, we think of fossilized prints that were left in the mud and can be easily seen with the naked eye. But it turns out we can also find footprints that have been hidden for thousands of years, including the footprints of ancient humans and mammoths and other creatures that were alive during the last Ice Age.

800px Woolly mammoth Mammuthus primigenius Mauricio Antón
Woolly mammoths roamed what we know now as New Mexico thousands of years ago, leaving their footprints behind. Image via Wikimedia.

Of course, we can only see these footprints under certain circumstances, and we need ground-penetrating radar to do it.

In an experiment at White Sands National Monument (WHSA) in New Mexico, a team of researchers employed ground-penetrating radar at the Alkali Flat of the ancient dried-up lake bed of Lake Otero in an effort to discover if the technology can be used to find pre-historic footprints as efficiently as it is used to find archaeological sites around the world.

White Sands Dreamland
White Sands National Monument in New Mexico is ideal for hunting ancient hidden footprints. Image via Wikimedia.

According to the authors of the study published by Scientific Reports:

Footprints provide evidence of an animal’s presence, pedal anatomy, abundance, co-association with other animals and behavioral ecology, and have been used to infer not only body size and mass, but also pedal anatomy and biomechanics. At some locations, especially in the American southwest, these important yet delicate fossils may hold the key to unanswered questions about human behavior during the upper Pleistocene, particularly those related to hunting activity, with footprints offering access to predator-prey interactions outside the more typical “site” locus of a kill or camp.

Basically, scientists can put together a scenario of what happened in an area during the Pleistocene based on footprints kind of like how forensic experts are able to use footprints and fingerprints to recreate what happened at a modern-day crime scene.

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The team used ground-penetrating radar, a non-invasive method, to find the footprints. Image via Scientific Reports.

What the team discovered is groundbreaking as the radar detected thousands of footprints, which means the area used to be an ancient highway of sorts along which humans and animals both traveled.

Our work demonstrates the effectiveness and efficiency of non-destructive GPR for detecting and documenting fossil footprints in soft sediments, including human tracks. Ichnofossils of extinct Rancholabrean fauna occur widely at WHSA and include tracks of Proboscidea (mammoth), Folivora (ground sloth), Carnivora (canid and felid), and Artiodactyla (bovid and camelid), as well as humans.

They occur on an extensive gypsum playa (Alkali Flat), the erosional relict of ancient Lake Otero, dating from the Upper Pleistocene. The sheer number of tracks, tens of thousands extending over large areas, allows animal and human-animal interactions via true ‘paleo-tracking’ to be deduced.

This valuable resource however is only intermittently and partially visible at the surface during specific moisture/salt conditions, and when visible may be covered quickly by drifting sand. The occasionally visible tracks are therefore known colloquially as ‘ghost tracks’. Given the scale of the site the resource management challenges are considerable.

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The team found ancient human footprints right under the sands. Image via Scientific Reports.

Footprints under footprints

In fact, the team was even able to find footprints under footprints, giving them an even better picture of the action that took place at the time. For example, the team found a human footprint inside a mammoth footprint.

The GPR survey reported here was undertaken along this trackway at a location where a series of proboscidean tracks, presumably Mammuthus columbi, cross in a westerly direction perpendicular to the two human trackways. Deformation in front of one of the mammoth’s manus tracks partially closed two tracks of the northbound human trackway, showing that the mammoth crossed that human trackway after it was made.

In turn, a single human footprint of the southbound human trackway is superimposed on a mammoth manus track, showing that the human crossed the mammoth track on the return. This provides a clear sequence for the tracks, demonstrating co-association. Though the time-lapse between each of the three track-making events is unknown, the mammoth track is temporally book-ended by the two human trackways (believed to be the same individual).

Current N. American dates for an initial human presence along with dates for mammoth extinction on the continent support a late Pleistocene biostratigraphical dating of the tracks. The exact outline of the mammoth track, believed to be made by a manus, is not well-defined but by analogy with extant Loxodonta africana would most likely correspond to a mature (>50 years) bull with a potential shoulder height of over 3 metres.

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Ancient human footprints, one of which was found inside a mammoth footprint, which you can see top right. Image via Scientific Reports.

“We never thought to look under footprints,” Cornell research scientist and lead study author Thomas Urban said in a statement. “But it turns out that the sediment itself has a memory that records the effects of the animal’s weight and momentum in a beautiful way. It gives us a way to understand the biomechanics of extinct fauna that we never had before.”

“Now, using geophysics methods, they can be recorded, traced, and investigated in 3-D to reveal Pleistocene animal and human interactions, history, and mechanics in genuinely exciting new ways,” co-author Stuart Manning added.

The team determined the prints belonged to a mammoth because they were able to measure the foot pressure of modern African elephants and compare the data, which showed similarities. They also excavated the footprints to confirm their findings.

In addition, the team also found footprints belonging to ancient sloths, camels, and canines. Clearly, the area where White Sands is located used to be a lush environment teeming with wildlife before it became a desert.

But the radar technology might be able to detect even older footprints belonging to dinosaurs, especially in similar sites in North America and Africa, making excavation unnecessary.

Taken to its logical extent, potentially thousands of plantar pressure records are waiting therefore to be collected at sites like WHSA and elsewhere in North America and Namibia without the need for tracks to be excavated. The potential here to enhance our understanding of the biomechanics of extinct animals may yield important information for developing more sophisticated biomechanical models from and for extant elephants and by analogy from anatomically similar dinosaurs such as sauropods. It may also improve the quality of geotechnical models applied to both elephants and mammoth tracks since it would allow estimated plantar pressures to be used rather than as now uniform indenters.

“There are bigger implications than just this case study,” Urban said. “The technique could possibly be applied to many other fossilized footprint sites around the world, potentially including those of dinosaurs. We have already successfully tested the method more broadly at multiple locations within White Sands.”

The sheer multitude of data scientists could gain from using ground-penetrating radar to find and study footprints is simply mind-blowing. This could allow us to study ancient migrations in ways we’ve never been able to before. The possibilities are endless. And as the technology improves and is more refined, we can expect better results in the future. We can already reconstruct life during pre-historic times, but now we could do it far more accurately and that’s an exciting prospect.

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Featured Image: Mammoth via Pixabay with footprints found via radar from YouTube screenshot

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