© ROB PONGSAJAPAN/CC BY 2.0Modern footprints speckle the Alkali Flat Trail at White Sands—and around the area, oodles of prehistoric footprints are buried beneath the sand.
Today, White Sands National Monument in
New Mexico is studded with dune fields, which are
constantly being shuffled and sculpted by the wind. Visitors can hike across the soaring, powdery mounds of gypsum, or even
barrel down them on a sled. The dunes seem to go on forever: They stretch for hundreds of square miles, and as the sand whips past, it's easy to imagine an infinity of white, rolling humps.
At the end of the last Ice Age, it looked a lot different. Neighboring Lake Otero was
beginning to evaporate, leaving behind selenite crystals that eroded into the sands of
Alkali Flat. In the waning days of the Pleistocene, a human, a ground sloth, and a mammoth trudged across the eastern side of that disappearing lake. Now, well over 10,000 years later, researchers from Cornell University, Bournemouth University, and the National Park Service are using ground-penetrating radar to study the tracks they left behind.
An animal can only die once, and when it does, there's a vanishingly slim chance that it will become a fossil: Far, far more often than not, an animal's carcass will decay and rot until there's little proof that it ever existed at all. While it's alive, though, a creature can stamp proof of itself all across the landscape. Ichnology is the study of those preserved tracks, burrows, and other "trace fossils" — and it's a way for researchers to visualize an animal's behavior and biomechanics without a body in sight.
Like any other fossils, trace fossils owe their existence to a bit of luck. "You need a surface that is soft enough to deform and leave an imprint, which is true for sand and mud," says Douglas Jerolmack, a geophysicist at the University of Pennsylvania who has worked extensively in White Sands but was not involved in this current research. "To preserve it, you also need the surface to 'lock in' that imprint somehow," he adds. A track might be submerged by other sediment faster than wind or water can obliterate it, for instance, or it may be cemented in place. Jerolmack suspects that's what happened in White Sands. "The upwelling of salt-saturated groundwater in the arid environment there leads to precipitation of salt that builds little bridges among sand grains and binds them together," he says.
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