About 310 million years ago in what's now Arizona, a primitive creature trundled along on all fours through towering sand dunes that spilled into the sea. Normally, this creature's tracks would have vanished like other footfalls on a beach. But in a rare case, the tracks hardened into sandstone—preserving this flash of ancient behaviour.
Exposed by a rockfall, the footprints are now the oldest ever discovered in the Grand Canyon, scientists reported this month at the Society of Vertebrate Paleontology's annual meeting in Albuquerque, New Mexico. The prints are also noteworthy for their strangeness: Each one is angled 40 degrees from the direction of walking, as if the animal had a diagonal gait.
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The red of cactus blooms pops in a Grand Canyon valley. Cut by the Colorado River over millions of years, the Grand Canyon is considered one of the finest examples of arid-land erosion in the world. The immense canyon is 446 kilometres long and averages 1,200 metres deep, but it is only 24 kilometres across at its widest.
PHOTOGRAPH BY BARRY TESSMAN
“Even if it was an ordinary trackway, it would be unusual”, says Stephen Rowland, a palaeontologist at the University of Nevada, Las Vegas, who is studying the tracks. “But in this case, it's doing a funny little side-walking step, line-dance kind of thing, which is weird”. (These fossil footprints in South Korea are likely the oldest known traces of a lizard running).
Rowland learned of the tracks by chance back in 2016, when a paleontologist friend of his was hiking the Grand Canyon with students. As they hiked Bright Angel Trail, the group saw a rock that had fallen out of the cliffside and cracked open.
The busted boulder had split along an inner seam, revealing a natural mould and cast of 28 footprints stretched out in a line more than a yard long. The friend let Rowland and park officials know, and a field crew pulled the rock off to the side. In May 2017, Rowland visited the prints, and in March 2018, he returned with San Diego State University geologist Mario Caputo to study them.
Step by step
The pair's findings are still preliminary; they hope to submit a formal scientific paper soon. In the meantime, several questions linger, including the dune-walker's identity.
Tantalisingly, these prints were made right around the time that reptiles' oldest ancestors started to diversify, and they resemble 299-million-year-old footprints found in Scotland made by early reptiles or amphibians with reptile-like proportions. If similar animals made both trackways, the Grand Canyon tracks could be the oldest footprints of their kind by more than 10 million years. (See a treasure trove of ancient human footprints found in Africa).
An illustration shows how an early reptile-like animal might have made the strange angled tracks in the sand.
ILLUSTRATION BY STEVE ROWLAND
The team is also debating why the footprints are angled so weirdly. Was the animal struggling to walk into a strong wind? Or did it angle its gait as it walked down a sand dune, so it didn't slip? Caputo's study of the sandstone will help tease out these scenarios.
“If the exposure allows it, it will enable me to say, okay, this organism was walking near the crest or the summit of this dune ... or perhaps in an area in between dunes”, he says. Such specifics will let researchers reconstruct a single moment in the life of an animal that died more than 300 million years ago—an impressively intimate scientific feat.
“With a skeleton with bones and teeth, you get lots of good information, but you don't actually see behaviour”, Rowland says. By contrast, “we've captured this animal walking”.