Fallen Boulder at the Grand Canyon Exposes 300-Million-Year-Old Footprints

The reddish boulder embedded with 314-million-year-old footprints.  (Image: Stephen Rowland)
The reddish boulder embedded with 314-million-year-old footprints. (Image: Stephen Rowland)

The oldest fossilised vertebrate footprints ever discovered in the Grand Canyon were found embedded in a recently fallen boulder located in plain sight.

Around 314 million years ago — long before the first dinosaurs appeared on Earth — a reptilian creature measuring around a foot long slowly angled its way upward along a windswept sand dune. Shortly after, the same four-legged animal, or possibly a similar one, took a more direct route, climbing straight up the dune at a slighter faster pace. The resulting footprints hardened after getting damp, then got covered in sand, which preserved them for hundreds of millions of years.

Today, we know the location of these ancient treks as the Manakacha Formation of the Grand Canyon. A boulder imprinted with these fossilised trackways recently fell from a Manakacha cliff, landing near the Near Bright Angel Trail. Norwegian geology professor Allan Krill noticed some peculiar features on this reddish rock while hiking with students in the area four years ago. Krill smartly sent photos of the boulder to Stephen Rowland, a paleontologist at the University of Nevada Las Vegas and first author of a new PLOS One study describing the trackways.

Illustration showing the two trackways on the boulder. (Image: S. M. Rowland et al., 2020)

“These are by far the oldest vertebrate tracks in Grand Canyon, which is known for its abundant fossil tracks,” explained Rowland in a U.S. National Parks Service press release. “More significantly, they are among the oldest tracks on Earth of shelled-egg-laying animals, such as reptiles, and the earliest evidence of vertebrate animals walking in sand dunes.”

The species responsible for the tracks could not be determined, but they were made by some kind of early amniote, which, as Rowland pointed out, are animals that lay fertilised eggs on land. Modern reptiles, mammals, and birds are descendants of these pioneering quadrupedal creatures.

Artist's depiction of the early amniote moving diagonally up a sand dune, creating the first of two trackways seen on the boulder. (Illustration: S. M. Rowland et al., 2020)

Other layers at the Grand Canyon, including the Coconino Sandstone and Hermit Shale, have also yielded vertebrate footprints, but the newly described tracks are about 15 million years older. These are also the first trackways ever discovered in the Manakacha Formation, which is dated to 314 million years old.

The first of the two tracks was made by an animal moving across the slope of a sand dune at a 20-degree angle, which it likely did to reduce the steepness of the climb, according to the new study. Reconstruction of the prints revealed a distinct mode of locomotion known as a lateral-sequence gait, in which legs on one side of an animal move in succession: rear leg followed by front leg, and then alternating to the other side. Cats walk this way when moving very slowly. This is now “the first documented occurrence of a lateral-sequence gait” dating back to this time period, proclaims the study.

The second track shows a set of prints that were made a few hours or days after the first and after some more sand had collected on the surface. This animal, possibly the same one, moved faster than the initial trackmaker but still quite slow at a rate of 0.09 m per second (0.1 meters per second). For the second track, the animal went straight up the slope using a more traditional diagonal-sequence gait, which is how dogs and cats walk when moving at normal walking speed.

The new fossil is also the earliest example of vertebrates moving across a sand dune, pre-dating the previous record by 8 million years, according to the researchers.

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As AZ Central reports, the fate of the boulder is uncertain. Rowland would like to see in put on display “in a controlled environment,” but it would have to be moved by a helicopter, in what would be a complex process. Hopefully hikers walking by this ancient relic will treat it with the respect it deserves until the boulder can be relocated.