ROCK FALL REVEALS ANCIENT FOOTPRINTS

FLAGSTAFF, Ariz. — It’s something like a modern-day chuckwalla, strolling in sand dunes on an island in what now is the Grand Canyon region.

That’s how Steve Rowland, professor emeritus of geology at the University of Nevada, Las Vegas, and his fellow researchers interpret fossil footprints that were revealed in a rock fall near a popular Grand Canyon hiking trail. They estimate the tracks are 313 million years old, give or take a half-million years.

At that age, they’d be among the oldest tracks of animals that lay eggs with a protective hard or leathery shell and the earliest evidence of vertebrate animals walking distinctively in sand dunes, Rowland, Mario Caputo and Zachary Jensen wrote in a research paper published this month.

“I think our interpretations will hold up very well,” Rowland said Monday.

Not everyone is convinced the footprints were created by a single, four-legged animal that has a lateral-sequence walk, where the legs on one side of the body move in succession, followed by the legs on the other side. Or, that the footprints mark the point in evolution where animals were able to lay eggs with protective shells outside water.

Still, the paper raises interesting questions, said Mark Nebel, the paleontology program manager at the Grand Canyon.

“Some of the conclusions likely are going to be controversial,” he said. ”There’s a lot of disagreement in the scientific community about interpreting tracks, interpreting the age of rocks, especially interpreting what kind of animal made these tracks.”

Rowland made out claw marks common among reptiles in the 28 footprints, which he said help tease out the scarce skeletal record.