(FOX) — Fossil footprints from 313 million years ago are the oldest to ever be found in the Grand Canyon.

A geology professor from Norway, Allan Krill, made the surprising discovery four years ago on a hike with students.

Lying next to the trail, in plain view of the many hikers, was a boulder containing conspicuous fossil footprints, National Park Service officials said this week in announcing the discovery.

An intrigued Krill snapped a photo and sent it to a colleague, Stephen Rowland, a paleontologist at the University of Nevada Las Vegas.

“These are by far the oldest vertebrate tracks in Grand Canyon, which is known for its abundant fossil tracks,” Rowland said.

“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,” he said.

The boulder with the tracks fell from a nearby cliff exposure of the Manakacha Formation, park officials said.

They said the presence of a detailed geologic map of the strata along the Bright Angel Trail, together with previous studies of the age of the Manakacha Formation, allowed the researchers to pin down the age of the tracks quite precisely to 313 +/- 0. 5 million years.

The findings were published Wednesday in the journal PLOS One.