World's Oldest Footprints Found in Nevada?

Kimberly Johnson
for National Geographic News

October 10, 2008

Scientists believe they have uncovered Earth's oldest known footprints in the mountains of Nevada—a fossil find that suggests animals have been walking around about 30 million years longer than previously thought, according to new research.

The controversial tracks—described by one skeptical scientist as "paired rows of dots"— may indicate animals had legs in the late Protozoic era, about 570 million years ago, according to lead researcher Loren Babcock.

The discovery is the strongest evidence to suggest animals were able to move about on their own appendages during the Ediacaran period, before the Cambrian period "explosion." During the Cambrian complex animals rapidly emerged and replaced simple multicellular animals, said the Ohio State University professor.

(Explore an interactive prehistoric time line.)

"We keep talking about the possibility of more complex animals in the Ediacaran—soft corals, some arthropods, and flatworms—but the evidence has not been totally convincing," he said in a statement.

"But if you find evidence, like we did, of an animal with legs—an animal walking around—then that makes the possibility much more likely."

Digging Up Dirt

The fossilized animal tracks were unearthed near Goldfield, Nevada, near Death Valley, in the Deep Spring rock formation.

The sedimentary formation—made of sandstone, siltstone, shale, and limestone layers—is about 600 million years old.

Babcock and others on his research team were analyzing rock dating back to the Ediacaran-Cambrian period when he flipped over a slab of sediment and found the supposed footprints.

"It was basically an accidental discovery," Babcock told National Geographic News after the findings were presented earlier this week at a meeting of the Geological Society of America in Houston, Texas.

The footprints are a couple of millimeters across and look like centipede leg marks, he said.

"It's like taking a dull pencil point and poking it into sediment," he said, noting that such a description may not be so far from how the marks were actually made.

About 570 million years ago, the area was covered by a shallow sea. The water had a mat-like surface made of sediment grains that were held together by a cohesive network of bacteria and fungi, which would have easily preserved animal tracks.

The tracks were determined to have been made in the Ediacaran period by comparing the "footprints" layer to previously dated layers with similar features.

"There is nothing we can use to [directly] obtain numerical age date," said Babcock, who acknowledges the findings could be hard for some to swallow.

"If everyone accepted it right off the bat, I'd be shocked," he said.

"Paired Rows of Dots"

Precambrian paleontologist Nick Butterfield said he was "deeply skeptical," about the conclusions drawn.

"From the description—paired rows of dots—it just doesn't sound like a trackway," the University of Cambridge scientist said in email.

"Centipedes and their ilk shuffle along and leave continuous traces in soft (sub-aerially exposed) sediments—they don't carefully step ahead, lifting each foot out of the mud to place it exactly in a previously made footprint," he said.

The probability the tracks were made in an underwater environment is even lower, he said.

"With a specific gravity essentially the same as water/mud, small aquatic invertebrates simply don't sink into the mud and don't leave footprints."