Ancient Footprints
© Nanjing Institute of Geology and Palaeontology (NIGP)
The trackways and burrows found in southern China's Dengying Formation, which dates to the Ediacaran period.
Neil Armstrong left the first footprint on the moon, on July 20, 1969. But what about Earth - when did animals first leave footprints here?

While we don't know exactly when animals first left tracks on our planet, the oldest footprints ever found were left between 551 million and 541 million years ago during the Ediacaran period, a new study finds. That's hundreds of millions of years before dinosaurs started roaming Earth, about 245 million years ago. The new findings suggest animals evolved primitive "arms" and "legs" earlier than previously thought.

The odd-looking prehistoric trackways show two rows of imprints that resemble a series of repeated footprints, the researchers said. The scientists found the trackways in the Dengying Formation, a site in the Yangtze Gorges area of southern China.

The trackways' characteristics indicate that a bilaterian animal - that is, a creature with bilateral symmetry that has a head at one end, a back end at the other, and a symmetrical right and left side - made the tracks. This sea-dwelling animal had paired appendages that raised its body above the ocean floor, the footprints left behind by its multiple feet suggest.

These footprints are located next to fossilized burrows. This means that the mystery animal might have periodically dug into the ocean floor's sediments and microbial matts, possibly to mine for oxygen and food, the researchers said.

Both the footprints and the borrows are known as "trace fossils," a term that refers to fossilized remnants that animals leave behind, such as fossilized poop, rather than fossils of the animals themselves. The newfound trace fossils are some of the earliest known evidence for animal appendages on record. Until now, it was thought that bilaterian animals, such as arthropods and annelid worms, first appeared during the Cambrian explosion, between 541 million and 510 million years ago, although some scientists suspected that these animals evolved earlier.

Now, the discovery of the trackways and burrows shows that animals with appendages lived during the Ediacaran period, the researchers said.

The study was published online today (June 6) in the journal Science Advances.