Fossils discovered in Western Australia provide evidence that animals may have been making tracks in the sand well over 550 million years ago, long before the appearance of the earliest known fossils of animals with hard body parts, such as shells or skeletons.

The fossils are burrows and trails left by creatures that lived on the ocean floor during the Ediacaran period, the last period of the Precambrian eon. Prior to this discovery, the oldest known body fossils of animals date to the beginning of the Cambrian period, about 541 million years ago.

"I think everyone expected the first animals to be microscopic," said Adolf Seilacher, a paleontologist at Yale University who was not involved in the discovery. "No one expected to find animals half a meter long leaving traces on the sea floor more than 550 million years ago. It's quite a discovery."

The new trace fossils come from two localities: Mistaken Point in the Canadian province of Newfoundland, and the Kimberley region of Western Australia. Those from Newfoundland are horizontal tubes and burrows excavated into microbial mats—thin, layered sheets of bacteria and sediment that covered much of the seafloor during the Ediacaran.

The Australian trace fossils, however, are more complex. In addition to U-shaped burrows similar to those found at Mistaken Point, the Kimberley region contains trails of distinctive triangular shape, which could have been made by bilaterally symmetrical animals moving across the seafloor. The researchers, led by paleontologist Bruce Runnegar of the University of California, Los Angeles, found similar triangular trace fossils in rocks dated to the earliest Cambrian period.

The Kimberley trace fossils are about half a meter in size, suggesting that the animals making them were comparatively large. Their three-dimensional preservation also indicates that the creatures were probably not soft-bodied, like jellyfish, but rather had sturdy bodies or shells, says Runnegar.

The researchers, who published their findings in the December issue of the journal Science, say that these trails may be the oldest evidence yet found for animals with true bilateral symmetry, a body plan found in all but a few animal groups today.

"We cannot be sure these traces were made by animals, but they sure look like animals," said Seilacher.