Tracks left in a B.C. sea floor half a billion years ago have been linked to a bizarre ancient predator with 66 legs.
It is very rare for fossil tracks to be preserved on the sea floor, and it's even rarer to be able to identify them, said Nicholas Minter, the lead author of the study published Wednesday in the journal Proceedings of the Royal Society B.
The footprints have given Canadian scientists the rare opportunity to indirectly observe the behaviour of a long-extinct animal known as Tegopelte gigas, which is distantly related to horseshoe crabs and millipedes.
"It's very exciting. It's like a detective story in some ways," said Jean-Bernard Caron, a curator at the Royal Ontario Museum in Toronto who first discovered the tracks in B.C.'s Yoho National Park in 2000.
"It really adds to our record of early life. We never found [evidence of the] activity of these animals in that way before."
The trail of footprints several metres long – known as a "trackway" — was the first ever found in the Burgess Shale, considered one of the most important fossil fields in the world. Most fossils found there have been the bodies of strange, undersea creatures that lived in equatorial seas during an ancient period known as the Cambrian Explosion.
The trackway was embedded in such a large slab of rock that Caron was only able to take a small piece of it back with him for analysis.
In 2008, he returned with a helicopter that airlifted out the trackway in carefully packed fragments so they could be shipped back to the museum.
Caron then sought help from Minter, a postdoctoral researcher at the University of Saskatchewan and Gabriela Mangano, a geology professor, at the same university, who had experience analyzing trackways and other types of "trace fossils" – fossil evidence of animals' activities rather than the remains of their physical bodies.
Minter had analyzed the tracks of related animals on land, but never ones from underwater. In order for such undersea tracks to be preserved, he said, they had to be made in very firm, stiff mud and then covered in sediments that could help protect them.
In this case, the tracks were "very distinctive," Minter said.
Based on the size of the trackways, the animal must have been about 30 centimetres long, and from their grouping, the animal that made them had to have had at least 25 pairs of legs.
"When you sort of compare that to all the animals that are known in the Burgess Shale, there's really only one suspect that's left," he said.
Tegopelte, known from fossil specimens, belonged to a group of animals that included a trilobites and was about the size of a flattened loaf of bread. It was an arthropod — a relative of insects, crabs and millipedes — that at first glance resembles a giant woodlouse. However, unlike many well-known arthropods, it had a soft carapace without segments.
"It's kind of hard to describe," Minter said. "The animals back then were all a bit strange."
Once the researchers had figured out what animal the tracks belonged to, they examined the fossils of Tegopelte to see how much its legs were able to move and match that to the prints themselves. They found that each of Tegopelte's legs made only brief contact with the ground in sequence, forming a wave of movement along the length of its body.
"We described it as skimming across the surface," Minter said. The tracks suggested Tegopelte could move very quickly and make tight turns, consistent with other evidence that it was a top predator within its ecosystem.