Fossilized footprints reveal the deadly caterpillar that ruled the oceans 500 million years ago

Many of the creatures that lived in the seas half a billion years ago were far too soft to leave behind many fossils. But their footprints remain, and one of the most striking finds is a giant, predatory sea millipede.

Our best window into the Cambrian Period of 500 million years ago is the Burgess Shale, a huge rock formation found in British Columbia that holds many fossils dating back to this ancient eon. While paleontologists have found plenty of fossils in the Burgess Shale, these represent only the tiniest fraction of all the sea life from the Cambrian Period, most of which had soft bodies that don't easily fossilize. But the Burgess Shale also holds evidence of ancient trails and burrows created by the organisms of the Cambrian period.

One such trackway holds the best evidence yet of an unusual predator: Tegopelte gigas. This ancient creature was somewhere between a caterpillar and a millipede, sporting a soft shell on its back and 33 pairs of legs. The creature could reach well over a foot long, which by Cambrian standards was positively gigantic. Researchers at the University of Saskatchewan and Royal Ontario Museum knew T. gigas had created this particular trail - the footprints of a 66-legged creature are pretty much unmistakable.

What's more, this new find actually reveal the place of this giant millipede in the Cambrian food chain. The trackway reveals that the creature moved rapidly over the seafloor, with its legs only ever briefly touching the sediment. The researchers say that a creature like this would only need such rapid movement if it was an apex predator that needed to quickly close in on its prey. This means it was a competitor to the Cambrian's swimming predators, and its presence would have gone a long way towards shaping the marine ecosystem and how the species within it evolved.

Via Proceedings of the Royal Society. Artist's impression by Marianne Collins.