Reconstruction of Hurdia victoria, a formidable predator from the Cambrian era. Fossils of the unique animal were discovered in the 505 million-year-old Burgess Shale, a UNESCO World Heritage Site in British Columbia. (Marianne Collins/Science/AAAS)

A team of scientists from Canada, Britain and Sweden has pieced together fossils of what they believe was a giant predator that roamed the seas 500 million years ago — a century after the fossils were first discovered.

In a time when most creatures were no bigger than a fingernail, the 20-cm-long Hurdia victoria was a giant, earning its nickname as the Tyrannosaurus rex of the Cambrian era, researchers report in the Friday issue of journal Science.

The fossils, found in British Columbia's Burgess Shale, were originally thought to belong to several different species, with some parts attributed to jellyfish, sea cucumbers and arthropods, a groups that includes crustaceans, spiders and insects.

American paleontologist Charles Walcott made the initial discovery in 1909 and described Hurdia victoria as a crustacean-like animal. But the truth is much more complicated, said Allison Daley, the study's co-author and a Burlington, Ont.,-native working on her PhD at Sweden's Uppsala University.

"Back when he found them, he thought they were all separate animals and gave them separate names," said Daley.

"It was only when my co-author, Desmond Collins, did collecting in the '80s and '90s that he discovered that all these different parts ... were actually different parts of just one animal, the hurdia animal."

Once Collins and Jean-Bernard Caron — both of the Royal Ontario Museum and the other Canadian co-authors of the study — started assembling these pieces, it became clear that they were looking at a creature unlike anything they had ever seen.

It has a pair of tiny claws on its head that likely were used to shovel food into its large and toothy mouth, similar to another group of predators from that time, of the genus A nomalocaris.

But its most distinct feature is the shell, or carapace, jutting from the front of its head. While crustaceans often have shells to protect the soft parts of the body, the H urdia shell is empty.

"It's very odd," Caron told CBC News. "We don't understand the connection of this part to the rest of the creature very well."

Caron said the carapace likely slowed the creature down, leading it to probably spend most of its time feeding closer to the sea floor.

Also contributing to the study were Graham Budd of Uppsala University and Gregory Edgecombe of the Natural History Museum in Britain.

The Burgess Shale, located high in the Rocky Mountains, has provided paleontologists with a treasure trove of fossils from about 505 million years ago. It was recognized as a UNESCO World Heritage Site in 1981.

With files from the Canadian Press