Researchers studied a fossil untouched for more than two decades at the Royal Ontario Museum in Canada and discovered the remains of the giant extinct bristle worm.
The new species is unique among fossil worms and possesses the largest jaws ever recorded in this type of creature, reaching over one centimetre in length and easily visible to the naked eye, researchers said.
Despite being only known from the jaws, comparison with living species suggests that this animal achieved a body length in excess of a metre.
"Gigantism in animals is an alluring and ecologically important trait, usually associated with advantages and competitive dominance," said Mats Eriksson from Lund University in Sweden.
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"The new species demonstrates a unique case of polychaete gigantism in the Palaeozoic, some 400 million years ago," he added.
The specimens were collected in 1994. Sample materials, from what proved to belong to the Devonian Kwataboahegan Formation, were brought back to the Royal Ontario Museum, where they have been stored until now.