In the tropical oceans of the Indo-Pacific, buried beneath the seafloor, dwells a nightmare predator named the bobbit worm. Known to grow up to three metres long, these worms are famous for swinging their bear-trap jaws so forcefully that they sometimes cut fish clean in half.

And it turns out the "terror worm" has a long family history. Fossil remains from Ontario, Canada reveal that a similarly huge snap-jawed relative of the bobbit worm, named Websteroprion armstrongi, lurked in prehistoric seas around 400 million years ago.

An artist's reconstruction of Websteroprion as a fish-hunting carnivore like its living cousin, the bobbit worm. Image: James Ormiston

Bobbit worms belong to a vast group called the polychaetes (or "bristle worms"), which contains several thousand species ranging in size from just a few millimetres to a few metres long. Today, these creatures are found all over the oceans, and besides the larger predators, they also include tiny free-swimming clam worms, long-lived cold seep tubeworms and bone-eating Osedax worms.

Worms are mostly squishy – even the big, scary ones – and squishy things don't fossilise very well. But one body part of Websteroprion was fit for fossilisation: its jaws. The hard mouthparts of bristle worms are called scolecodonts, and they're usually so small that scientists need a microscope to study them. Not Websteroprion. This worm's jaw parts are ten times larger than those of the average bristle worm.

The fossil remains of Websteroprion consisted of fossilised jaw parts as well as impressions in the ancient mud. Image: Luke Parry

The fossils of Websteroprion were dug up back in 1994 amidst the remains of corals, trilobites and other sea-dwellers in the Hudson Bay Basin of Ontario. But it wasn't until just recently that a team of Swedish and British palaeontologists realised what these fossils were, and gave the animal its name.

"This is an excellent example of the importance of looking in remote and unexplored areas for finding new exciting things, but also the importance of scrutinising museum collections for overlooked gems," says David Rudkin of the University of Bristol, one of the scientists who studied the new worm.

Websteroprion has the largest polychaete worm jaws in the fossil record: more than two centimetres across when wide open. And the fact that multiple specimens of the same size were found proves this wasn't just a single mutant giant. Determining the creature's full length is difficult, but by comparing jaw-to-body ratios in living worms, the scientists estimate these ancient titans reached between one and two metres (3-6 feet) long, comparable to the largest bristle worms today.  

But figuring out what exactly this worm was doing with its terrifying jaws isn't as straightforward as you might think. "Although it would perhaps be easy to assume that W. armstrongi had a predatory, carnivorous mode of feeding based on the 'fierce' appearance of the jaws, it has been shown that [modern] jaw-bearing polychaetes exhibit a wide range of feeding habits," the researchers explain in their study.

Living species of bristle worms may use powerful jaws to catch and kill prey, to snag dead plants or animals from the water, or to scrape algae off rocks. Websteroprion may have looked vicious, but without other parts of its anatomy, it's impossible to say whether it was a predator, scavenger or herbivore.

3D reconstruction of the jaw parts of Websteroprion. Not something you want to see coming your way if you're a small sea creature! Image: Luke Parry

Also unknown is why this species grew so big. Was its large size a defence against predators or competition? Was there something about its environment that encouraged it to attain such lengths? Or did the species have some special feature of its anatomy or lifestyle that allowed it to dwarf its fellow worms? We still don't know.

"[Gigantism is] a poorly understood phenomenon among marine worms and has never before been demonstrated in a fossil species," says Mats Eriksson of Lund University, lead author on the study.

What Websteroprion does show is that whatever it is that makes a family of tiny sea worms evolve metres-long species, it has been happening for a long, long time. Who knows what other squishy giants await discovery in the fossil record?