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Penis worms have been burrowing into ocean sediments since the famous Burgess Shale was laid down 505 million years ago.

These gruesome creatures are capable of turning their mouths inside out and dragging themselves around by their tooth-lined throat.

Now scientists have found that these bizarre dental structure could be key in helping find previously unknown penis worm species from all over the world.

Penis worms have been burrowing into ocean sediments since the famous Burgess Shale was laid down. Pictured on the right is a fossil of a penis worm from the 508 million year old Burgess Shale. On the left is an image of a modern day penis worm

As penis worms are among the first ever predators, discovery of new species is important for helping scientists understand how animal life on Earth developed.

Reconstructing the teeth of penis worms, or priapulids, in fine detail has enabled researchers to compile a 'dentist's handbook' of the species.

The researchers used electron microscopy to examine the structure of the teeth of these creatures, which first emerged during the 'Cambrian explosion'.

This was a period of rapid evolutionary development about half a billion years ago, when most major animal groups first appear in the fossil record.

The teeth of these Cambrian priapulids had different shapes. Some were shaped like a cone fringed with tiny prickles and hairs, some were shaped like a bear claw while others like a city skyline.

The carnivorous Penis Worm, or Ottoia, known from 500-million-year-old fossils, was a fearsome beast: it could turn its mouth inside out to reveal a tooth-lined throat that looked like a cheese grater. Pictured are various teeth specimens found in the Burgess Shale 

Reconstructing the teeth of penis worms, or priapulids, in fine detail has enabled researchers to compile a 'dentist's handbook' of the species

During the Cambrian, most animals were soft-bodied, like worms and sponges.

But it is difficult to know for certain how far certain species were distributed across the Earth at the time.

'As teeth are the most hardy and resilient parts of animals, they are much more common as fossils than whole soft-bodied specimens,' said Dr Martin Smith, a postdoctoral researcher in Cambridge's Department of Earth Sciences and the paper's lead author.

'But when these teeth - which are only about a millimetre long - are found, they are easily misidentified as algal spores, rather than as parts of animals.

'Now that we understand the structure of these tiny fossils, we are much better placed to a wide suite of enigmatic fossils.'

Both modern and Cambrian penis worms have spent their lives burrowing into the sediment beneath the ocean since they first appeared 500 million years ago.

During the Cambrian, penis worms were voracious predators, gobbling up anything that crossed their path, including worms, shrimp and other marine creatures.

The left image shows a reconstruction of the Penis Worm Ottoia in its burrow. Pictured on the right is a close-up of the everted tooth-covered 'throat' of Ottoia, in a fossil from the Burgess Shale

The teeth of these Cambrian priapulids had different shapes. Some were shaped like a cone fringed with tiny prickles and hairs (pictured), some were shaped like a bear claw while others like a city skyline

They were able to turn their mouths inside out to reveal a tooth-lined throat that looked like a prehistoric cheese grater.

These teeth were not just used for eating food, however.

By turning their mouths inside out, penis worms could also use their teeth as miniature grappling hooks, using them to grip a surface and then pulling the rest of their bodies along behind.

'Modern penis worms have been pushed to the margins of life, generally living in extreme underwater environments,' said Smith.

'But during the Cambrian, they were fearsome beasts, and extremely successful ones at that.'

For this study, the researchers examined fossils of Ottoia, a type of penis worm, about the length of a finger, which lived during the Cambrian.

The fossils originated from the Burgess Shale in Western Canada, the world's richest source of fossils from the period, full of weird-looking creatures that have helped scientists understand how animal life on Earth developed.

Using high resolution electron and optical microscopy, they were able to expose the curious structure of Ottoia's teeth for the first time.

By reconstructing the structure of these teeth in detail, the researchers were then able to recognise fossilised teeth of a number of previously-unrecognised penis worm species from all over the world.

'Teeth hold all sorts of clues, both in modern animals and in fossils,' said Smith.

'It's entirely possible that unrecognised species await discovery in existing fossil collections, just because we haven't been looking closely enough at their teeth, or in the right way.'

The fossils originated from the Burgess Shale in Western Canada, the world's richest source of fossils from the period, full of weird-looking creatures that have helped scientists understand how animal life developed