Evolution has produced some pretty weird creatures, and the penis worm is surely at the top of our list.

The penis worm, named after its penis-like shape, first emerged about 500 million years ago. Back then, they were among the most common critters around. And penis worms were vicious. Their mouths were covered with sharp hooks, teeth, and spines that they used to kill prey and as a mode of transportation. When a penis worm needed to move, it would sink its teeth into the ground and then drag itself along toward its prey.

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Here's a video of a modern-day penis worm in action:

Now, science has figured out more about the mysterious and frightening penis worms. Today, a group at the University of Cambrige published a study in the journal Paleontology that catalogs the penis worms's teeth. The researchers took fossils they found in Canada and used some fancy imaging techniques to analyze their chompers at super-high resolutions. The study found that, according to the varieties of teeth found in the fossil record, there were once many more types of penis worms than originally thought.

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Here's what the teeth look like:

The penis worm's wild and weird teeth. Wiley/University of Cambridge

Once you get over how impressively strange and gnarly they are, these teeth can actually be a boon for science. Scientists have looked to the penis worms' teeth for insights into what life may have been like on Earth millions of years ago.

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"As teeth are the most hardy and resilient parts of animals, they are much more common as fossils than whole soft-bodied specimens," said Martin Smith, the lead author the study, in a statement. "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." The study, he said, allowed them to "compile a 'dentist's handbook' that will help paleontologists recognize a range of early teeth preserved in the fossil record."

Penis worms still exist today, but "modern penis worms have been pushed to the margins of life, generally living in extreme underwater environments," said Smith. It's hard out there for a penis worm.

Daniela Hernandez is a senior writer at Fusion. She likes science, robots, pugs, and coffee.