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Penis worms wore shells hundreds of millions of years before hermit crabs did

Fascinating fossils show the marine worms figured out a smart survival strategy.

This artist's illustration shows how a penis worm might have inhabited a shell over 500 million years ago.
Prof Zhang Xiguang, Yunnan University

Penis worms earned their goofy name, thank you very much. They might resemble a private part of the body, but don't hold that against them. They're fascinating marine worms that have now written a new chapter in ancient animal history.

We're all familiar with the house-hopping habits of modern-day hermit crabs, which will move themselves into different shells. An international research team discovered penis worm fossils from the Cambrian period in China that show the worms were doing the same thing over 500 million years ago.

The fossils show the worms took up residence in snail shells. "The researchers established that Cambrian predators were plentiful and aggressive, that forced the penis worms to move into empty shells, leading to the invention of 'hermit' lifestyle," Durham University said in a statement on Tuesday.

This fossil of a penis worm found in China dates back to the Cambrian period.

Zhang Xiguang, Yunnan University

The penis worms' shell-wearing activities predate hermit crabs by hundreds of millions of years. Durham paleontologist Martin Smith is the co-author -- along with researchers from Yunnan University -- of a study on the fossils published in the journal Current Biology this week.

Priapulida worms, if you prefer the more formal name, are still around. Even that name is a nod to their appearance. It comes from Priapus, a Greek fertility god known for his outsized member.

The worms in the study are called Eximipriapulus and the fossil record shows them inside conical shells belonging to now-extinct animals called Hyoliths. 

The researchers discovered four different examples of the worms in shells, each oriented and positioned the same way. The paper describes this as "the first direct evidence of a 'hermiting' life strategy." 

The penis worms of yore tell a story about survival and how animals adapt to the pressures of predators. It's a trick hermit crabs figured out, too. When you've got a soft body, a bit of armor can make the difference between living and becoming a snack.