Millions of years before hermit crabs developed their famous trick, penile worms used it successfully.

British and Chinese researchers come to this surprising conclusion after examining fossil remains recovered in China. They came across four penile worms that had crawled into four shells 500 million years ago, after which disaster struck and they died in the shell and eventually fossilized. “The worms were all snug, in the same position and direction, in the same type of shell,” said researcher Martin Smith. It leads the researchers to a startling conclusion. “The only logical explanation is that these shells were their homes.”

hermit crabs

It immediately brings to mind the rather famous hermit crabs. These decapod crustaceans have the habit of lifting empty snail shells onto their backs to protect their vulnerable abdomen. A golden move. But they are not the first to come up with that idea; penile worms existed hundreds of millions of years before the first hermit crabs even saw the light of day, though. However, they don’t use snail shells, Smith and colleagues argue, but cone-shaped shells of the now-extinct hyolites.

Fossil remains reveal that penile worms hid in shells millions of years ago. Image: Prof Zhang Xiguang, Yunnan University.

Surprise

“It comes as a surprise,” says Smith. “Because not long before these penile worms were alive, there was nothing more complex on Earth than seaweed and jellyfish. So it’s mind-boggling to see that so soon after the first complex animals made appearances, we’re already seeing complex and dangerous ecologies typically associated with much younger geological periods.”

many enemies

The fact that the penile worms went in search of a home can easily be explained. They lived during the Cambrian era; the era in which the Cambrian explosion – also called the Big Bang of life – took place. Where the earth previously harbored fairly simple life forms, the organisms became a lot more complex and diverse in this era. For these penis worms it must have resulted in their enemies increasing to such an extent that they saw no other option but to ‘squat’ empty houses.

Still, the fact that penile worms embraced this approach 500 million years ago remains quite remarkable. Researchers have never seen the previously recovered fossilized penile worms do. And also the penile worms that live to this day, as far as we know, don’t care. Nor are we aware of any other organisms that developed this approach so early in Earth’s history. The fact that the penile worms did that in the aftermath of the Cambrian explosion once again reveals how flexible and fast the evolutionary process can be, according to the researchers.