The penis worm Eximipriapulus inhabiting a hyolith shell.Illustration: Prof Zhang Xiguang, Yunnan University
Hermit crabs move into other animals’ discarded shells as shelter, because the ocean is a treacherous place, full of hungry predators. Half a billion years ago, another creature evolved the same defensive strategy, suggests new fossil evidence.
But this was no crab. It was a penis worm—an animal that looks precisely how you’d expect.
Penis worms are cylindrical, invertebrate animals found in the phylum Priapulida, and they often do resemble their genital namesakes, but the similarities stop at the general shape. Unlike penises (thankfully), priapulids have an extensible proboscis and a strong, everting throat lined with sharp teeth. These days, they live humble lives burrowing into sediment, feeding on detritus or capturing small invertebrates. But penis worms have a deep evolutionary history, stretching back over 500 million years, shortly after an event where animal life experienced an incredible burst of body plan evolution and diversity (the “Cambrian Explosion”).
It was from this time period that four strange fossils emerged from rock in eastern Yunnan Province, China, discovered by a team of researchers at Yunnan University in Kunming. The fossils appeared to be penis worms in the ancient Eximipriapulus genus, but their bodies were closely overlapping with the remains of conical shells.
“It’s not immediately obvious what’s happening, it takes a little bit of careful interpretation,” said Martin Smith, a paleontologist at Durham University in the U.K. who collaborated with the Yunnan University team.
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Close inspection of the fossils revealed a remarkable window into ecosystems in the deep past. The worms—only a smidge longer than a fingernail—seem to be using empty shells found in their environment as permanent shelters, the researchers report in a new paper in Current Biology.
Alternative explanations, like the worms using the shells as temporary shelters for molting or egg laying, aren’t as likely, the researchers argue. The worms so closely match their funnel-shaped accommodations in size that the shells were probably carefully selected, much like a hermit crab’s relationship with a shell.
The worm was probably ‘hermiting’ in a similar manner, hiding out from predators and looking like an anxious, uncooked sausage stuffed in a traffic cone. “Defense really is the reasonable explanation,” said Smith.
No living priapulid worm is known to behave like this, and neither is any animal from this early in the fossil record. Hermiting has evolved multiple times in different animal groups, but the worms predate some of the first known examples of the strategy by around 300 million years. They’re so old that the animals whose shells they’re using—hyoliths—have been extinct for hundreds of millions of years.
Fossil of a partially extended penis worm in a hyolith shell.Photo: Prof Zhang Xiguang, Yunnan University
The discovery adds to growing evidence that penis worms used to be quite a bit more diverse in their habits. Scientists already knew, for instance, that some priapulids were particularly fearsome, acting as flesh-rending predators in their ancient ecosystems.
The finding also paints a picture of ocean ecosystems shortly after the Cambrian Explosion that is very different from what has largely been assumed by researchers, according to Smith. Predators and the adaptations prey animals use to evade them were thought to be fairly low-key compared to the ocean ecosystems that came later.
But if the penis worms were seeking protection, something back then was probably trying to eat them.
“Maybe the Cambrian was actually quite a nasty place, a dangerous place” said Smith, noting that carnivorous arthropods and their creepy, shrimp-mustached relatives prowled Cambrian seas.
Half a billion years ago, animal-rich ecosystems not that different from today’s may have burst onto the scene, rather than slowly developing over hundreds of millions of years.
An adult Priapulus caudatus, a modern priapulid speciesPhoto: Bruno C. Vellutini (Other)
“This is an important find,” said Ben Slater, a paleobiologist at Uppsala University in Sweden not involved with this research, adding that the discovery “demonstrates that while the species and animal groups that dominated Cambrian ecosystems were very different, they interacted in very familiar ways: in other words, different actors following the same script.”
Sarah Jacquet, a paleontologist at the University of Missouri in Columbia also not involved with the new paper, said the hermiting behavior may be enshrined in the fossil record more than scientists have yet detected, “but the chance of preserving it is extremely low.”
She noted that the findings might steer researchers towards re-evaluating fossils of other ancient shelled species to see if any creatures wearing the armor have been inadvertently ignored.
