Ancient penis worms requisitioned shells for self-defense, fossil display

The penile worm Eximipriapulus inhabiting a hyolith shell.

Hermit crabs settle in abandoned shells of other animals for shelter, because the ocean is a treacherous place, full of hungry predators. Half a billion years ago, another creature developed the same defensive strategy, suggests new fossil evidence.

But it wasn’t crab. It was a penile worm—an animal that looks exactly what you would expect.

Penile worms are cylindrical invertebrate animals found in the phylum Priapulida, and they Often resemble their genital namesakes, but the similarities end with the general shape. contrary to penis (luckily) priapulids have an expandable proboscis and a strong, evergreen throat lined with sharp teeth. Today, they lead a humble life digging in sediment, feeding on detritus, or capturing small invertebrates. But penile worms have a deep evolutionary history, dating back to 500 million years ago, shortly after an event in which animal life experienced an incredible explosion of evolution and bodily diversity (the “Cambrian explosion”).

It was from this period that four strange fossils emerged from the rock in the eastern province of Yunnan, China, discovered by a team of researchers from Yunnan University in Kunming. The fossils appeared to be penile worms in the ancient Eximipriape kind, but their bodies overlapped closely with the remnants of conical shells.

“What is happening is not immediately obvious, it takes a bit of careful interpretation,” said Martin Smith, paleontologist at Durham University in the UK who collaborated with the Yunnan University team.

Close inspection of the fossils has revealed a remarkable window into ecosystems from the distant past. Worms-a little longer than a fingernail—appear to use empty shells found in their environment as permanent shelters, researchers report in a new paper in Current biology.

Alternative explanations, such as worms using the shells as temporary shelters for molting or laying eggs, are not as likely, the researchers argue. The worms correspond so closely to their funnel-shaped dwellings that the shells were probably carefully selected, much like the relationship of a hermit crab with a shell.

The worm was probably “hermit” in the same way, hiding from predators and looking like an anxious, raw sausage stuffed into a traffic cone. “Defense is really the reasonable explanation,” said Black-smith.

No living priapulid worm is known to behave this way, nor any early animal in the fossil record. The hermitage has evolved several times in different groups of animals, but the worms predate some of the earliest examples of the strategy of about 300 million years ago. They are so old that the animals whose shells they use—hyoliths—have been extinct for hundreds of millions of years.

Fossil of a penile worm partially extended in a hyolith shell.

fossil of a partially extended penis Earthworm in a hyolith shell.
Photo: Professor Zhang Xiguang, Yunnan University

The discovery adds to growing evidence that penile worms were a little more diverse in their habits. Scientists already knew, for example, that certain priapulids were particularly formidable, acting like predators ripping flesh in their old ecosystems.

The discovery also paints a picture of ocean ecosystems soon after the Cambrian explosion that is very different from what has been widely assumed by researchers, according to Black-smith. Predators and the adaptations prey animals use to escape they were thought to be quite inconspicuous compared to ocean ecosystems that came later.

But if the worms on the penis were looking for protection, then something at the time was probably trying to eat them.

“Maybe the Cambrian was actually a pretty mean place, a dangerous place” noted Smith, noting that carnivorous arthropods and their scary parents with shrimp mustache roamed the Cambrian seas.

Half a billion years ago, animal-rich ecosystems not so different from those of today may have burst onto the scene, rather than slowly developing over hundreds of millions of years. .

An adult Priapulus caudatus, a modern species of priapulid

“This is an important discovery,” said Ben Slater, a paleobiologist at Uppsala University in Sweden, not involved in this research, adding that the discovery “demonstrates that while the species and groups of animals that dominated Cambrian ecosystems were very different, they interacted in very different ways. familiar: in other words, different actors following the same scenario.

Sarah Jacquet, a paleontologist at the University of Missouri at Columbia, is also not involved in the new article., noted hermit behavior may have been recorded in the fossil record more than scientists have yet detected, “but the chances of preserving it are extremely low.”

She noted that the findings could steer researchers toward reassessing fossils from other ancient shelled species to see if there is any creatures wearing armor havee been inadvertently ignored.

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