Hermit crabs are often the face of borrowed-shell real estate, but they might have gotten the idea from some other unexpected creatures – ancient penis worms.
Don’t worry: the images with this story are (mostly) safe for work.
Researchers from Durham University, UK, and Yunnan University, China, analysed vivid, 500-million-year-old fossils and found that the penis worm (Priapulida) may have invented the “hermit” lifestyle during the Cambrian period, millions of years before crabs showed up on the scene.
The researchers studied collections of the Guanshan fossil deposits – famous for incredible preservation of soft tissue, such as worm bodies – along with traditional shelly materials that make up other parts of the fossil record.
They found that four specimens of the penis worm Eximipriapulus were preserved hidden inside the conical shells of long-extinct hypoliths.
“The worms are always sitting snugly within these same types of shells, in the same position and orientation”, explains Dr Martin Smith, co-author of the study, published in Current Biology.
Penis worms live in shell-ters
The abundant and aggressive predators that dominated the Cambrian era likely threatened penis worms immensely, but it seems that where there’s a will[y] there’s a way, because these shy guys adapted by sheathing their sword-like selves and moving into a new housing market. Presumably, the prime real-estate afforded armoured protection for their soft boneless bodies.
“The only explanation that made sense was that these shells were their homes – something that came as a real surprise,” says Smith.
“Not long before these organisms existed, there was nothing alive more complex than seaweeds or jellyfish: so it’s mind-boggling that we start to see the complex and dangerous ecologies usually associated with much younger geological periods so soon after the first complex animals arrive on the scene.”
This type of living arrangement has never before been documented in any living or fossilised penis worm specimen. Nor has it been observed in any other organism living before the Mesozoic Marine Revolution in the age of dinosaurs, when predators learned how to crush shells.
With this in mind, the fact that such a trait evolved independently during the Cambrian explosion of predation – which established the rise of modern animal parts – marks the astounding speed and flexibility of penis worm evolution.
Or maybe “houses” – with all the knobs and handles – were just a little cheaper and more available back then.
Deborah Devis is a science journalist at Cosmos. She has a Bachelor of Liberal Arts and Science (Honours) in biology and philosophy from the University of Sydney, and a PhD in plant molecular genetics from the University of Adelaide.
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