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Consider this evolutionary dilemma faced by the aquatic and the squishy: How do you survive in oceans full of hostile, predatory animals?
Squid rely on speed or camouflage. Snails develop complex shells. Hermit crabs borrow these intricate shells when other animals aren’t using them and swap them for larger models as they grow.
Paleontologist Martin Smith of the University of Durham in England said this sheltering strategy is believed to have emerged 180 million years ago, during the Jurassic Period, when the ancestors of hermit crabs emerged in the oceans. But in a study Published Monday in Current Biology, Dr. Smith and colleagues suggest that the practice of hiding in borrowed shells actually dates back hundreds of millions of years, to the birth of complex ecosystems.
The first monks in question were priapulan worms, not crabs—the carnivorous, ocean mud-dwellers commonly and inevitably known as penis worms. Dr. “They’re a neat group of animals with an unfortunate name,” Smith said. “They have a really strange morphology: a somewhat finger-like body and a throat covered with small inverted teeth. Which is disgusting.”
Priapulan worms appeared more than 500 million years ago during the Cambrian period, when animal life rapidly evolved in sophistication, complexity, and size. Saying that worms played an important role in the Cambrian seas, Dr. Smith said: The beginning of the Cambrian period was officially marked by the first appearance of its nests, which helped mix oxygen with the seafloor and made it more hospitable to other living species. life.
Dr. Smith said it was previously unknown that none of the 20 known or extinct species lived in the discarded shells of other animals. However, over the past decade, a team of paleontologists led by Zhang Xi-guang at Yunnan University in China has collected finely detailed Cambrian fossils from approximately 510 million-year-old deposits in Yunnan Province, China. Among these sediments, first discovered in 1999, researchers found a surprise: Traces of four priapulan worms were wrapped in the empty, conical shells of the tiny tentacled creatures. hyolites.
Dr. “The shells are almost exactly the same size as the worms inside, and the worms are always in the same direction — nestled inside with their mouths sticking out,” Smith said. “This made us think it was a biological relationship, not just luck.”
“It’s a compelling case,” said Javier Ortega-Hernandez, a Harvard paleontologist who specializes in invertebrates and was not involved in the study. Fossils showing the two species interacted could be deceiving, he said, as the bodies could be swept together by water currents or collapsing mud hills. But it’s hard to argue about the repeated existence of worms neatly stuck inside the shells, he said.
Dr. “The fact that the mouth opening faces the outside of the shells would also support this interpretation, since the mouth might instead be expected to look inside if the hyolith was feeding on carcass,” Ortega-Hernandez said.
Dr. Smith said the discovery goes against a generally accepted story that monk behavior was the product of an era known as the Mesozoic marine revolution. Beginning about 240 million years ago, countless predators have developed innovations that make life difficult for crustaceans and create an evolutionary arms race.
Dr. “We’re seeing fish with big crushing teeth emerge, we’re seeing crabs and lobsters with big crackling claws,” Smith said. “It’s a bit like going from wandering around with a bow and arrow to someone inventing a machine gun.”
Dr. Smith said researchers traditionally believe the Cambrian oceans had less intense predation as opposed to them. But end discoveries The numbers of large invertebrate predators suggested that the oceans were by no means a safe place for small animals.
Dr. “There is ample evidence that some groups are overly specialized for different types of diets,” Ortega-Hernandez said.
Dr. The new discovery is exciting in part because borrowing shells requires a certain degree of behavioral complexity, Smith said. Some species of modern hermit crabs are notoriously inventive when it comes to choosing new homes, including using human trash and litter. queue by size to change shells. Dr. While penis worms aren’t “noted for their brains,” Smith said, Cambrian fossils imply that they had sensory abilities to search for shells that were just the right size.
Derek Briggs, curator of invertebrate paleontology at the Yale Peabody Museum of Natural History and who was not involved in the study, said the finding suggests that life in the early Cambrian seas was more complex than previously thought. “There were many more habits of life in the Cambrian than we know.”
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