A fossil bed of clam-like animals from a half-billion years ago is covered in tube-dwelling organisms. These suggest the tube dwellers were parasites, scientists now report.
More than 500 million years ago, tube-dwelling creatures spent their lives stuck to the shells of clam-like sea animals called brachiopods (BRAK-ee-oh-podz). Scientists now believe those tube dwellers may be the earliest known parasites.
Parasites are organisms that must live in or on other organisms to stay alive. And their host pays a price.
Explainer: How a fossil forms
Usually, parasites don’t become fossils, says Tommy Leung. That’s because their bodies are often small and soft, he explains. Leung is a parasite specialist who did not take part in the new study. He works at the University of New England in Armidale, Australia.
Parasites are “an integral part of life on Earth,” he says. But it’s been hard to tell when the parasite lifestyle emerged. It likely was very, very long ago, he notes. Today, he notes, “Practically every living thing has some kind of parasitic thing living on or in them.”
Five years ago, scientists reported finding one early parasite. It was a type of tongue worm. Fossils showed that these organisms lived on sea crustaceans some 425 million years ago. Earlier fossils had only hinted at possible parasites.
Scientists Say: Fossil
Now, a fossil bed of brachiopods in Yunnan, China, offers strong evidence for parasites from almost 100 million years earlier. Zhifei Zhang is a paleontologist at Northwest University in Xi’an, China. He was part of a team that described these parasites June 2 in Nature Communications. The fossilized animals they studied date back to 512 million years ago.
Thousands of brachiopod fossils at the site had been clustered in sediment once covered by the sea. Hundreds of them had tube-shaped structures anchored to the outside of their shells. The mouthlike parts of the tubes fanned out along the shell’s opening edges. These tubes appeared only on brachiopods — never alone or on other animals. This suggests that the tube-dwelling creature needed the brachiopod to survive.
This fossilized clam-like sea animal is encrusted with tubes that housed a possible type of tube-dwelling parasite. CREDIT: ZHIFEI ZHANG/NORTHWEST UNIV.
The brachiopods were likely filter feeders. That means they caught whatever food drifted into their open shells. Zhang and his colleagues wondered if the tube-dwellers had been snatching food at the shell’s edge, before the brachiopod could eat it. If true, then tube-covered brachiopods would get less food. And that means they should weigh less than brachiopods without the tube dwellers.
To investigate, Zhang’s group estimated the mass of brachiopods with and without tubes. Tube-free ones almost always were heavier than their tube-covered companions. And that was true regardless of how many tubes were present.
