Fossils Reveal the World’s Most Persistent Parasite

Those tiny “question marks” etched into ancient shells? They’re not random scratches—they’re the calling cards of a 480-million-year-old parasite!

A new study Published in iScience has revealed that a parasite still plaguing modern oysters today has been around far longer than anyone realized—nearly half a billion years.

The research used high-resolution 3D micro-CT scans to peer inside 480-million-year-old bivalve trace fossils from Morocco, a site famed for its exquisitely preserved marine life. The scans uncovered a series of distinctive etchings, both on the shells’ surfaces and hidden within.

“The marks weren’t random scratches,” said Karma Nanglu, former postdoctoral researcher, now an assistant professor at University of California at Riverside, who led the study. “We saw seven or eight of these perfect question mark shapes on each shell fossil. That’s a pattern.”

Co-Author Javier Ortega-Hernández, professor of organismic and evolutionary biology and curator of invertebrate paleontology at the Museum of Comparative Zoology, said deciphering the marks wasn’t easy. “It took us a while to figure out the mystery behind these peculiar-looking traces. It was as if they were taunting us with their question mark-like shape,” he said. “But as often happens, we came across the answer while deep in obscure literature before our eureka moment.”

The researchers, including postdoc Sarah Losso ('24) and Ph.D. candidate Madeleine Waskom, determined the culprit was a marine bristle worm from the spionid family – soft-bodied parasites that live on and feed from mussels and oysters without consuming their flesh. “They parasitize the shells of bivalves like oysters, not the flesh of the animals themselves,” said Nanglu. “But damaging their shells may increase oyster death rates.”

The fossils belonged to early clam relatives that lived during the Ordovician Period, a time of dramatic evolutionary expansion. “This is a time when ocean ecosystems got more intense,” Nanglu said. “You see the rise of mobility, predation, and, clearly, parasitism.”

Evidence pointed strongly to the spionid worms. One image in particular, from a study of modern worms, showed the exact same shape inside a shell. “That was the smoking gun,” Nanglu said.

To visualize the fossils, the team used high-resolution 3D micro-CT scans which revealed even more parasitized shells buried within the rock layers. “We never would’ve seen this without the scanner,” Nanglu said.

The traces show the worm’s life cycle: larvae settling on shells, dissolving a small area to attach, and burrowing deeper as they grew, creating the signature question mark. “If it’s not a spionid, then it’s something we’ve never seen before,” Nanglu said.

The find offers a rare evolutionary insight. “This group of worms hasn’t changed its lifestyle in nearly half a billion years,” said Nanglu. “We tend to think of evolution as constant change, but here’s an example of a behavior that worked so well, it stayed the same through multiple mass extinction events.”

Today, spionid worms continue to damage oyster shells, impacting commercial fisheries. “This parasite didn’t just survive the cutthroat Ordovician period, it thrived,” Nanglu said. “It’s still interfering with the oysters we want to eat, just as it did hundreds of millions of years ago.”

Morocco’s fossil beds continue to preserve such ancient interactions. “You’re lucky to get any record of an animal from that long ago,” Nanglu said. “But to see evidence of two animals interacting? That’s gold.”

Funding provided by: NSF CAREER award no. 2047192 “Ecological turnover at the dawn of the Great Ordovician Biodiversification Event—Quantifying the Cambro-Ordovician transition through the lens of exceptional preservation.”