A 480-million-year-old parasite still infects oysters today

Researchers reporting in iScience used high-resolution 3D imaging to examine 480-million-year-old fossil shells from Morocco, a site famous for its exceptionally preserved marine life. The scans uncovered a pattern of unusual markings etched both on the surfaces of the shells and inside them.

"The marks weren't random scratches," explained Karma Nanglu, a paleobiologist at UC Riverside and lead author of the study. "We saw seven or eight of these perfect question mark shapes on each shell fossil. That's a pattern."

Javier Ortega-Hernandez, a Harvard evolutionary biologist and co-author, recalled the team's initial confusion. "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."

Ancient Worms Behind the Mystery Marks

After comparing the markings with modern examples, the scientists concluded that they were made by a soft-bodied marine bristle worm belonging to a group known as spionids. These worms, still common today, bore into the shells of mussels and oysters but typically don't kill their hosts outright.

"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 studied came from early relatives of modern clams that lived during the Ordovician Period, a time of rapid ecological expansion when marine life became increasingly mobile, predatory, and parasitic. "This is a time when ocean ecosystems got more intense," Nanglu said. "You see the rise of mobility, predation, and, clearly, parasitism."

A Half-Billion-Year Parasitic Lineage

The researchers considered other explanations for the distinctive marks, such as self-inflicted shell growth patterns or traces from unrelated organisms. However, the evidence most strongly matched spionid activity.

"There's one image in particular, from a study of modern worms, that shows exactly the same shape inside a shell," Nanglu said. "That was the smoking gun."

The finding offered more than just an identification -- it provided 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."

Peering Inside Fossils with High-Tech Scans

To reveal the interior structures of the shells, the team used a technique similar to medical CT imaging, called micro-CT scanning. This high-resolution method allowed them to visualize internal burrows and hidden shells embedded within the rock layers, which were stacked like a layered cake.

"We never would've seen this without the scanner," Nanglu said.

The Parasite's Ancient Life Cycle

The worm's life cycle helped confirm its identity. The researchers determined that it likely began as a larva that attached itself to a shell, dissolved a small spot to secure its position, and then tunneled deeper as it grew -- creating the recognizable question mark shape.

No other known species leaves this precise trace. ""If it's not a spionid, then it's something we've never seen before," Nanglu said. "But it would have to have evolved the same behavior, in the same place, in the same way."

A Survivor Across Deep Time

Remarkably, the same burrowing behavior continues in today's oceans. Although spionid worms do not consume their hosts directly, the damage they inflict on oyster shells still increases mortality rates in modern 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."

Fossil Time Capsules of Ancient Life

The Moroccan fossil site where these discoveries were made is celebrated for capturing behavior frozen in time. Other fossils there have preserved scenes of animals interacting, such as creatures feeding on the remains of squid-like ancestors.

"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."