Researchers have uncovered fossil evidence from Morocco showing that spionid worms, which parasitize today's oysters, were already boring into ancient bivalve shells 480 million years ago. High-resolution scans revealed distinctive question mark-shaped burrows, linking ancient and modern parasitic behavior. The discovery, published in iScience, highlights an unchanged lifestyle persisting through mass extinctions.
In a study published in the journal iScience, paleobiologist Karma Nanglu from the University of California, Riverside, and her team examined 480-million-year-old fossil shells from Morocco, a site renowned for its well-preserved Ordovician Period marine life. Using micro-CT scanning—a technique akin to medical CT imaging—they visualized internal structures, uncovering seven or eight perfect question mark-shaped markings on each shell.
"The marks weren't random scratches," Nanglu explained. "We saw seven or eight of these perfect question mark shapes on each shell fossil. That's a pattern."
The team, including co-author Javier Ortega-Hernandez from Harvard, initially puzzled over the traces. "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," Ortega-Hernandez said. After consulting obscure literature, they identified the burrows as the work of spionid worms, soft-bodied marine bristle worms that still infest bivalves like oysters and mussels today.
These ancient worms targeted early relatives of modern clams during the Ordovician Period, a time of ecological intensification marked by rising mobility, predation, and parasitism. "This is a time when ocean ecosystems got more intense," Nanglu noted. "You see the rise of mobility, predation, and, clearly, parasitism."
Spionids do not consume their hosts' flesh but damage shells, potentially raising mortality rates. "They parasitize the shells of bivalves like oysters, not the flesh of the animals themselves," Nanglu said. "But damaging their shells may increase oyster death rates."
The worms' lifecycle—starting as larvae that dissolve shell spots and tunnel inward—produced the signature shapes, unmatched by other organisms. "There's one image in particular, from a study of modern worms, that shows exactly the same shape inside a shell," Nanglu added. "That was the smoking gun."
This finding reveals a parasitic strategy unchanged for nearly half a billion years, surviving multiple mass extinctions. "This group of worms hasn't changed its lifestyle in nearly half a billion years," Nanglu observed. "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."
The Moroccan site preserves rare behavioral snapshots, including ancient interactions between species. As Nanglu put it, "You're lucky to get any record of an animal from that long ago. But to see evidence of two animals interacting? That's gold."