A new study has unexpectedly discovered that a common parasite of modern oysters actually started infecting bivalves hundreds of millions of years before the dinosaurs went extinct.

The research, published in iScience, used high-resolution 3D scans to look inside 480-million-year-old shells from a Moroccan site known for its exceptionally well-preserved sea life. The scans revealed a series of distinctive patterns etched both on the surface of the fossils and hidden inside them.

“The marks weren’t random scratches,” said Karma Nanglu, a UC Riverside paleobiologist who led the research. “We saw seven or eight of these perfect question mark shapes on each shell fossil. That’s a pattern.”