Half-billion-year-old parasite still threatens shellfish
Rare fossils show worms survived multiple mass extinction events
University of California - Riverside
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Adult marine shell-boring spionid polychaete.
view moreCredit: Vasily Radishevsky/ Far Eastern Branch of the Russian Academy of Sciences
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.”
“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,” said Javier Ortega-Hernandez, paper co-author, Harvard evolutionary biologist and curator at the university's Museum of Comparative Zoology where the fossils used in this study reside.
“But as often happens, we came across the answer while deep in obscure literature before our eureka moment,” he said.
The research team determined the marks are the work of a soft-bodied marine bristle worm, still common in today’s oceans. The worms, which belong to a group called the spionids, live and feed on mussels and oysters without killing them, though they are still destructive.
“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 shells examined in the study belonged to an early relative of modern clams that thrived during the Ordovician, a period of rapid ecological change.
“This is a time when ocean ecosystems got more intense,” Nanglu said. “You see the rise of mobility, predation, and, clearly, parasitism.”
The researchers considered the possibility that the question marks on the fossils were made by the shellfish themselves or by some other kind of organism. But the evidence was strongest for the spionid explanation.
“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.”
Beyond the thrill of identification, the discovery offers a rare evolutionary insight.
“This group of worms hasn’t changed its lifestyle in nearly half a billion years,” Nanglu said. “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.”
To get a look inside these question mark-shaped traces, the researchers used a method similar to a medical CT scan but much more detailed, called micro-CT scanning. This revealed another discovery, that more bivalves with more parasites were hidden from view inside the rock, where the fossil layers were stacked like a multilayered cake.
“We never would’ve seen this without the scanner,” Nanglu said.
The parasite’s life cycle also offered a key clue to its identity. It appears to have followed a consistent pattern: beginning life as a larva, settling onto a host shell at a specific time and place, then dissolving a small area to anchor itself. As it grew, it burrowed farther into the shell, forming the distinctive question mark shape.
No other known animal creates this exact pattern. “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.”
The same shell-burrowing behavior seen in the fossils still affects oysters today. Though spionid worms don’t feed on the animals directly, the structural damage they cause can lead to higher mortality in 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.”
The fossil site in Morocco is renowned for offering snapshots of long-lost behavior. Other finds include animals on the remains of squid-like creatures, providing rare evidence of ancient inter-species interactions frozen in time.
“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.”
Spionid traces on fossilized bivalve shells.
Credit
Javier Ortega-Hernandez/Harvard University
Journal
iScience
Article Title
A 480-million-year-old parasitic spionid annelid
UTEP dinosaur discovery extends known range of ancient species
University of Texas at El Paso
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Jason W. Ricketts, Ph.D., associate professor in The University of Texas at El Paso’s Department of Earth, Environmental and Resource Sciences, discovered fossils – identified as belonging to the dinosaur Tenontosaurus – while conducting unrelated fieldwork at Indio Mountains Research Station, a UTEP-owned natural laboratory near Van Horn, Texas.
view moreCredit: UTEP
EL PASO, Texas (Nov. 4, 2025) – A dinosaur fossil discovery by a researcher from The University of Texas at El Paso may expand the known range of a species that roamed the Earth approximately 115 million years ago.
Jason W. Ricketts, Ph.D., associate professor in UTEP’s Department of Earth, Environmental and Resource Sciences, discovered the fossils – identified as belonging to the dinosaur Tenontosaurus – while conducting unrelated fieldwork at Indio Mountains Research Station (IMRS), a UTEP-owned natural laboratory that spans more than 41,000 acres in southeastern Hudspeth County, about 26 miles southwest of Van Horn, Texas.
“I wasn’t out looking for fossils that day,” said Ricketts. “I was studying the rocks in the area when I noticed fragments weathering out of soft shale. There was no need to excavate – I simply picked them up. It was an unexpected and exciting find, and my family even came out with me to help collect the pieces.”
Ricketts details the discovery in a non-peer reviewed paper titled, An Ornithopod Dinosaur from the Lower Cretaceous of West Texas, which was recently published by the New Mexico Museum of Natural History and Science. Spencer G. Lucas, Ph.D., paleontology curator at the museum, and Sebastian G. Dalman, a doctoral student at Montana State University, coauthored the study.
Dinosaur fossil discoveries in West Texas are rare, and the finding of fossilized bones, rather than just footprints, is especially uncommon, Ricketts said. The fossils were found as individual fragments, with the largest identified as part of a femur leg bone. Although incomplete, the fossils have significant scientific value, he said. Before this discovery, the nearest places where similar fossils had been found were Montana, Idaho, Arizona, and other parts of Texas.
“This discovery extends the known range of Tenontosaurus farther southwest than previously documented,” Ricketts explained. “Until now, fossils of this species were known primarily from localities farther north and east, such as Utah and Wyoming. This finding shows that Tenontosaurus lived as far south as West Texas.”
Tenontosaurus was a medium-sized, plant-eating dinosaur that lived during the Early Cretaceous period. The discovery site adds an important piece to the puzzle of how these dinosaurs dispersed across North America and adapted to diverse environments, the team said.
Liz Walsh, Ph.D., interim dean of the UTEP College of Science, praised the discovery as an example of how curiosity-driven research can lead to major insights.
“This find highlights the importance of fieldwork and the role of dedicated researchers like Dr. Ricketts in uncovering new chapters of Earth’s history,” Walsh said. “It’s also a reminder that major discoveries can happen when we least expect them.”
While the fossil fragments are still being studied, Ricketts hopes the discovery will inspire further exploration in West Texas, a region that remains largely underexplored for dinosaur fossils.
“This discovery shows that there’s still much to learn about our region’s prehistoric past,” he said. “It’s a privilege to contribute even a small piece to that bigger story.”
About The University of Texas at El Paso
The University of Texas at El Paso is America’s leading Hispanic-serving university. Located at the westernmost tip of Texas, where three states and two countries converge along the Rio Grande, 84% of our 26,000 students are Hispanic, and more than half are the first in their families to go to college. UTEP offers 171 bachelor’s, master’s and doctoral degree programs at the only open-access, top-tier research university in America.
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