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Though their wriggling, oozing forms may provoke a shudder, worms seem like uniquely fragile creatures—prone to getting flattened underfoot, flash-dried on a sidewalk, or crushed in the beaks of the early birds that patrol the dawn. And yet, scientists have recently discovered one species of worm that would inspire the envy of any survivalist: It endured multiple mass extinction events across a half billion years without ever changing its lifestyle.
Karma Nanglu, a University of California, Riverside, paleobiologist, says she and her colleagues were studying ancient bivalves who lived hundreds of millions of years before the dinosaurs went extinct when they noticed a series of mysterious question mark shapes etched into the fossil shells. They began asking themselves some questions about the traces of this symbol of inquiry, curiosity, and doubt.
“It took us a while to figure out the mystery behind these peculiar-looking traces,” said Javier Ortega-Hernandez, a Harvard University evolutionary biologist, in a statement. “It was as if they were taunting us with their question mark-like shape.” Ortega-Hernandez is a curator at the university’s Museum of Comparative Zoology, which houses the fossils studied by Nanglu’s team.
Nanglu, Ortega-Hernandez, and their collaborators eventually came across their answer in some obscure scientific literature: One image from a study of modern worms showed the identical shape carved into another shell. “That was the smoking gun,” said Nanglu. The slithering question marks, they realized, were made by a family of soft-bodied marine bristle worms that still writhe through today’s oceans. The scientists published their results in iScience.
Read more: “Strange Worms Are Taking Their Place on Your Family Tree”
The bristle worms, which sport little bristles across the lengths of their bodies, belong to a group known as the spionids, which live and feed on mussels and oysters. They aren’t very considerate of their hosts: They damage the bivalve shells, which can eventually kill the creatures that inhabit them. The shells on which the scientists found the worm traces belonged to an ancestor of modern clams that thrived during the Ordovician, a period of time that stretched from 485 million to 444 million years ago, when ocean ecosystems became intensely competitive, featuring increasing movement, predation, and parasitism.
“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.”
At first the team thought the question mark shapes were made by the shellfish themselves or some other organism. But the evidence began to pile up that it was, in fact, the work of the freeloading bristle worm.
To get a better look at the questionable traces the worms left, the researchers used an imaging device called a micro-CT scan: What they found was a layer cake of fossils stacked one on top of the other, bivalves infected with parasites all the way down.
The parasite’s life cycle also offered some hints. It appeared to follow a consistent pattern, the scientists found. In the larval stage, it would settle onto a host shell, destroying a bit of the shell in the process and then burrowing deeper and deeper, creating that unique question mark trace. Spionids are the only animals known to leave behind this mysterious symbol in their wake.
Parasitism: It’s a survival strategy that may never die. 
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Lead image: Nanglu, K., et al. iScience (2025).
