For more than two decades, a 300-million-year-old blob-like fossil from Illinois held a distinctive title in both scientific records and popular science: the earliest known octopus ever discovered. That designation has now been formally stripped away, after a team of paleontologists produced concrete evidence that the fossilized creature was never an octopus at all — and actually belongs to a different branch of the cephalopod family tree.
Discovered in the fossil-rich Mazon Creek deposit, roughly 50 miles southwest of Chicago, the fossil known as *Pohlsepia mazonensis* was first identified as an early octopus by paleontologists in 2000. That finding upended long-held scientific consensus about octopus evolution, pushing back the estimated origin of eight-tentacled cephalopods by more than 200 million years; previously, the oldest confirmed octopus fossil dated back only 90 million years. This massive chronological gap between *Pohlsepia* and the next confirmed octopus fossil left the scientific community with persistent doubts about the original classification.
“It’s a very difficult fossil to interpret,” explained lead researcher Thomas Clements, a zoologist at the University of Reading. “To look at it, it kind of just looks like a white mush. Superficially, if you’re a cephalopod researcher focused on octopuses, it does bear a resemblance to a deep-sea octopus.” The hand-sized fossil, preserved in rock after the creature died in Carboniferous period seas long before dinosaurs evolved, retained few clear distinguishing features that could resolve the debate — until Clements and his research team turned to advanced imaging technology to uncover hidden traits.
To unpack the “mystery of the weird blob,” the team used a synchrotron, a cutting-edge device that accelerates electrons to generate light beams far brighter than the sun, allowing researchers to examine internal structures within the fossil rock without damaging the specimen. What they found inside settled the long-running debate: a ribbon-like structure of teeth called a radula, a shared feature of all mollusks including both octopuses and nautiluses, but with a distinct arrangement that rules out an octopus classification.
While octopuses typically have either seven or nine teeth per row on their radula, the *Pohlsepia* fossil had exactly 11 teeth per row — a tooth pattern that perfectly matches a known fossil nautiloid, *Paleocadmus pohli*, previously recovered from the same Mazon Creek deposit. Nautiluses are shelled cephalopods, distant relatives of octopuses that still exist in modern oceans today. Clements says the original misidentification likely occurred because the creature’s hard shell decomposed before fossilization, leaving only a soft, blob-like impression of the tentacled animal that confused early researchers.
“This has too many teeth, so it can’t be an octopus,” Clements noted. “And that’s how we realize that the world’s oldest octopus is actually a fossil nautilus, not an octopus.”
Following the publication of the team’s findings this week in the peer-reviewed journal *Proceedings of the Royal Society B*, Guinness World Records announced it will update its listings to remove *Pohlsepia mazonensis* from its title as the earliest known octopus. “The scientists have made a fascinating discovery,” said Guinness Managing Editor Adam Millward. “We will be resting the original ‘oldest octopus fossil’ title and look forward to reviewing this new evidence.”
The fossil, named for its original discoverer James Pohl, is currently held in the collection of Chicago’s Field Museum. While the reclassification means the museum has lost its claim to hosting the world’s oldest octopus, Clements says the collection gains an even rarer specimen: the oldest known fossil of a nautilus that preserves soft tissue, a rare find for organisms whose bodies mostly decay over time. “The Field Museum have a small collection of these ancient nautiluses, which I think as a cephalopod worker is probably the best thing ever,” Clements added. The Field Museum has been contacted for comment on the reclassification and has not yet released a public response.