World's Oldest 'Octopus' Fossil Reclassified as Nautilus

Scientists use advanced imaging to determine the 300-million-year-old specimen is not an octopus after all.

Apr. 9, 2026 at 8:19am

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A highly textured abstract painting in earthy tones of green, blue, and brown, featuring sweeping geometric arcs, concentric circles, and precise botanical spirals, conveying a sense of the complex evolutionary forces that shaped the prehistoric marine life of the cephalopod family.Advanced imaging techniques reveal new insights into the ancient evolutionary history of cephalopods, overturning long-held assumptions about the origins of the octopus.Chicago Today

A 300-million-year-old fossil previously identified as the world's oldest octopus has been reclassified by scientists as a relative of the nautilus, a cephalopod with both tentacles and a shell. Researchers used synchrotron imaging to discover the fossil had too many teeth to be an octopus, matching instead a known nautiloid species from the same fossil-rich region of Illinois.

Why it matters

The original identification of the 'Pohlsepia mazonensis' fossil as the earliest known octopus upended scientific understanding of octopus evolution, suggesting they emerged much earlier than previously thought. This new evidence corrects that misclassification and restores the large gap in the octopus fossil record.

The details

The fossilized creature, about the size of a human hand, was found in the Mazon Creek area of Illinois and first identified as an octopus in 2000. However, a team led by University of Reading zoologist Thomas Clements used advanced synchrotron imaging to look inside the fossil and found it had 11 teeth per row on its radula, rather than the 7 or 9 teeth typical of octopuses. The teeth matched those of a known nautiloid fossil from the same region, leading the researchers to conclude the 'Pohlsepia mazonensis' specimen is actually an ancient relative of the nautilus.

  • The 'Pohlsepia mazonensis' fossil was discovered in the Mazon Creek area of Illinois, about 50 miles southwest of Chicago.
  • The fossil was first identified as the world's oldest octopus in 2000.
  • The new research reclassifying the fossil as a nautilus relative was published in April 2026.

The players

Thomas Clements

The lead researcher behind the new findings and a zoologist at the University of Reading.

James Pohl

The discoverer of the 'Pohlsepia mazonensis' fossil, which is now in the collection of the Field Museum in Chicago.

Pohlsepia mazonensis

The fossilized 300-million-year-old tentacled sea creature that was previously identified as the world's oldest octopus.

Paleocadmus pohli

The fossil nautiloid species whose teeth were found to match those of the 'Pohlsepia mazonensis' specimen.

Field Museum

The Chicago museum that houses the 'Pohlsepia mazonensis' fossil.

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What they’re saying

“It's a very difficult fossil to interpret. To look at it, it kind of just looks like a white mush.”

— Thomas Clements, Lead Researcher

“If you look at it and you are a cephalopod researcher and you're interested in everything octopus, it does superficially look a lot like a deep-water octopus.”

— Thomas Clements, Lead Researcher

“It's a huge gap. And so that big gap got researchers sort of questioning, 'Is this thing actually an octopus?'”

— Thomas Clements, Lead Researcher

“This has too many teeth, so it can't be an octopus. And that's how we realize that the world's oldest octopus is actually a fossil nautilus, not an octopus.”

— Thomas Clements, Lead Researcher

“We will be resting the original 'oldest octopus fossil' title and look forward to reviewing this new evidence.”

— Adam Millward, Managing Editor, Guinness World Records

What’s next

The Field Museum, which houses the 'Pohlsepia mazonensis' fossil, has been approached for comment on the reclassification of the specimen.

The takeaway

This discovery corrects a major misunderstanding about the evolution of octopuses, restoring the large gap in the fossil record that the original identification had upended. It also highlights the value of advanced imaging techniques in re-evaluating long-held scientific assumptions about ancient specimens.