For 25 years, a single, squashed, 300-million-year-old fossil sat comfortably at the top of the cephalopod hall of fame. Pohlsepia mazonensis had a Guinness World Record. It had a reputation. Scientists cited it as the oldest known octopus on Earth. And the whole time, it was something else entirely.
New research published in Proceedings of the Royal Society B confirms that Pohlsepia was never an octopus. It was a nautiloid—a shelled cephalopod more closely related to the nautiluses still cruising the ocean today than to anything with eight arms and attitude.
When the fossil was first discovered in 2000 in the Mazon Creek Lagerstätte in Illinois, it was already in rough shape. The creature had been decomposing for weeks before mud swallowed it, locking it in for geological eternity. Paleontologists saw what looked like eight limbs, two eyes, and an ink sac and called it an octopus. Fair enough, given the tools they had at the time. The problem is that rotting does weird things to a body.
What finally cracked the case? Teeth. Tiny, ancient, hidden teeth.
How Scientists Used Teeth to Prove the Octopus Wasn’t an Octopus
Using synchrotron imaging—a technique that fires X-rays produced by particle accelerators through dense objects, producing light billions to trillions of times brighter than a hospital X-ray—Thomas Clements and his team at the University of Reading found 11 minuscule tooth-like structures lined up in a row inside the fossil. That’s a radula, a ribbon-like feeding structure covered in denticles found exclusively in mollusks.
Octopuses typically have seven or nine elements per row. Nautiloids have 13. Pohlsepia landed at 11, and the shape of those structures looked far more nautiloid than octopus. The lack of ink sac didn’t help its case either—no melanosomes, no pigment, no ink sac.
“The world’s most famous octopus fossil was never an octopus at all,” Clements told ScienceAlert. “It was a nautilus relative that had been decomposing for weeks before it became buried and later preserved in rock, and that decomposition is what made it look so convincingly octopus-like.”
When researchers compared their findings to other fossils from the same site, the radula matched Paleocadmus pohli, a nautiloid already identified at Mazon Creek—meaning Pohlsepia was never even a distinct species. Just a badly decomposed specimen of something scientists had already correctly named.
The implications rewrite the timeline on both ends. Nautiloid soft tissue preservation now extends back another 220 million years. The earliest confirmed evidence for actual octopuses moves forward by roughly 150 million years.
Science getting it wrong and then correcting itself isn’t a scandal, it’s the whole point. The researchers in 2000 worked with what they had. The researchers in 2025 had a particle accelerator.
The oldest, most celebrated answer in the room still deserves a second look. Even if it has a Guinness World Record.
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