Eleven teeth per row. Octopuses have seven or nine.
For twenty-six years, a small white smear of rock sat in a drawer at the Field Museum in Chicago, holding what scientists believed was a remarkable distinction: the oldest octopus ever found. That claim is now gone. A team of researchers has determined that the creature, a hand-sized blob pulled from an Illinois fossil bed, was never an octopus to begin with.
The fossil in question is called Pohlsepia mazonensis, named for James Pohl, the man who discovered it. It came from the Mazon Creek area, roughly fifty miles southwest of Chicago, a stretch of terrain that has been yielding ancient specimens for decades — creatures from a time before dinosaurs, some 300 million years ago. When paleontologists first classified Pohlsepia as an octopus in 2000, it was a significant moment. The next oldest confirmed octopus fossil is only about 90 million years old, meaning this one, if genuine, represented a gap so enormous it forced researchers to rethink the entire evolutionary timeline of the eight-armed cephalopods.
That gap, it turns out, was a clue something was wrong. Thomas Clements, a zoologist at the University of Reading and the lead author of the new study, said the sheer size of the discrepancy kept nagging at researchers. A 210-million-year jump between the oldest and second-oldest known specimen of any animal is the kind of anomaly that invites scrutiny. "It's a huge gap," Clements said, and that gap kept people asking whether the identification had been right in the first place.
The fossil itself offered little help to the naked eye. Clements described it plainly: it looks like white mush. To someone who studies cephalopods and is primed to see an octopus, the shape can superficially suggest one — particularly a deep-water variety. But superficial resemblance is not classification, and the team needed to look deeper, literally.
They turned to a synchrotron, a machine that accelerates electrons to generate beams of light far more intense than sunlight, capable of illuminating the interior of a rock without destroying it. Inside the fossil, they found a radula — a ribbon-like structure lined with teeth, present in all mollusks. The radula had eleven teeth per row. Octopuses have either seven or nine. That single anatomical detail was decisive. "This has too many teeth, so it can't be an octopus," Clements said.
What it appears to be instead is a nautiloid — a relative of the nautilus, a shelled cephalopod that still exists today. The tooth pattern matched that of a fossil nautiloid called Paleocadmus pohli, found in the same Mazon Creek deposits. Clements believes the original misidentification happened because the creature decomposed and shed its shell before fossilization, leaving behind only soft tissue that, without its defining hard structure, looked ambiguous enough to be mistaken for something else entirely.
The findings were published this week in the journal Proceedings of the Royal Society B. Guinness World Records, which had listed Pohlsepia mazonensis as the earliest known octopus, confirmed it would retire that designation. Managing Editor Adam Millward called the discovery fascinating and said the organization would review the new evidence before considering any updated record.
At the Field Museum, Paul Mayer, who oversees the collection of fossil invertebrates, said he was somewhat surprised by the reclassification, though he acknowledged that doubts about the original identification had circulated almost from the moment the 2000 paper was published. He also noted that new imaging technologies have brought fresh attention to the Mazon Creek collection, and he expects more discoveries to follow.
Clements, for his part, wants the museum to see this as a gain rather than a loss. Pohlsepia mazonensis may no longer be the world's oldest octopus, but it now holds a different title: the oldest known soft-tissue nautilus fossil on record. For a researcher who studies ancient cephalopods, that is not a consolation prize. The search for the true oldest octopus, meanwhile, remains open — and the bar has just been reset to 90 million years ago.
Notable Quotes
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.— Thomas Clements, University of Reading zoologist and lead researcher
People have been questioning whether it was an octopus ever since the original paper was first published in 2000.— Paul Mayer, manager of fossil invertebrate collections at the Field Museum
The Hearth Conversation Another angle on the story
So the fossil was in a museum for decades before anyone figured out it wasn't what they thought it was?
It had been sitting in the Field Museum's collection since its discovery, and doubts started almost immediately after the original 2000 paper — but nobody had the tools to settle the argument until now.
What changed?
The synchrotron. It lets you look inside a rock without cracking it open, using light so intense it can reveal structures too small or faint to see any other way. That's how they found the teeth.
And the teeth were the whole case?
Essentially, yes. Eleven teeth per row. Octopuses have seven or nine. That's not a gray area — it's a hard anatomical boundary, and Pohlsepia was on the wrong side of it.
Why did anyone think it was an octopus in the first place?
The shell was gone. Nautiluses have shells; octopuses don't. But if a nautiloid decomposes before it fossilizes and loses that shell, what's left is soft tissue that can look, to an eager eye, like something else entirely.
Does this change how scientists think about octopus evolution?
It resets the clock significantly. The oldest confirmed octopus fossil is now only about 90 million years old. Whether octopuses existed before that and simply haven't been found yet is still an open question.
Is the Field Museum upset about losing the record?
Clements doesn't think they should be. They now hold something arguably rarer — the oldest soft-tissue nautilus fossil in the world. That's not nothing.
What does Guinness do with a record like this?
They retire it. They said they'd review the new evidence before considering what comes next. For now, the title of world's oldest octopus fossil simply doesn't exist.