The oldest octopus on Earth had two too many teeth.
For more than two decades, a palm-sized blob of fossilized tissue sitting in a drawer at the Field Museum in Chicago held one of paleontology's more improbable titles: the oldest known octopus on Earth. That distinction is now gone — not because a better candidate turned up, but because the creature wasn't an octopus to begin with.
The fossil in question, known as Pohlsepia mazonensis, was pulled from the Mazon Creek formation in northeastern Illinois, roughly 50 miles southwest of Chicago. The area is one of the most productive fossil beds in the world, preserving soft-bodied creatures from a period roughly 300 million years ago — long before dinosaurs, long before most of what we'd recognize as modern life. When paleontologists first described Pohlsepia in 2000, they saw eight limbs, a flattened body, and features that looked, at least superficially, like a deep-water octopus. Guinness World Records agreed, and the title stuck.
But the identification never sat entirely comfortably with researchers. The next oldest confirmed octopus fossil is only about 90 million years old, leaving a gap of more than 200 million years between Pohlsepia and everything else in the octopus lineage. That kind of discontinuity tends to make scientists nervous. "It's a huge gap," said Thomas Clements, a zoologist at the University of Reading who led the new investigation. "And so that big gap got researchers sort of questioning, 'Is this thing actually an octopus?'"
Clements and his team decided to look more carefully — not with the naked eye, but with a synchrotron, a machine that accelerates electrons to near-light speed and generates beams of light far more intense than the sun. Aimed at the fossil rock, that light revealed something hidden inside: a radula, the ribbon of teeth that all mollusks use to process food. The radula was intact enough to count. Each row had 11 teeth. Octopuses have either seven or nine. "This has too many teeth, so it can't be an octopus," Clements said.
What it is, the team concluded, is a nautiloid — a relative of the nautilus, the shelled cephalopod that still cruises the deep Pacific today. The tooth pattern matched that of a fossil nautiloid called Paleocadmus pohli, found in the same Mazon Creek deposits. The leading theory for why Pohlsepia was misidentified in the first place is that the creature decomposed and shed its shell before fossilization, leaving behind only the soft tissue — the blob — with no obvious shell to signal what it actually was.
The findings were published this week in the journal Proceedings of the Royal Society B. Guinness World Records responded promptly, announcing it would retire the original listing. Managing Editor Adam Millward called it "a fascinating discovery" and said the organization would review the new evidence before any updated record is considered.
At the Field Museum, where Pohlsepia mazonensis has lived in the invertebrate fossil collection since its discovery — named, incidentally, for the man who found it, James Pohl — the reaction was measured. Paul Mayer, who manages the museum's fossil invertebrate holdings, said he was "a little surprised" by the reclassification, but acknowledged that skepticism about the octopus identification had been present almost from the beginning. He also noted that new imaging technologies have been drawing fresh scientific attention to the Mazon Creek collection, and expressed hope that more discoveries would follow.
Clements, for his part, thinks the museum has reason to be pleased rather than deflated. Pohlsepia may no longer be the world's oldest octopus, but it is now the oldest known soft-tissue nautilus fossil — a distinction that carries its own considerable weight. "The Field Museum have a small collection of these ancient nautiluses," he said, "which I think as a cephalopod worker is probably the best thing ever."
The question of when octopuses actually first appeared on Earth remains open. The 200-million-year gap in the fossil record is back, unresolved, waiting for the next strange blob someone pulls out of the ground.
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, Field Museum manager of fossil invertebrate collections
The Hearth Conversation Another angle on the story
So the fossil was sitting in a museum for over two decades with the wrong label on it?
Essentially, yes. It was identified in 2000, the record was official, and the debate simmered quietly in the background the whole time.
What made scientists keep doubting it?
Mostly that gap — 200 million years between this fossil and the next oldest octopus. That kind of discontinuity in the record is a red flag. Evolution leaves traces, and there were none in between.
And the synchrotron is what finally settled it?
It let them see inside the rock without destroying anything. They found the radula — the tooth ribbon — and counted. Eleven teeth per row. Octopuses max out at nine. That was the answer.
Is it strange that a tooth count could overturn a 25-year-old classification?
It's actually very clean science. Morphology is how you identify species. One clear anatomical mismatch and the whole edifice shifts.
What does this mean for the octopus origin story?
It reopens the question entirely. The 200-million-year gap is back. We still don't know when octopuses first appeared, and now there's no fossil anchor at the early end.
Does the Field Museum lose something here?
On paper, yes — a world record. But Clements made the point that they now hold the oldest soft-tissue nautilus fossil in the world. That's not nothing.
Why does Mazon Creek keep producing these kinds of finds?
The conditions there were unusual — rapid burial in iron-rich sediment that preserved soft tissue, not just bones and shells. Most fossil sites don't do that. Mazon Creek is genuinely rare.