A nautiloid that fooled everyone for twenty-five years.
For nearly a quarter century, a small, unremarkable fossil pulled from an Illinois rock formation held one of paleontology's most coveted titles: the oldest known octopus. Now that title has been revoked. The creature known as Pohlsepia mazonensis, it turns out, was never an octopus at all.
When researchers first described Pohlsepia in 2000, the fossil seemed to push the origin of octopuses back to the late Carboniferous period, somewhere between 311 and 306 million years ago. That was a problem, because every other line of evidence pointed to octopuses, squid, and cuttlefish — collectively the crown coleoids — diverging much later, during the Jurassic. Pohlsepia was an outlier that demanded explanation. For two decades, the field largely accepted the identification while quietly noting the tension it created.
The fossil comes from the Mazon Creek Lagerstätte, a celebrated deposit in Illinois where organisms were buried in iron-rich river mud roughly 300 million years ago. The high iron content caused a mineral called siderite to precipitate around decaying bodies, sealing them inside hard nodules. The catch is that this process doesn't produce three-dimensional casts. It produces flat, two-dimensional stains — faint smears of chemistry against dark rock. Interpreting anatomy from such material, as Thomas Clements of the University of Leicester put it, is a bit like reading a Rorschach test.
The original researchers thought they saw a fused head and mantle, an arm crown, symmetrical fins, and a pair of eyespots, with no trace of a shell. They called it a cirrate octopod — one of the finned, deep-water octopuses. Superficially, Clements acknowledged, it does look the part. But the fossil was missing features you'd expect: no single row of suckers, no arm cirri, no internal shell vestige. Some cryptic pale stains around the body were dismissed as fluids that leaked out during burial.
The first serious crack in the octopus story came in 2019, when a separate team went looking for melanosomes — the organelles responsible for eye pigment — in the supposed eyespots. Melanosomes preserve well in other Mazon Creek fossils. In Pohlsepia, they found nothing. The chemistry didn't match coleoid eyes at all. That result left the identification shaky but unresolved.
Clements and his colleagues decided to settle the question with everything available. They took the Pohlsepia holotype — both halves of the original concretion — to the SOLEIL synchrotron facility in Paris and subjected it to a battery of techniques: high-energy X-ray fluorescence mapping, scanning electron microscopy, micro-CT scanning, and multispectral imaging across multiple wavelengths. The goal was a complete chemical portrait of the fossil.
The results dismantled the octopus case piece by piece. The fins were just contours of the body outline, with no tissue folding. The appendage-like structures were vague and yielded no evidence of eight distinct arms. Micro-CT scans found no biomineralized hooks, which coleoids of that era are known to have possessed. The famous ink sac turned out to be a lump of clay and pyrite. The eyespots were mineral deposits — shallow impressions filled with a dark pyritic matrix, chemically inert, biologically meaningless.
What cracked the case open was something found near the mouth. The synchrotron X-rays penetrated just 30 micrometers into the surrounding rock — barely a hair's width — but that was enough to reveal a radula: the chitinous, toothed, conveyor-belt-like feeding organ used by mollusks to scrape food. A radula confirmed Pohlsepia was a mollusk. The teeth told the rest of the story. Researchers counted the individual tooth impressions across the radula's rows and found a precise arrangement: a central rachidian tooth, pairs of lateral teeth, marginal teeth, and smooth marginal plates — at least 11 distinct elements per row. That exact configuration belongs to nautiloids, the ancient shelled relatives of modern cephalopods.
Pohlsepia mazonensis, Clements concluded, was a nautiloid that died, decomposed, shed its shell, and was pressed flat into the mud — preserved as an ambiguous stain that fooled everyone for twenty-five years. The study is published in Proceedings of the Royal Society B.
The fossil loses its octopus crown but gains something new: it is now the oldest unequivocal evidence of nautiloid soft tissue in the entire Paleozoic fossil record. Clements called it both the hardest and the most enjoyable fossil he has ever worked on. His team plans to turn the same suite of imaging techniques on other long-neglected Mazon Creek specimens — a deposit he describes as full of strange fossils that haven't been seriously examined in years. As for Pohlsepia itself, Clements thinks the story isn't finished. The oldest nautiloid, he noted, is worth studying further.
Notable Quotes
Pohlsepia mazonensis is a nautiloid that died, decomposed, lost its protective shell and was subsequently flattened into the mud and preserved as an ambiguous stain.— Thomas Clements, paleontologist, University of Leicester
Mazon Creek is full of very interesting and very weird fossils which have not been looked at for a long time, and all of these new techniques can be used on them.— Thomas Clements, University of Leicester
The Hearth Conversation Another angle on the story
So the fossil wasn't wrong — the interpretation was wrong?
Exactly. The rock held real information. The problem was that the information was so faint, so chemically subtle, that the original researchers were essentially pattern-matching against a smear.
Why did it take until now to catch the mistake?
Partly technology, partly inertia. The synchrotron imaging they used in Paris didn't exist in a practical form for this kind of work in 2000. And once a fossil gets a famous label, it tends to keep it until someone is motivated to really push.
What was the actual turning point — the thing that broke the octopus story open?
The radula. Everything else chipped away at the identification, but finding that toothed feeding organ was the positive proof. It told you not just what Pohlsepia wasn't, but what it was.
Why does it matter that the eyespots had no melanosomes?
Because eyes leave a chemical signature. Melanin-producing organelles preserve well in these rocks — they've been found in other Mazon Creek fossils. Their complete absence in Pohlsepia meant those spots were never eyes. They were just mineral coincidences.
Does this change anything about how we understand octopus evolution?
It actually resolves a tension that had been sitting in the field for twenty-five years. Pohlsepia was always an uncomfortable outlier. Without it, the timeline for crown coleoid divergence fits the rest of the evidence much more cleanly.
What does it mean that this is now the oldest nautiloid soft tissue fossil?
Nautiloids are ancient and widespread, but their soft parts almost never preserve. Having a confirmed soft-tissue record from 300 million years ago is genuinely rare — it's a different kind of scientific value than the octopus claim ever was.
Is there a lesson here about how paleontologists should handle ambiguous fossils?
Probably that ambiguity should stay on the table longer. The original team noted the problems — the missing suckers, the odd stains — but the overall gestalt of the fossil was persuasive enough to override the doubts. That's a very human thing to do.
What happens to Mazon Creek now?
Clements seems genuinely excited about it. He described the deposit as full of strange, understudied fossils. If the same imaging suite gets applied systematically, there could be more surprises — in either direction.