The World’s Oldest ‘Octopus’ Fossil Wasn’t An Octopus At All

A Hidden Set of Teeth Just Rewrote 150 Million Years of Octopus History

For 25 years, a smudgy, palm-sized fossil pulled from a strip mine in Illinois held a title few questioned: the oldest octopus ever found. It became a textbook example, a key calibration point for genetic dating studies of cephalopod evolution. Now an international team has used high-powered X-ray technology to peer beneath the fossil’s surface and find something its describers never could see: a hidden set of teeth showing the creature wasn’t an octopus. It was a decomposed relative of the nautilus, that ancient, spiral-shelled animal still drifting through tropical oceans.

This reclassification, published in Proceedings of the Royal Society B, strips the fossil of its identity and removes the calibration point behind claims that octopuses appeared before 252 million years ago. Without this fossil anchoring the timeline, the evolutionary story of octopuses snaps back into line with other evidence pointing to a later origin, during the age of dinosaurs.

Decades of Doubt Surrounded the ‘Octopus’ Identity

Named Pohlsepia mazonensis, the fossil was first described in 2000 from the Mazon Creek deposit in Illinois, a site known for preserving soft-bodied creatures inside lumps of iron-rich rock. Dated to roughly 311 to 306 million years ago, it appeared to show a sac-like body, paired eye spots, fins, and arms, features that led its describers to classify it as a deep-sea octopus and push the history of octopus-like animals back over 150 million years.

Doubts crept in almost immediately. Researchers noted the fossil lacked features expected of that type of octopus, including an internal shell remnant and a single row of suckers. A 2019 study found no pigment structures in the supposed eye spots, structures that readily preserve elsewhere in Mazon Creek fossils. Some scientists questioned whether the creature was even a mollusk. Excluding the fossil from genetic dating analyses pushed the origin of modern octopuses to between 200 and 145 million years ago, matching the oldest undisputed octopus fossils. The debate stayed stuck without new physical evidence.

Hidden Teeth Exposed a Nautilus Identity, Not an Octopus

Lead author Thomas Clements, of the University of Leicester and University of Reading, and colleagues brought the fossil to the SOLEIL synchrotron facility in France, a ring-shaped particle accelerator generating intense X-ray beams. A focused beam at the surface excited chemical elements in the fossil and rock, causing them to emit signals that revealed structures invisible to the naked eye.

Mapping iron across the mouth region initially hit a wall, since the fossil and rock shared the same iron content. But the sharpness of the scan produced a high-resolution surface map anyway. Hidden beneath the surface sat a radula, the ribbon-like strip of tiny teeth mollusks use to scrape and process food, with beak-like jaw structures nearby. That alone confirmed something uncertain for years: the creature was definitely a mollusk.

Detail in the teeth sealed the case. Researchers counted at least 11 tooth structures per row, including a central tooth and pairs of side and curved outer teeth. Modern octopuses have 7 or 9 elements per row, with markedly different teeth. Nautiluses have 13, and the fossil’s teeth closely matched Paleocadmus pohli, a nautilus relative already known from the same deposit. Researchers think a small pair of plates was probably lost to preservation, a gap common in other Mazon Creek nautilus fossils, which would bring the total to the full 13-part nautiloid row.

Decay Explains the Octopus-Like Appearance

Why did this nautilus relative look so much like an octopus? Decay is the answer. Not every carcass at Mazon Creek was buried quickly; some sat decomposing on the seafloor first. Many supposed octopus features, the arm-like structures, fin-like lobes, and eye spots, were either too degraded to interpret or explainable as decayed nautilus anatomy. Researchers propose the ‘fins’ could instead be decayed retractor muscles or digestive gland lobes already documented in living nautiluses.

A missing shell fits the same picture. Decay studies show a nautilus’s soft body can separate from its shell before burial, though researchers cannot say when that happened here; they found no shell material on the fossil. Additional Mazon Creek fossils resembling the specimen also preserved tooth ribbons tying them to nautilus relatives. Imaging found no confirmed siphon, no evidence of eight distinct arms, no arm hooks, and no pigment in the eye spots.

Researchers reclassified Pohlsepia mazonensis as a junior synonym of Paleocadmus pohli, meaning the two names refer to the same species and the older name takes priority.

Octopus Evolution Timeline Shifts to the Age of Dinosaurs

Because this specimen anchored many early genetic studies estimating when major cephalopod groups split apart, its reclassification removes the calibration point behind those ancient-origin estimates. Molecular studies now point instead to the octopus lineage arising in the Mesozoic, matching fossil evidence of gradual shell reduction in ancient octopus ancestors during the Jurassic. It also weakens the case for reading certain controversial Cambrian fossils, from over 500 million years ago, as early relatives of squid and octopuses.

Still, the fossil gains a different distinction: the only confirmed soft tissue from a nautilus relative of this age, older than any other by roughly 220 million years. A smudge in a lump of ironstone quietly rewrote its own history, and this time, the teeth did the talking.

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