300-million-year-old fossil known as the world’s ‘oldest octopus’ was not an octopus after all

For more than 20 years, an octopus fossil called Pohlsepia mazonensis enjoyed an unusual kind of celebrity. Textbooks and the Guinness Book of Records described it as the world’s “oldest known octopus.”

Dated to roughly 300 million years old, it appeared to overhaul the accepted timeline for when octopuses first emerged.

Yet in science, even famous finds can be reopened. When researchers reassessed the specimen, they uncovered a surprise concealed within the rock: the animal was not an octopus after all.

This reversal was not based on speculation. It relied on technology capable of looking through stone and revealing details that had previously been impossible to detect.

Finding Pohlsepia mazonensis in Illinois

First formally described in 2000 after being discovered in Illinois, Pohlsepia mazonensis initially seemed to display hallmark octopus traits.

Researchers thought they could make out eight arms and soft-bodied structures consistent with what is seen in living octopuses.

That reading held sway for decades, and it effectively moved the known origin of octopuses back by around 150 million years.

Even so, not everyone was convinced. The difficulty was straightforward: any attempt to check the fossil more thoroughly risked harming it.

That limitation eased with the use of synchrotron imaging. In this approach, beams of light brighter than the sun are used to scan inside rock without breaking it open. It functions like an extremely powerful X-ray, but with far finer detail.

Tiny teeth inside the rock

Once the scans were complete, the research team spotted something in the Pohlsepia mazonensis fossil that had never been observed before: minute teeth.

That single detail altered the interpretation entirely. The teeth belonged to a radula, a ribbon-like feeding structure found in molluscs. Crucially, the tooth count revealed what the animal really was. The fossil contained at least 11 tooth-like elements in each row.

That pattern does not fit octopuses, which typically have seven or nine. Nautiloids, by contrast, have about 13, and the fossil aligned far more closely with them.

Pohlsepia mazonensis reclassified as a decomposing nautiloid relative

Dr Thomas Clements, lead author and Lecturer in Invertebrate Zoology at the University of Reading, said the result overturned an assumption that had stood for years.

“It turns out the world’s most famous octopus fossil was never an octopus at all,” Clements explained. “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.”

He noted that Pohlsepia mazonensis was first identified as an octopus 25 years ago, but that modern imaging methods exposed what was hidden beneath the rock’s surface and finally resolved the mystery.

The researchers describe the specimen as the oldest soft-tissue evidence of a nautiloid yet found, and argue that it provides a much clearer view of when octopuses truly first appeared on Earth.

Clements also said that revisiting disputed fossils with new techniques can reveal small pieces of evidence that lead to “really exciting discoveries.”

When decay rewrites anatomy

The fossil’s deceptive appearance was not a random fluke. Its form had already been altered well before anyone examined it.

After the animal died, decomposition began. Soft tissues moved, collapsed, and distorted. As time passed, those shifts produced an outline that closely resembled an octopus. By the time the remains were fossilised, the reshaping had already occurred.

The decisive evidence came from within the rock itself. Scientists matched the tiny teeth to those of Paleocadmus pohli, a known nautiloid species discovered at the same site. That connection confirmed the fossil’s real identity.

The find also established a new benchmark: it is now the oldest known example of nautiloid soft tissue, extending that record by roughly 220 million years.

Rethinking octopus history

With Pohlsepia mazonensis removed from the octopus record, researchers now place the emergence of octopuses much later. Current evidence points towards the Jurassic period, rather than a date hundreds of millions of years earlier.

This change also reshapes how scientists view the separation between octopuses and close relatives such as squids. Rather than an extremely ancient split early in Earth’s history, it most likely occurred during the Mesozoic era.

Dr Clements captured the significance succinctly: “It’s amazing to think a row of tiny hidden teeth, hidden in the rock for 300 million years, have fundamentally changed what we know about when and how octopuses evolved.”

Lessons from Pohlsepia mazonensis

Fossils rarely provide an unambiguous message. They preserve the effects of time, pressure, and decay, leaving evidence that is often fragmentary and, at times, misleading.

However, new analytical tools are transforming what can be learnt. Synchrotron imaging, for instance, allows scientists to return to longstanding finds and investigate fresh questions without damaging the specimens. Each reassessment can yield unexpected results.

This episode demonstrates that even celebrated fossils may still conceal vital information. In this case, a single hidden feature-undetected for hundreds of millions of years-was enough to revise part of the story of evolution.

Pohlsepia mazonensis is therefore likely to be remembered not simply as a “mistake”, but as a warning about reading decomposed fossils too literally, and as an example of what modern imaging technology can bring to light.

The complete study appeared in Proceedings of the Royal Society B Biological Sciences.