The world’s “oldest octopus” was never an octopus
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The world’s “oldest octopus” was never an octopus
Date:
April 7, 2026
Source:
University of Reading
Summary:
A famous “oldest octopus” fossil has been exposed as a case of mistaken identity. Advanced imaging revealed hidden teeth showing it was actually related to a nautilus, not an octopus. The confusion came from decay that altered its shape before fossilization. This discovery rewrites part of evolutionary history, pushing the true origin of octopuses much later in time.
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FULL STORY
A reconstruction drawing of Pohlsepia mazonensis. Credit: Dr. Thomas Clements, University of Reading
A well-known 300-million-year-old fossil once believed to be the oldest octopus ever discovered has been reclassified after new analysis revealed it is something entirely different. The specimen had even earned a place in the Guinness Book of Records, but scientists now say that distinction was based on a misinterpretation.
The confusion traces back to events that happened long before the fossil formed. As the animal decayed hundreds of millions of years ago, its body changed in ways that later made it resemble an octopus when preserved in rock.
Advanced Imaging Reveals Hidden Teeth
Researchers used cutting-edge synchrotron imaging to examine the inside of the fossil in detail. This powerful technique allowed them to detect tiny structures that cannot be seen with the naked eye. Inside the rock, they found small tooth-like features that changed everything.
The fossil, known as Pohlsepia mazonensis, is not an octopus. Instead, it belongs to a group related to modern Nautilus, marine animals that have multiple tentacles and a distinctive external shell.
The findings, published today (April 8, 2026) in Proceedings of the Royal Society B, resolve a long-standing mystery about octopus evolution that has puzzled scientists for decades. The discovery also provides the earliest known example of preserved soft tissue from a nautiloid and removes the fossil's status as the "oldest octopus" from the record books.
Decay Led to a Scientific Misidentification
Dr. Thomas Clements, lead author and Lecturer in Invertebrate Zoology at the University of Reading, said: "It turns out the world's most famous octopus fossil was never an octopus at all. 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.
"Scientists identified Pohlsepia as an octopus 25 years ago, but using modern techniques showed us what was beneath the surface to the rock, which finally cracked the case. We now have the oldest soft tissue evidence of a nautiloid ever found, and a much clearer picture of when octopuses actually first appeared on Earth.
"Sometimes, reexamining controversial fossils with new techniques reveals tiny clues that lead to really exciting discoveries."
Reexamining a Fossil From Illinois
The fossil was originally discovered in Illinois, USA, and first described in 2000. It quickly became important in studies of cephalopod evolution, with scientists interpreting its features as evidence of eight arms, fins, and other traits associated with octopuses. This pushed the known origin of octopuses back by about 150 million years.
Over time, some researchers questioned this interpretation, but there was no reliable way to investigate further until recently. In the new study, scientists applied synchrotron imaging, which uses extremely bright beams of light to detect hidden structures within rock. The approach has been compared to performing a modern forensic investigation on a specimen that is 300 million years old.
Fossil Teeth Reveal Nautiloid Identity
The scans uncovered a radula, a ribbon-like feeding organ lined with rows of tiny teeth that is found in molluscs. The number and arrangement of these teeth provided a crucial clue. The fossil showed at least 11 tooth-like structures per row, which does not match octopuses, which have seven or nine. Nautiloids, in contrast, typically have 13.
The teeth closely matched those of a known fossil nautiloid species, Paleocadmus pohli, which has been found at the same site. Based on this evidence, the researchers concluded that the animal had partially decomposed before fossilization, altering its appearance and leading to the earlier misidentification.
New Timeline for Octopus Evolution
The Nautilus, a shelled marine animal still living today, is often described as a "living fossil" because of its ancient lineage. The Paleocadmus specimens from the Mazon Creek site in Illinois now represent the oldest known example of nautiloid soft tissue in the fossil record, surpassing the previous record by about 220 million years.
This discovery significantly changes the timeline for octopus evolution. Current evidence now suggests that octopuses appeared much later, during t