The “world’s oldest octopus” was actually a 300-million-year-old fossil impostor hiding its secret in tiny teeth.
A fossil long celebrated as the world’s oldest octopus has now been revealed to be an entirely different marine animal. The 300-million-year-old specimen, which even appeared in the Guinness Book of Records, was misidentified because of changes that happened to the animal’s body before it became fossilized.
Researchers discovered that the fossil, known as Pohlsepia mazonensis, was not an octopus at all. Instead, it belonged to a group related to the modern Nautilus, a shelled sea creature with many tentacles.
The breakthrough came after scientists used advanced synchrotron imaging to examine hidden structures inside the fossil rock. Their scans uncovered tiny preserved teeth that exposed the fossil’s true identity.
The findings were published in Proceedings of the Royal Society B. The study resolves a decades-long mystery surrounding octopus evolution and also provides the oldest known example of preserved nautiloid soft tissue ever discovered. As a result, the fossil’s title as the world’s “oldest octopus” is no longer valid.
Hidden Teeth Solve a Decades-Long Mystery
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.”
Fossil Scans Reveal New Details
The fossil was discovered in Illinois, USA, and first described in a scientific study in 2000. Researchers originally believed the specimen displayed eight arms, fins, and other octopus-like features. That interpretation suggested octopuses existed roughly 150 million years earlier than scientists had previously thought.
Over the years, some paleontologists questioned whether the fossil had been correctly identified, but there was no reliable way to confirm those doubts until now.
In the new research, scientists used synchrotron imaging, a method that relies on beams of light brighter than the sun to detect structures hidden inside rock. The team compared the process to carrying out a modern forensic investigation on a fossil that is 300 million years old.
Ancient Nautiloid Mistaken for Octopus
The scans revealed a radula, a ribbon-like feeding structure lined with rows of teeth that is unique to mollusks. The number and arrangement of those teeth immediately ruled out an octopus.
Researchers found at least 11 tooth-like structures in each row. Octopuses typically have seven or nine teeth per row, while nautiloids have 13.
The teeth closely matched those of Paleocadmus pohli, a fossil nautiloid species already known from the same Illinois fossil site. Scientists concluded the animal had partially decomposed before fossilization, dramatically altering its appearance and making it resemble an octopus.
Octopus Evolution Timeline Changes
The Nautilus, which still exists today, is often called a “living fossil” because its lineage stretches far back into Earth’s history. The Paleocadmus fossils from the Mazon Creek site in Illinois now represent the oldest known preservation of nautiloid soft tissue in the fossil record, surpassing the previous record by about 220 million years.
The discovery also changes scientists’ understanding of when octopuses first evolved. Current evidence now points to octopuses appearing later, during the Jurassic period.
Researchers also now believe the evolutionary split between octopuses and their ten-armed relatives, including squids, occurred during the Mesozoic era rather than hundreds of millions of years earlier.
Dr. Clements said: “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.”
Reference: “Synchrotron data reveal nautiloid characters in Pohlsepia mazonensis, refuting a Palaeozoic origin for octobrachians” by Thomas Clements, Imran Alexander Rahman, Alan R. T. Spencer, Christian Klug, Dirk Fuchs, Isabelle Rouget, Isabelle Kruta, Sebastian Schöder, Jack Wittry, Orla G. Bath Enright and Pierre Gueriau, 8 April 2026, Proceedings of the Royal Society B Biological Sciences.
DOI: 10.1098/rspb.2025.2369
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