A famous 300 million-year-old fossil once believed to be the oldest octopus ever discovered has been reclassified after new analysis reveals it to be something entirely different. This specimen was even included in the Guinness Book of World Records, but scientists now say this distinction is based on a misunderstanding.
This confusion goes back to events that occurred long before fossils were formed. The animal’s body changed hundreds of millions of years ago as it decomposed, and later began to resemble an octopus when it was preserved in rock.
Revealing hidden teeth with advanced image processing
The researchers used state-of-the-art synchrotron imaging to take a closer look inside the fossil. This powerful technology now allows us to detect tiny structures that are invisible to the naked eye. They discovered tiny tooth-like features in the rock that changed everything.
The fossil, known as Paulsepia mazonensis, is not an octopus. Instead, it belongs to a group related to modern nautiluses, which are marine animals with multiple tentacles and a distinctive outer shell.
The results of this investigation were announced today (April 8, 2026). Proceedings of the Royal Society Ba long-standing mystery about octopus evolution that has puzzled scientists for decades is solved. The discovery also provides the oldest known example of preserved soft tissue from a nautilus, removing the fossil’s status as the “oldest octopus” from the record books.
Corruption led to scientific misconceptions
Dr Thomas Clements, lead author and lecturer in invertebrate zoology at the University of Reading, said: “It turns out that the world’s most famous octopus fossil wasn’t an octopus at all. It was a relative of a nautilus that had been decomposing for several weeks before being buried and later preserved in rock. And it was that decomposition that made it look so convincingly like an octopus.”
“Scientists identified Paulsepia as an octopus 25 years ago, but they used modern technology to uncover what lay beneath the rock’s surface and finally got to the bottom of it. Now we have the oldest soft tissue evidence of a nautiloid ever discovered, and we have a clearer picture of when octopuses actually first appeared on Earth.”
“When we reexamine controversial fossils with new techniques, we sometimes find small clues that lead to really exciting discoveries.”
Reexamining Illinois fossils
This fossil was originally discovered in Illinois, USA, and was first described in 2000. The fossil quickly became important in the study of cephalopod evolution, with scientists interpreting its features as evidence of eight arms, fins, and other traits associated with octopuses. This pushes the known origins of the octopus back to about 150 million years ago.
Over time, some researchers have questioned this interpretation, but until recently there was no reliable way to investigate further. In the new study, scientists applied synchrotron imaging, which uses extremely bright light beams to detect hidden structures within rocks. This approach has been likened to conducting modern forensic investigations on specimens that are 300 million years old.
Fossil teeth reveal the true identity of nautiloids
The scans revealed the radula, a ribbon-like feeding organ with rows of tiny teeth found in molluscs. The number and placement of these teeth provided important clues. The fossil shows at least 11 tooth-like structures per row, which is inconsistent with octopuses, which have seven or nine teeth. In contrast, nautiloids typically have 13.
The teeth closely matched those of a known fossil nautilus species, Palaeocadomus poli, found at the same site. Based on this evidence, the researchers concluded that the animal partially decomposed before fossilizing, changing its appearance and leading to early misidentifications.
A new timeline of octopus evolution
The nautilus is a marine animal that still lives in its shell and is often referred to as a “living fossil” due to its ancient lineage. The Paleocadomus specimen from the Mason Creek site in Illinois is currently the oldest known example of nautilus soft tissue in the fossil record, exceeding previous records by about 220 million years.
This discovery significantly changes the evolutionary timeline of octopuses. Current evidence suggests that octopuses appeared much later, during the Jurassic period. Scientists now believe that the evolutionary divergence between octopuses and 10-armed relatives such as squids occurred in the Mesozoic era, rather than hundreds of millions of years ago.
Dr Clements said: “It’s amazing to think that a tiny hidden row of teeth hidden in a rock for 300 million years has fundamentally changed our knowledge of when and how octopuses evolved.”
