World’s Oldest Octopus Fossil Is Not Actually an Octopus

The application of advanced synchrotron imaging has led to the reclassification of one of the world’s most famous fossils, revealing that a specimen long believed to be the oldest known octopus was actually an entirely different animal. The 300-million-year-old fossil, known as Pohlsepia mazonensis, has been identified as a nautiloid, a shelled cephalopod related to the modern nautilus.

The discovery, published on April 8, 2026, in Proceedings of the Royal Society B, corrects a scientific misinterpretation that had persisted for 25 years. The specimen had previously earned a place in the Guinness Book of Records as the oldest octopus species ever discovered, based on its physical appearance in the rock.

Synchrotron Imaging and Hidden Structures

Researchers from the University of Reading utilized cutting-edge synchrotron imaging to resolve the mystery. This high-powered imaging technique allows scientists to examine the internal structures of fossils in extreme detail, detecting minute features that remain invisible to the naked eye.

By peering inside the rock, the research team discovered small tooth-like features, specifically a radula and a beak. These anatomical structures are characteristic of nautiloids rather than octopuses, providing the definitive evidence needed to reclassify the specimen.

The use of this technology revealed that the fossil’s outward appearance was a result of natural processes rather than its original anatomy. According to paleobiologist Thomas Clements of the University of Reading, the animal had been decomposing for weeks before it was buried and preserved in rock.

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.

Thomas Clements, University of Reading

Impact on Evolutionary Timelines

The reclassification of Pohlsepia mazonensis necessitates a significant redrawing of the cephalopod family tree. Because the fossil is 300 million years old, its new identity as a nautiloid pushes the record of nautiloid soft tissue evidence back by approximately 220 million years.

Conversely, this discovery removes a critical early data point for octopus evolution. The earliest verified evidence for the appearance of octopuses on Earth is now pushed forward by approximately 150 million years.

The specimen was originally discovered in 2000 within the Mazon Creek Lagerstätte in Illinois. For over two decades, it served as a fundamental piece of evidence in describing how octopuses evolved, until the application of modern imaging techniques proved the identity was mistaken.

Technological Correction of the Fossil Record

The case of Pohlsepia mazonensis highlights the role of imaging technology in correcting long-standing errors in paleontology. The confusion stemmed from how the body changed during decay hundreds of millions of years ago, which altered the shape of the animal to resemble an octopus during the fossilization process.

By utilizing synchrotron imaging, researchers can now bypass the misleading external shapes caused by decay and focus on internal biological markers. This approach provides a clearer picture of when different cephalopod groups first appeared and how their physical characteristics evolved over time.

The findings resolve a mystery that had puzzled scientists for decades, demonstrating that even record-breaking discoveries can be overturned when new analytical tools are applied to old specimens.