Earliest known octopus is not an octopus after all

A 300-million-year-old fossil, long considered the world’s oldest octopus, has been reclassified as a different species. Researchers at the University of Reading employed cutting-edge technology to examine the fossil, uncovering microscopic teeth that link it to modern nautiloids—creatures with external shells and multiple tentacles.

Revealing hidden clues

Previously, scientists had assumed the fossil displayed characteristics typical of an octopus, such as eight arms and fins. However, synchrotron imaging, which uses light brighter than sunlight, revealed internal structures that contradicted this belief. The teeth found within the fossil matched those of a nautiloid species discovered at the same location in Illinois.

“It turns out the world’s most famous octopus fossil was never an octopus at all,” said Dr Thomas Clements, a lecturer in invertebrate zoology at the University of Reading. “It was a nautilus relative that had been decomposing for weeks before it became buried and later preserved in rock. That decomposition process is what made it look so convincingly octopus-like.”

The fossil, named Pohlsepia mazonensis, was once featured in the Guinness Book of Records as the earliest known octopus. This latest study, published after 25 years of prior research, has provided the first soft tissue evidence of a nautiloid from such an ancient period. The findings also adjust the timeline for octopus evolution, placing their origin in the Jurassic period rather than earlier.

Scientists now believe the split between octopuses and their 10-armed relatives occurred during the Mesozoic era, not hundreds of millions of years prior. This re-evaluation highlights how new techniques can reshape our understanding of ancient life, even when fossils have been studied for decades.