Scientists have found that a 300-million-year-old fossil once called the world’s oldest octopus is not an octopus after all. The fossil, named Pohlsepia mazonensis, was re-examined in a new study published on 8 April 2026. Researchers now believe it was a different sea animal related to the modern nautilus.
The fossil was discovered in Mazon Creek in Illinois, in the United States, and was first formally described in 2000. At that time, scientists thought it had eight arms, fins and other features like an octopus. Because of this, it was even listed in the Guinness Book of Records as the earliest known octopus.
A team from the University of Reading used powerful scanning technology to look inside the fossil more closely. They used beams of light brighter than the sun to reveal tiny details hidden in the rock. Inside, they found very small teeth that matched those of a fossil nautiloid found at the same site.
This showed that the creature could not have been an octopus. Instead, it was likely a relative of the nautilus, a sea animal with many tentacles and an outer shell. Scientists think the animal had been rotting for weeks before it was buried, which made it look more like an octopus.
The new finding means octopuses may have appeared much later than this fossil suggested. Experts now say the evidence supports the idea that octopuses first appeared during the Jurassic period. Lead researcher Dr Thomas Clements said studying old fossils with new tools can uncover tiny clues and lead to exciting discoveries.
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