The fossil was originally found at a site at Mazon Creek in the US state of Illinois, and was formally named and described in 2000.
At the time, scientists thought the fossil showed eight arms, fins and other features typical of an octopus.
However, scientists at the University of Reading decided to use the latest technology to take a look inside the fossil.
They used beams of light, brighter than the sun, to scan the fossil, which revealing new details within the rock.
They discovered tiny teeth that matched those of a fossil nautiloid that was found at the same site in the US, meaning the fossil wasn’t actually an octopus.
Experts say these findings mean that data now supports the theory that octopuses first appeared during the Jurassic period, much later than previously thought.
Lead author of the study, Dr Thomas Clements, from the University of Reading explained: “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.
“Sometimes, re-examining controversial fossils with new techniques reveals tiny clues that lead to really exciting discoveries,” Dr Clements added.
