World’s Oldest Octopus Fossil Is Not an Octopus

New scientific research has overturned the long-held identification of a 300-million-year-old fossil previously recognized as the world’s oldest octopus. The specimen, known as Pohlsepia mazonensis, has been reclassified as a relative of the nautilus, a cephalopod characterized by having both tentacles and a shell.

The findings, published on April 9, 2026, resolve a long-standing scientific debate regarding the origins of eight-tentacled cephalopods. The fossil had previously been listed by Guinness World Records as the earliest known octopus, a designation that suggested these creatures emerged significantly earlier than other evidence indicated.

Advanced Imaging Resolves the “Weird Blob”

The reclassification was led by Thomas Clements, a zoologist at the University of Reading. The fossil is described as a blob roughly the size of a human hand, discovered in the Mazon Creek area of Illinois, approximately 50 miles southwest of Chicago. This region is noted for being rich in fossils from the period before dinosaurs.

Due to its physical state, the fossil has proven exceptionally difficult for researchers to interpret. According to Clements, the specimen kind of just looks like a white mush when viewed normally, though it superficially resembles a deep-water octopus to those specializing in cephalopods.

To penetrate the mystery of the specimen, Clements and his team employed a synchrotron. This technology uses fast-moving electrons to generate beams of light brighter than the sun, allowing scientists to look inside the fossil rock with extreme precision and identify the creature’s true biological lineage.

Impact on Evolutionary Timelines

The initial identification of Pohlsepia mazonensis as an octopus in 2000 had significant implications for evolutionary biology. It suggested that octopuses, squid, and cuttlefish evolved over 150 million years earlier than previously believed.

However, the new evidence reveals a massive chronological gap in the fossil record. With the removal of this 300-million-year-old specimen from the octopus lineage, the next oldest known octopus fossil dates back only about 90 million years.

It’s a huge gap. And so that big gap got researchers sort of questioning, ‘Is this thing actually an octopus?

Thomas Clements, University of Reading zoologist

By correctly identifying the creature as a nautilus relative, scientists are rewriting the understanding of when octopuses actually evolved, removing a misleading data point that had skewed the perceived timeline of cephalopod development for over two decades.