New findings published this week challenge a long-running scientific label attached to a famous fossil from Mazon Creek, Illinois: the “world’s oldest octopus” title. Instead, scientists said the 300-million-year-old remains should be reclassified as a nautilus relative, ending a debate that began when the specimen was first described as an octopus in 2000.
The fossil at the center of the re-evaluation is Pohlsepia mazonensis, described as a blob about the size of a human hand. It was found in the Mazon Creek area, about 50 miles southwest of Chicago, which preserves fossils from a period before dinosaurs walked the Earth.
University of Reading zoologist Thomas Clements, who led the new study, said the fossil has long been difficult to interpret and has been the subject of scientific debate. In an interview with the Associated Press, he said, “It’s a very difficult fossil to interpret,” adding, “To look at it, it kind of just looks like a white mush.” He also described why it could be mistaken for a deep-water octopus: “If you look at it and you are a cephalopod researcher and you’re interested in everything octopus, it does superficially look a lot like a deep-water octopus.”
Clements and his team turned to synchrotron imaging, using the technique to look inside the fossil rock in greater detail. Their work identified a radula, a ribbon of teeth known in mollusks including nautiluses and octopuses, and counted teeth in each row. The researchers found that each row had 11 teeth—an arrangement that Clements said does not match what is seen in octopuses.
“This has too many teeth, so it can’t be an octopus,” Clements said. He concluded that the specimen was therefore not the earliest known octopus but “a fossil nautilus, not an octopus.” The study’s teeth also matched those of a fossil nautiloid, Paleocadmus pohli, previously found in the same area.
The reclassification also connects to earlier uncertainty about how the fossil formed. Clements said the mistaken identification may have happened because the creature decomposed and lost its telltale shell before it was fossilized, complicating identification for researchers using external features alone.
The new research, published in the journal Proceedings of the Royal Society B, had an immediate effect beyond academic circles. Guinness World Records said it will no longer list Pohlsepia mazonensis as the earliest known octopus, citing the new conclusions. Managing Editor Adam Millward said the scientists had made “a fascinating discovery,” and added, “We will be resting the original ‘oldest octopus fossil’ title and look forward to reviewing this new evidence.”
The Field Museum in Chicago holds the specimen in its collections, and museum staff said they expected classification questions to persist. Paul Mayer, manager of the museum’s collection of fossil invertebrates, told the Associated Press he was “a little surprised” by the change, but said, “people have been questioning whether it was an octopus ever since the original paper was first published in 2000.” Mayer also said that advances in scientific investigation helped renew interest in the Mazon Creek fossils.
Clements said the museum should treat the update as a chance rather than a setback, arguing that the specimen now represents the oldest soft-tissue nautilus in the world. He said the Field Museum has a small collection of these ancient nautiluses, and called it, “as a cephalopod worker,” “probably the best thing ever.”