LONDON — A 300-million-year-old tentacled sea creature has lost its crown as the world’s oldest octopus, after scientists found evidence that it’s not an octopus at all.

Newly published research concludes that fossilized remains listed by Guinness World Records as the earliest known octopus belong instead to a relative of a nautilus, a cephalopod with both tentacles and a shell.

University of Reading zoologist Thomas Clements, the lead researcher behind the new findings, said the fossil, Pohlsepia mazonensis, has long been the subject of scientific debate.

“It’s a very difficult fossil to interpret,” he said. “To look at it, it kind of just looks like a white mush.

“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.”

The creature, a blob about the size of a human hand, was found in the Mazon Creek area of Illinois, about 50 miles southwest of Chicago, that is rich in fossils from a period before dinosaurs walked the Earth.

Its identification by paleontologists as an octopus in 2000 upended ideas about the evolution of the eight-tentacled cephalopods, suggesting they emerged much earlier than previously thought. The next oldest-known octopus fossil is only about 90 million years old.

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

To solve the mystery of the “weird blob,” Clements and his team used a synchrotron — which uses fast-moving electrons to create beams of light brighter than the sun — to look inside the fossil rock. They found a ribbon of teeth known as a radula that is common to all mollusks, including nautiluses and octopuses. Each row had 11 teeth. Octopuses have either seven or nine.

“This has too many teeth, so it can’t be an octopus,” Clements said. “And that’s how we realize that the world’s oldest octopus is actually a fossil nautilus, not an octopus.”

The teeth matched those of a fossil nautiloid called Paleocadmus pohli that had been found in the same area. Clements said the mistaken identification may have happened because the creature decomposed and lost its telltale shell before it was fossilized, complicating identification.

As a result of the findings published this week in the journal Proceedings of the Royal Society B, Guinness World Records said it will no longer list Pohlsepia mazonensis as the earliest known octopus.

Managing Editor Adam Millward said the scientists had made “a fascinating discovery.”

“We will be resting the original ‘oldest octopus fossil’ title and look forward to reviewing this new evidence,” he said.

Pohlsepia mazonensis is named for its discoverer James Pohl, and is in the collection of the Field Museum in Chicago.

Clements said the museum should not be disappointed by the new evidence, which means it now has “the oldest soft tissue nautilus in the world.

“The Field Museum have a small collection of these ancient nautiluses, which I think as a cephalopod worker is probably the best thing ever,” he said.

The museum has been approached for comment.