Ancient ocean mystery deepens as long-misidentified fossil is finally revealed by researchers

More than two decades after scientists identified a fossil as the world's oldest octopus - officials now say it wasn't one at all.

A recent study published in the journal Proceedings of the Royal Society B found that the fossil Pohlsepia mazonensis - a 300-million-year-old sea creature found in the Mazon Creek fossil beds of northeastern Illinois - was closer to a nautilus than an octopus.

Researchers now believe the creature was a relative of the nautilus, which is a shell-covered cephalopod with tentacles.

NEW 'HELL HERON' SPINOSAURUS SPECIES FOUND IN REMOTE SAHARA 70 YEARS AFTER FOSSIL CLUE

Cephalopods are a class of marine animals that includes octopuses, squids and cuttlefish, and are known for their tentacles and advanced nervous systems - and for lacking rigid skeletons.

The fossil was identified as the world's oldest octopus in 2000 - but is now considered the oldest soft-tissue nautilus in the world.

University of Reading zoologist Thomas Clements, the lead researcher of the new study, told The Associated Press the fossil is a "very difficult [one] to interpret."

He added, "To look at it, it kind of just looks like a white mush."

ARCHAEOLOGISTS FIND ARTIFACTS OLDER THAN STONEHENGE BENEATH BRITAIN'S HOUSES OF PARLIAMENT: 'INCREDIBLY RARE'

"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 determining factor, Clements said, was its teeth - which researchers examined using a synchrotron to peer inside the fossil.

They found that each row had 11 teeth - more than the seven or nine typically found in octopuses.

TEST YOURSELF WITH OUR LATEST LIFESTYLE QUIZ

"This has too many teeth, so it can't be an octopus," Clements said.

"And that's how we realized that the world's oldest octopus is actually a fossil nautilus, not an octopus."

The fossil had the same teeth as a nautiloid called Paleocadmus pohli, an ancient creature found in the same area.

CLICK HERE TO SIGN UP FOR OUR LIFESTYLE NEWSLETTER

Researchers say the mix-up likely occurred because the creature decomposed and lost its shell before it was fossilized.

The next oldest-known octopus fossil is about 90 million years old - around 210 million years younger than the fossil Pohlsepia mazonensis.

"It's a huge gap," Clements said, noting it had long raised questions.

CLICK HERE FOR MORE LIFESTYLE STORIES

"And so that big gap got researchers sort of questioning, 'Is this thing actually an octopus?'"

The fossil is currently held in the Field Museum in Chicago. 

Paul Mayer, who manages the museum's fossil invertebrate collections, said he was "a little surprised" by the new classification, but acknowledged that scientists have questioned the finding for years.

"People have been questioning whether it was an octopus ever since the original paper was first published in 2000," Mayer told the AP.

He added that the news "is great for our collections, and hopefully new discoveries will be made and new stories will be revealed."

The Associated Press contributed reporting.