The World’s Oldest ‘Octopus’ Fossil Isn’t an Octopus After All—Here’s What Led to the Discovery
“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."
For 25 years, a hand-sized sea creature embedded in Illinois rock held the title of the world’s oldest octopus. Now, thanks to a machine that shoots extremely powerful beams of light through stone, scientists have discovered the fossil is something else entirely — a relative of the nautilus.
The fossil, known as Pohlsepia mazonensis, was originally identified as an octopus in 2000. That classification was a bombshell. It pushed the origin of eight-armed cephalopods back far earlier than scientists had believed, to roughly 300 million years ago — a time before dinosaurs walked the Earth. The next oldest known octopus fossil is roughly 90 million years old.
“It’s a huge gap,” said lead researcher Thomas Clements, per The Associated Press. “And so that big gap got researchers sort of questioning, ‘Is this thing actually an octopus?’”
Hidden teeth told the real story of the fossil
Clements’ team examined the fossil using a synchrotron, a machine that produces extremely powerful beams of light by accelerating electrons to very high speeds. This allowed them to see inside the rock without breaking it open.
What they found changed everything. Inside the fossil, the researchers discovered a tooth-bearing ribbon called a radula, a feature shared by all mollusks such as nautiluses and octopuses. But the arrangement of the teeth stood out: each row contained 11 teeth, while modern octopuses typically have either seven or nine per row.
“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 were consistent with those of a fossil nautiloid named Paleocadmus pohli, which had been discovered in the same region. Clements suggested that the original misidentification likely occurred because the animal’s shell broke down before fossilization, removing a key feature that would have made it easier to recognize.
A ‘white mush’ that fooled experts
Clements was candid about just how tricky this fossil is to read.
“It’s a very difficult fossil to interpret,” he said, per AP. “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,” he added.
The fossil was found in the Mazon Creek area of Illinois, about 50 miles southwest of Chicago, a region rich in fossils from a period before dinosaurs walked the Earth. It was named after its discoverer, James Pohl, and is housed in the Field Museum in Chicago.
Guinness pulls the fossil record
Following the results, which were published this week in the Proceedings of the Royal Society B, Guinness World Records announced that it would remove Pohlsepia mazonensis from its listing as the oldest known octopus.
Managing Editor Adam Millward described the researchers’ work as an “intriguing discovery.” He added that the organization would temporarily retire the title of “oldest octopus fossil” while it reviews the new findings.
Not everyone was shocked about the findings
Paul Mayer, who oversees the Field Museum’s fossil invertebrate collection, said he was somewhat surprised by the reclassification as a nautiloid. However, he also noted that the fossil’s identity as an octopus had been questioned repeatedly since the original study was published in 2000.
The skepticism had good reason. When paleontologists first identified it as an octopus, the classification challenged earlier views about how eight-armed cephalopods evolved, indicating they appeared far earlier than scientists had believed. That 200-million-year gap between this fossil and the next oldest octopus specimen was always difficult for researchers to explain.
What this means for octopus origins
The reclassification doesn’t erase what scientists know about octopus ancestry. Cephalopods — the broader group that includes octopuses, squids and nautiluses — first appeared in the oceans roughly 500 million years ago. The specific lineage leading to modern octopuses is thought to be at least 300 million years old, though direct fossil evidence is limited because these animals have soft bodies that rarely preserve well. The earliest known octopus-like fossils date to around 160–200 million years ago.
But without Pohlsepia mazonensis in the record book, the timeline of confirmed octopus fossils just got significantly shorter. And the title of world’s oldest octopus? For now, it belongs to no one.
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