A 300-million-year-old fossil long celebrated as the world's oldest octopus has been reclassified as something else entirely—a development that underscores both the self-correcting nature of scientific inquiry and the challenges of interpreting ancient life from fragmentary remains.
The specimen, Pohlsepia mazonensis, was identified as an octopus in 2000 and subsequently listed by Guinness World Records as the earliest known example of the eight-tentacled cephalopod. That classification suggested octopuses emerged far earlier in evolutionary history than scientists had previously believed. Now, newly published research reveals the fossil actually belongs to a nautilid—a relative of the modern nautilus—fundamentally altering what we understand about cephalopod evolution.
The Evidence That Changed Everything
University of Reading zoologist Thomas Clements, lead researcher on the study published this week in the journal Proceedings of the Royal Society B, explained the difficulty in interpreting the fossilized remains. "It's a very difficult fossil to interpret," Clements 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, described as a blob roughly the size of a human hand, was discovered in the Mazon Creek area of Illinois, approximately 50 miles southwest of Chicago—a region renowned for fossils from the period before dinosaurs inhabited Earth. The 2000 identification created what Clements called "a huge gap" in the fossil record: the next oldest-known octopus fossil is only about 90 million years old, leaving a 210-million-year chasm that prompted researchers to question whether the original classification was correct.
To resolve the mystery, Clements and his team employed advanced technology: a synchrotron that uses fast-moving electrons to create beams of light brighter than the sun. This allowed them to examine the fossil's internal structure without damaging it. They discovered a ribbon of teeth called a radula—a feeding structure common to all mollusks, including both nautiluses and octopuses. The critical difference lay in the number: each row contained 11 teeth. Octopuses, by contrast, have either seven or nine.
"This has too many teeth, so it can't be an octopus," Clements explained. "And that's how we realize that the world's oldest octopus is actually a fossil nautilus, not an octopus." The tooth pattern matched those of a fossil nautiloid called Paleocadmus pohli discovered in the same area.
How the Mistake Occurred
Clements suggested the original misidentification likely resulted from decomposition. The creature apparently lost its telltale shell before fossilization, which complicated identification and led paleontologists to focus on the remaining soft tissue. The fossil, named for its discoverer James Pohl, resides in the collection of the Field Museum in Chicago.
Official Recognition of the Correction
Guinness World Records has acknowledged the new findings. Managing Editor Adam Millward stated that the scientists had made "a fascinating discovery," adding: "We will be resting the original 'oldest octopus fossil' title and look forward to reviewing this new evidence."
Clements noted that while the reclassification removes an octopus record, it grants the Field Museum a different distinction: 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.
Why This Matters:
This reassessment demonstrates how scientific understanding evolves through rigorous examination and technological advancement. The correction affects our understanding of cephalopod evolution and the timeline of when major groups of marine life diversified. More broadly, the case illustrates the importance of peer review and the willingness of the scientific community to revise previous conclusions when new evidence emerges. The use of advanced imaging technology to examine fragile fossils without destruction represents progress in how we study Earth's ancient past. For institutions like the Field Museum, the reclassification reframes their specimen's significance rather than diminishing it, highlighting how the same object can contribute meaningfully to scientific knowledge in different ways. This episode also underscores the challenges inherent in paleontological work, where incomplete remains and preservation conditions can lead even careful researchers to provisional conclusions that later investigation proves incomplete.