A 300-million-year-old sea creature has officially lost its status as the world’s oldest octopus after recent scientific evidence revealed the specimen is not an octopus at all. The fossil, long celebrated as a milestone in cephalopod evolution, has been reclassified as a relative of the nautilus.
The specimen, known as Pohlsepia mazonensis, had previously held a prestigious spot in the records of prehistoric life. However, research published this week in the journal Proceedings of the Royal Society B concludes that the creature was a cephalopod with tentacles and a shell, rather than the shell-less octopus it was believed to be for over two decades.
For paleontologists, the reclassification does more than just correct a label. it fundamentally alters the timeline of how eight-tentacled creatures evolved. When the fossil was first identified as an octopus in 2000, it suggested these animals appeared hundreds of millions of years earlier than previously thought. With this discovery, a massive evolutionary gap reappears, as the next oldest confirmed octopus fossil dates back only about 90 million years.
The challenge of the “white mush”
Located in the Mazon Creek area of Illinois, roughly 50 miles southwest of Chicago, the region is a goldmine for fossils from the Carboniferous period, long before the rise of the dinosaurs. The Pohlsepia mazonensis specimen is a hand-sized “blob” that has historically defied uncomplicated interpretation.
Thomas Clements, a zoologist at the University of Reading and lead researcher on the project, described the fossil as being notoriously difficult to analyze. According to Clements, to the naked eye, the specimen looks like “white mush.” He noted that for a researcher specifically searching for octopus ancestors, the fossil superficially resembles a deep-water octopus, which likely contributed to the initial misidentification.
The confusion was compounded by the state of the creature’s remains. Clements suggested the animal likely decomposed and lost its protective shell before the fossilization process began, leaving behind a soft-bodied form that mimicked the appearance of a modern octopus.
Solving the mystery with synchrotron imaging
To move beyond visual guesswork, Clements and his team turned to high-energy physics. They employed a synchrotron—a massive particle accelerator that uses fast-moving electrons to generate beams of light significantly brighter than the sun. This technology allowed the team to peer inside the fossilized rock with unprecedented precision.
The breakthrough came when the team located the radula, a ribbon-like structure of teeth common to most mollusks. By counting the teeth in each row of the radula, the researchers found a definitive biological marker: the specimen had 11 teeth per row.
This number provided the “smoking gun” for the reclassification. Octopuses typically possess either seven or nine teeth per row. Because the specimen had too many teeth to be an octopus, the team compared the dental structure to other fossils found in the same Illinois region. They found a perfect match with Paleocadmus pohli, a known fossil nautiloid.
Comparing the Cephalopod Classifications
| Feature | Previous Classification | Current Classification |
|---|---|---|
| Species Group | Octopus | Nautilus relative |
| Estimated Age | ~300 Million Years | ~300 Million Years |
| Dental Count | Assumed 7 or 9 teeth | 11 teeth (Verified) |
| Key Characteristic | Soft-bodied/Shell-less | Soft tissue nautilus |
Impact on records and museums
The fallout of the research has reached beyond the laboratory. Guinness World Records, which had listed Pohlsepia mazonensis as the earliest known octopus, announced it will remove the title. Managing Editor Adam Millward called the findings a “fascinating discovery” and stated the organization would be resting the title while reviewing the new evidence.
The fossil is currently housed in the collection of the Field Museum in Chicago. While the museum loses a record-breaking octopus, it gains a different kind of prestige. Clements pointed out that the institution now possesses the oldest known soft tissue nautilus in the world.
For cephalopod specialists, the presence of these ancient nautiluses in the Field Museum’s collection is a significant asset for studying the early diversification of mollusks and the environmental conditions of the prehistoric Illinois coastline.
The research team continues to analyze the Mazon Creek fossils to better understand the transition of cephalopods from shelled creatures to the highly intelligent, shell-less hunters seen in modern octopuses. The next stage of research will likely focus on filling the 210-million-year gap between this nautilus relative and the first confirmed octopus fossils.
Do you think high-tech imaging will uncover more “hidden” identities in our museums? Share your thoughts in the comments below.
