A 300-million-year-old fossil once believed to be the world’s oldest octopus has been reclassified, after scientists concluded it was not an octopus at all.
New research suggests the specimen, known as Pohlsepia mazonensis, actually belongs to a relative of the nautilus—a shelled cephalopod—rather than an eight-armed octopus. The fossil had previously been listed by Guinness World Records as the earliest known octopus, sparking debate over its true identity.
University of Reading zoologist Thomas Clements, who led the study, said the fossil has long been difficult to interpret due to its poor preservation. He described it as a “very difficult fossil” that can appear deceptively like a soft-bodied octopus at first glance.
Originally discovered in the Mazon Creek fossil beds in Illinois, the specimen dates back to a time before dinosaurs. Its classification as an early octopus in 2000 significantly shifted scientific thinking about cephalopod evolution, suggesting octopuses appeared much earlier than previously believed.
However, Clements and his team used advanced imaging techniques, including synchrotron scanning, to examine the fossil in detail. They identified a radula—a tooth-like structure found in molluscs—but found it had 11 teeth per row, unlike octopuses, which have seven or nine.
This key difference led researchers to conclude the creature was not an octopus. Instead, it matches characteristics of a fossil nautiloid, resolving long-standing uncertainty over the “mysterious blob” fossil.
Scientists say the discovery not only corrects the evolutionary record but also highlights the challenges of interpreting poorly preserved ancient remains.
