For two decades, a 300-million-year-old fossil discovered near Chicago was celebrated as the world’s oldest octopus. Now, advanced X-ray technology has revealed it was never an octopus at all.
The specimen, known as Pohlsepia mazonensis, was found in the Mazon Creek fossil site in Illinois and had long intrigued paleontologists due to its apparent arms, fins and possible ink sac—features that resembled those of a cephalopod. However, its age placed it roughly 150 million years before the earliest known octopuses, raising persistent questions about early cephalopod evolution.
Using powerful synchrotron micro-CT scans—described by researchers as “the most powerful X-rays in the world”—scientists from the University of Reading in the UK examined the fossil’s internal structure without damaging the surrounding rock. The scans uncovered a radula, a microscopic feeding structure composed of rows of teeth, hidden within the matrix.
The fossil contained about 11 teeth per row, a number inconsistent with modern octopuses, which typically have seven to nine. Instead, the dental pattern closely matched that of extinct nautiloids, relatives of the modern nautilus.
Study lead Thomas Clements explained that the creature had begun to decompose before burial, and this decay process softened and distorted its tissues, creating a superficial resemblance to an octopus. “It wasn’t a true octopus,” he said. “It was a decomposing nautiloid relative that was preserved in a way that mimicked octopus anatomy.”
The findings, published April 8, 2026, in the Proceedings of the Royal Society B, resolve a long-standing mystery in paleontology and underscore how advanced imaging techniques are transforming fossil analysis. Even as no longer holding the title of the world’s oldest octopus, the fossil remains a significant specimen for understanding the diversity and evolution of early cephalopods.
