Study Reclassifies 300-Million-Year-Old Fossil Previously Identified as Oldest Octopus

A recent study published in the journal Proceedings of the Royal Society B has reclassified a 300-million-year-old fossil previously identified as the world's oldest octopus. The fossil, named Pohlsepia mazonensis, was discovered in the Mazon Creek fossil beds of northeastern Illinois. Researchers determined it is more closely related to a nautilus than an octopus.

Cephalopods, the class of marine animals that includes octopuses, squids, and cuttlefish, are characterized by tentacles and advanced nervous systems without rigid skeletons. The nautilus is a shell-covered cephalopod with tentacles. The fossil was initially classified as an octopus in 2000 but is now regarded as the oldest soft-tissue nautilus.

The lead researcher from the University of Reading described the fossil as difficult to interpret, noting it appears as a white mush superficially resembling a deep-water octopus to cephalopod specialists.

“To look at it, it kind of just looks like a white mush. " — Thomas Clements, University of Reading zoologist, 2026 (New York Post) Researchers used a synchrotron to examine the fossil's internal structure, focusing on its teeth. They found each row contained 11 teeth, exceeding the seven or nine typically seen in octopuses.”

This tooth count led to the conclusion that the fossil could not be an octopus. The teeth matched those of Paleocadmus pohli, a nautiloid from the same region.

The mix-up likely resulted from the creature decomposing and losing its shell before fossilization, obscuring its nautilus-like features.

This reclassification addresses a significant chronological gap in cephalopod evolution. The next oldest known octopus fossil dates to about 90 million years ago, 210 million years younger than Pohlsepia mazonensis. This large discrepancy had prompted questions about the original identification.