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.

Analysis of Fossil Features 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.

Reasons for Initial Misidentification 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.

“It’s a huge gap... '" — Thomas Clements, University of Reading zoologist, 2026 (New York Post) The fossil is housed at the Field Museum in Chicago. The museum's fossil invertebrate collections manager noted that the classification had been questioned since the 2000 paper.”

People have questioned the octopus identification since the original 2000 publication. The manager expressed surprise at the new findings but viewed them positively for the museum's collections.

Implications for Cephalopod Research This discovery refines understanding of early cephalopod evolution, highlighting the presence of nautiloids in the fossil record much earlier than previously thought for soft-tissue preservation.

It underscores the challenges in interpreting ancient fossils without advanced imaging techniques. The study emphasizes the importance of detailed anatomical analysis, such as tooth structure, in distinguishing between cephalopod lineages. Future research may uncover additional fossils to fill evolutionary gaps.

The Field Museum holds the specimen, which continues to contribute to paleontological studies. Researchers anticipate that new technologies will reveal more about similar ancient marine life.