Oldest Octopus Fossil May Actually Be a Nautilus

A fossil previously identified as the world’s oldest octopus has been reclassified as a nautilus, correcting a scientific record that stood for more than two decades. The sea creature, which lived approximately 310 million years ago, was long thought to be the most ancient evidence of an octopus, but new research indicates it was actually a partly decomposed nautilus.

The findings, published April 8, 2026, in Proceedings of the Royal Society B, resolve a long-standing discrepancy in the cephalopod fossil record. The reclassification clarifies the evolutionary timeline for octopuses and nautiluses, removing a specimen that had created a significant chronological gap in paleontological data.

The Misidentification of Pohlsepia mazonensis

In 2000, researchers described a fossil discovered near Chicago that featured a round body, a tangle of arms, and finlike structures on one end. Based on these characteristics, the specimen was classified as an octopus and given the name Pohlsepia mazonensis.

Due to its age of over 300 million years, the fossil gained significant notoriety and was featured in the Guinness Book of Records as the oldest known octopus fossil.

However, this classification presented a challenge for paleontologists. While the Pohlsepia fossil dated back roughly 310 million years, other well-documented fossil octopuses did not appear in the geologic record until at least 150 million years ago.

It’s been a real trouble for paleontologists to try to understand how Pohlsepia fits into our understanding of octopus evolution

Thomas Clements, paleontologist at the University of Reading

Technological Breakthroughs in Analysis

To resolve the mystery, Thomas Clements and his colleagues at the University of Reading employed high-powered X-rays to examine the fossil. This technology allowed the researchers to illuminate specific chemicals within the minerals that had formed around the animal’s soft tissues before they decayed.

The imaging provided a clearer view of the creature’s anatomy, leading to the discovery of a preserved radula. The radula is a rasping tongue common to many mollusks, including snails, chitons, and cephalopods.

Clements identified the radula as the only unequivocal character this fossil has, providing the critical evidence needed to determine the animal’s true identity.

Evolutionary Context of Cephalopods

The determination that the fossil was a partly decomposed nautilus aligns more closely with the known evolutionary history of cephalopods. Nautiloids are known for their extreme morphological stability, with fossil records indicating minimal changes over the past 500 million years.

Nautiluses are members of the family Nautilidae and are often described as living fossils because they are the only extant members of the subclass Nautiloidea.

The Nautilidae family is characterized by several distinct features:

• Involute or convoluted shells that are generally smooth.
• Compressed or depressed whorl sections.
• A tubular siphuncle, generally located centrally.
• Straight to sinuous sutures.

By removing Pohlsepia mazonensis from the octopus lineage, scientists can now better understand the actual emergence and diversification of octopuses without the interference of a misidentified 310-million-year-old specimen.