A prehistoric mystery found in the soil near Chicago has been solved, and the result changes the history books. For more than two decades, a 310-million-year-old specimen known as Pohlsepia mazonensis was cited as the oldest fossil octopus ever discovered. However, new evidence suggests that the oldest fossil octopus is probably another animal entirely.
The reclassification, detailed in research published April 8 in Proceedings of the Royal Society B, indicates that the creature was not an early octopus but actually a partly decomposed nautilus. This shift in identity does more than just rename a specimen; it removes a massive, perplexing anomaly from the evolutionary timeline of cephalopods.
The fossil had long been a source of contention among paleontologists. Its age—dating back to the Carboniferous period—created a staggering gap in the geologic record. Whereas Pohlsepia was dated to roughly 310 million years ago, other well-documented fossil octopuses did not appear until at least 150 million years ago. This left a void of 160 million years that scientists struggled to explain.
“It’s been a real trouble for paleontologists to endeavor to understand how Pohlsepia fits into our understanding of octopus evolution,” said Thomas Clements, a paleontologist at the University of Reading in England.
The Breakthrough in the ‘Rasping Tongue’
To resolve the identity crisis, Clements and his team turned to advanced imaging technology. Using high-powered X-rays, the researchers were able to illuminate specific chemicals within the minerals that had formed around the creature’s soft tissues before they decayed. This allowed the team to see a structure that had previously remained hidden: the radula.
The radula is a chitinous, rasping tongue used by many mollusks—including snails, chitons, and cephalopods—to feed. Because soft tissues rarely survive the fossilization process, the discovery of a preserved radula provided the first piece of “unequivocal” evidence for the animal’s true identity.
The team focused on the dental architecture of the radula. In modern octopuses, the radula typically features seven or nine teeth per row. However, the Pohlsepia specimen revealed at least 11 teeth per row. This specific count is far more consistent with a nautilus, an ancient shelled cephalopod that still exists today as a “living fossil.”
Further comparison showed that these teeth closely mirrored those of Paleocadmus pohli, an extinct species of nautilus found at the same fossil site. This correlation led the team to conclude that Pohlsepia was, in fact, a member of the nautilus lineage.
Why the Fossil Looked Like an Octopus
The original classification in 2000 was based on the specimen’s round body and a tangle of arms. The lack of a shell—the defining characteristic of a nautilus—led researchers to believe it was an octopus. However, the new study suggests that the “octopus-like” appearance was a result of post-mortem decay.
According to Clements, the animal had begun to rot before it was fully fossilized. This process of decomposition often causes the soft tissues to distort, creating the illusion of different anatomical structures. The missing shell is a common occurrence in decaying nautiluses; modern observations show that dead nautiluses often separate from their shells as they decompose.
Alexander Pohle, a paleontologist at Ruhr University Bochum in Germany who was not involved in the study, noted that there had been “serious doubts about the alleged octopod identity of Pohlsepia for some time.” He described the new, detailed perform as a successful resolution to the long-standing debate.
Impact on Cephalopod Evolution
The reclassification of Pohlsepia significantly alters the understood timeline of cephalopod evolution. By removing this specimen from the octopus lineage, the “start date” for octopuses as a distinct group is pushed forward, making them much younger than 310 million years.
This adjustment “relaxes” the evolutionary timeline, removing the need for scientists to explain a 160-million-year gap in the fossil record. It suggests that the divergence of octopuses from their ancestors happened later and more gradually than the Pohlsepia fossil had previously implied.
| Feature | Previous Classification (Octopus) | New Classification (Nautilus) |
|---|---|---|
| Estimated Age | ~310 Million Years | ~310 Million Years |
| Radula Teeth per Row | 7 to 9 (Expected) | 11+ (Observed) |
| Shell Status | Absent (Natural) | Absent (Due to Decay) |
| Timeline Impact | 160M year gap in record | Consistent with known records |
While the debate over this specific fossil may be settled, the study highlights the precarious nature of soft-tissue paleontology. Because muscles and organs rarely survive in the same way bones or shells do, early classifications are often subject to revision as imaging technology improves.
Looking forward, the research team remains open to the possibility that even more precise data may emerge. Clements noted that in another decade or two, a “new piece of kit” might allow scientists to “zap” the fossil again and extract even deeper anatomical truths.
For now, the scientific community can move forward with a more cohesive understanding of how these intelligent marine creatures evolved, free from the distorting influence of a decomposed nautilus. We invite you to share your thoughts on this discovery in the comments below.
