Franz Anthony / Clements et al., Proc. R. Soc. B., 2026
Reconstruction of a decomposing Paleocadmus prior to burial in the Mazon Creek marine basin
What was considered the oldest fossil of an octopus is, after all, just a poorly preserved specimen of an octopus. Paleocadmus pohli, a known species of nautiloid.
A fossil once celebrated as the world’s oldest octopus has been unmasked as an impostorleading scientists to rethink the evolutionary chronology of cephalopods.
The 300 million year old specimen, Pohlsepia mazonensiswas discovered in 2000 at the Mazon Creek fossil site in Illinois, and was later recognized by the Guinness Book as the oldest octopus. However, a new report, published in the journal Proceedings of the Royal Society B, reveals that the fossil is not an octopus, but a nautiloida shelled marine animal related to modern nautiluses.
This reclassification significantly alters the family tree of cephalopods. By correcting the fossil’s identity, scientists pushed back evidence of soft tissue in nautiloids by approximately 220 million years, while bringing forward the confirmed emergence of octopuses by about 150 million years.
According to paleobiologist Thomas Clements of the University of Reading, the initial misidentification of the fossil was due to its unusual state of preservation. The animal had been decomposing for weeks before burial, causing its remains to resemble octopus features such as eight limbs, eyes and even what appeared to be an ink sac.
For decades, some scientists questioned the classification, but definitive proof remained elusive. Advances in imaging technology have now allowed researchers re-examine the fossil without damaging it. Using synchrotron imaging, an advanced X-ray technique much more powerful than conventional medical tomography, the team was able to observe the rock’s interior in unprecedented detail.
This analysis revealed a crucial anatomical feature: a row of 11 tiny structures similar to teeth known as radula, a feeding organ found in molluscs. The number and shape of these structures were decisive. Although octopuses typically have fewer elements in their radula and nautiloids slightly more, the fossil’s radula closely matched the shape seen in known nautiloid species.
Other evidence has undermined the original octopus classification. THE supposed ink sac did not have melanosomesthe pigmentary structures expected in such an organ.
Comparisons with other fossils from the same site revealed an even more surprising conclusion. The radula was very similar to that of the Paleocadmus moveda known species of nautiloid. Researchers now believe that Pohlsepia mazonensis is not a separate speciesbut simply a poorly preserved specimen of P. pohli.
