For more than two decades, a small, blob-like fossil from an Illinois coal mine was held as the ‘oldest known octopus’ on Earth. Now, a closer look has stripped it of that distinction.
The fossil, named Pohlsepia mazonensis, was unearthed at the Mazon Creek Lagerstätte — a remarkably preserved fossil site in Illinois dating back roughly 311 to 306 million years — and described in 2000 as a primitive cirrate octopus. That single specimen went on to reshape ideas about when octopuses and their relatives first evolved, pushing their origins into the Palaeozoic era. But a new analysis reveals the creature was not an octopus at all — it was a decomposed nautiloid, researchers report in Proceedings of the Royal Society B.
The finding “refutes a Palaeozoic origin for octobrachians,” the authors write, supporting instead the view that crown octopuses first appeared in the mid-to-late Mesozoic — roughly 150 million years later than Pohlsepia had implied.
The reidentification rests on a tiny feeding structure called a radula — essentially a toothed tongue used by molluscs to process food — that had been concealed beneath the surface of the fossil’s rocky matrix for over two decades. Using synchrotron micro-X-ray fluorescence at the SOLEIL synchrotron facility in France, the researchers mapped the fossil’s chemistry in extraordinary detail, “definitively confirming a molluscan affinity.”
What they found was a radula with at least 11 distinct elements per transverse row — far more than the 7 or 9 typically seen in octopuses and their coleoid relatives, and consistent with the 13-element rows characteristic of nautiloids. The shape of the teeth also closely matched those of Paleocadmus pohli, a nautiloid already known from the same fossil site.
Additional techniques — including micro-CT scanning, scanning electron microscopy, and multispectral imaging — further undermined the octopus interpretation. Structures previously identified as arms, fins, and eyespots either lacked supporting chemical evidence or could not be confirmed as genuine anatomical features. The researchers concluded that the animal had likely been in an advanced state of decay before being entombed in iron carbonate, leaving its soft tissues ambiguous and, ultimately, misleading.
“The only unequivocal evidence of nautiloid soft tissue in the Palaeozoic fossil record,” the authors note.
Citations
T. Clements et al. Synchrotron data reveal nautiloid characters in Pohlsepia mazonensis, refuting a Palaeozoic origin for octobrachians. Proceedings of the Royal Society B. Published online April 8, 2026. DOI: 10.1098/rspb.2025.2369
