The discovery of the radula, a chitinous, serrated, conveyor-like tongue that molluscs use to break up food, was the definitive proof that Pohlsepia was indeed a mollusk. A more accurate answer came in the form of teeth. The synchrotron scan was sharp enough to allow the researchers to count tiny, individual tooth marks in the transverse rows of the radula. They observed a central rachidian tooth, a pair of lateral teeth, marginal teeth, and smooth marginal plates that add at least 11 distinct elements to each row. The only animals that fit this exact 11-element configuration are Nautiloids, the ancient, crustacean relatives of modern cephalopods.
“Pohlsepia mazonensis is a nautiloid that dies, disintegrates, loses its protective shell, and is subsequently flattened into mud and preserved as an indistinct blob,” Clements said.
Changing titles
But while the fossil’s “oldest octopus” designation is likely gone, it has managed to gain new advantages. “The first one is certainly the most difficult fossil I’ve ever worked on,” Clements said. “The second one would probably be the most fun fossil I’ve ever worked on.”
The results of his study also made Pohlsepia the oldest unequivocal evidence of nautiloid soft tissues found in the entire Paleozoic fossil record. “I would say unequivocally, because other nautiloid soft tissue fossils are a little questionable,” Clements said.
His team now plans to use the same advanced imaging on other fossils. “Mazon Creek in particular is full of very interesting and very strange fossils that haven’t been looked at in a long time, and you know all these new techniques can be used on them,” Clements said.
He thinks that even for Pohlsepia herself, this is not the end of the story. “This is now the oldest nautiloid, so it would be interesting to study it further.”
Clements’ study of Pohlsepia mazonensis was published in the journal Proceedings of the Royal Society B Biological Sciences: https://doi.org/10.1098/rspb.2025.2369
