Finding the radula, a chitinous, serrated, conveyor-belt-like tongue used by mollusks to scrape up food, was definitive proof that Pohlsepia was indeed a mollusk. The more accurate answer came in the form of teeth. The synchrotron scan was so fast that the researchers counted tiny, individual tooth marks in the transverse rows of the radula. They observed a central rhachidian tooth, pairs of lateral teeth, marginal teeth, and smooth marginal plates, consisting of at least 11 distinct elements per row. The only animals matching this exact 11-element configuration are nautiloids, ancient, shell-bearing relatives of modern cephalopods.
“Pohlsepia mezonensis is a nautiloid that died, disintegrated, lost its protective covering and was later flattened in the mud and preserved as an indistinct stain,” Clements said.
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But while the fossil’s “oldest octopus” badge is likely gone, it managed to earn itself some new superlatives. “The first one is definitely the hardest fossil to work on that I’ve ever worked on,” Clements said. “The second one will probably be the most fun fossil I’ve ever worked on.”
The results of their study made Pohlsepia the oldest unequivocal evidence of nautiloid soft tissues found in the entire Paleozoic fossil record. “I would say obvious 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 noticed for a long time, and you know, all these new techniques can be used on them,” Clements said.
He believes that even for Pohlsepiya, this is not the end of the story yet. “This is the oldest nautiloid ever found, so it will be interesting to study it further.”
Clements’ study on Pohlsepia mezonensis is published in the Proceedings of the Royal Society B Biological Sciences: https://doi.org/10.1098/rspb.2025.2369
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