The ‘oldest fossil octopus’ is probably another animal
The over 300-million-year-old Pohlsepia may be a nautilus called Paleocadmus
A Paleocadmus nautilus decays on the seafloor more than 300 million years ago in what will become Illinois. The separated shell is visible in the background. Other animals are visible, such as the marine worm Esconites zelus (foreground) and Bandringa rayi, a relative of sharks (back left).
Dr. Thomas Clements, University of Reading
The oldest fossil octopus isn’t an octopus at all.
That’s the conclusion from new research on a perplexing fossil previously thought to be the most ancient record of an octopus. The findings — published April 8 in Proceedings of the Royal Society B — suggest the roughly 310-million-year-old fossilized sea creature was actually a partly decomposed nautilus. Such a reclassification has implications for scientists’ understanding of the evolution of octopuses, nautiluses and cephalopods as a whole.
In 2000, researchers described an odd fossil found not far from Chicago. It had a round body, finlike structures on one end and a tangle of arms. The fossil was classified as an octopus and named Pohlsepia mazonensis. But that classification produced a conspicuous time gap, given its age of over 300 million years, says paleontologist Thomas Clements at the University of Reading in England. Fossil octopuses were well-known, but not until far later in the geologic record — at least 150 million years.
“It’s been a real trouble for paleontologists to try to understand how Pohlsepia fits into our understanding of octopus evolution,” he says.
When revisiting the mystery mollusk, Clements and his colleagues used high-powered X-rays on the fossil that illuminated different chemicals within the preserved minerals that formed around the soft tissues prior to their decay, giving the researchers a clearer view of Pohlsepia. This technique also revealed a clue about the animal: a preserved radula, the rasping tongue found in many mollusks, including snails, chitons and cephalopods.
“That was the big breakthrough,” says Clements. “Because it’s the only unequivocal character this fossil has.”
The radula is made up of many rows of teeth. Octopuses have seven or nine teeth per row, but Pohlsepia showed at least 11. This is more consistent with a nautilus, an ancient shelled cephalopod that survives as a “living fossil” in oceans today. Pohlsepia’s teeth resembled those on fossilized nautilus radulae belonging to an extinct species, found at the same fossil site, called Paleocadmus pohli. Clements and his team think this is the true identity of the paleontological puzzle.
“There had been serious doubts about the alleged octopod identity of Pohlsepia for some time,” says Alexander Pohle, a paleontologist at Ruhr University Bochum in Germany not involved with the study. “It’s great to see this debate settled with such detailed work!”
The fossil’s preserved soft tissues may not look particularly nautilus-like because it had started to rot before it was fossilized. Rot may also explain why the animal was missing its shell. There are examples of dead modern nautiluses separating from their shells as they decayed, says Clements.
A reassigning of the fossil as a nautilus would mean that octopuses as a group are much younger than 310 million years old, an age that would have meant that cephalopods overall arose quite early in mollusk evolution. The reclassification of “Pohlsepia” relaxes this evolutionary timeline.
It is possible that future technology will reveal even more about the fossil, says Clements.
“Maybe in 10 or 20 years’ time, a new piece of kit will come along and someone will zap Pohlsepia again and be like, ‘Oh, we can now definitely work out what this thing is.’”
