‘World’s oldest octopus’ fossil isn’t an octopus after all

The mystery of a controversial fossil, once believed to be the oldest known octopus, has finally been resolved.

First described in the early 2000s, Pohlsepia mazonensis was initially thought to push the origins of octopus, squid and cuttlefish back by over 150 million years. However, doubts had been raised over its identity, with researchers struggling to find key features of octopuses in the 310-million-year-old fossil.

New research, led by Dr Thomas Clements at the University of Reading, reveals that this is because it’s not an octopus at all. Instead, it’s likely to be a nautiloid – a member of the group containing the modern nautiluses and their extinct relatives.

“25 years ago, scientists identified Pohlsepia as an octopus,” Thomas says. “Now, using modern techniques to look beneath the surface of the rock, we’ve finally cracked the case.”

“We now have the oldest soft tissue evidence of a nautiloid ever found, and a much clearer picture of when octopuses actually first appeared on Earth. Sometimes, re-examining controversial fossils with new techniques reveals tiny clues that lead to really exciting discoveries.”

Dr Imran Rahman, one of our researchers who co-authored the paper, adds that the soft tissue preserved in the fossil record gives us a better idea of what ancient nautiloids looked like.

“While Pohlsepia might not be an octopus, it’s still a very important fossil preserving the kinds of soft tissues we don’t normally get in the fossil record,” explains Imran. “This helps us piece together the picture of what Pohlsepia might have looked like when it was alive.”

“There are still outstanding challenges for interpreting this animal’s soft parts, but our study helps demonstrate the power of modern analytical techniques for analysing exceptionally preserved fossils to find out more about their anatomy.”

The study was published in the journal Proceedings of the Royal Society B.

An uncertain octopus

The early history of octopuses has long been a mystery. This is because they’re largely made of soft tissue and so don’t fossilise well. In contrast, hard-shelled cephalopods, such as the extinct ammonites and belemnites, are much more common in the fossil record.

Pohlsepia mazonensis seemed to be an incredibly rare exception. Discovered in Mazon Creek in Illinois, USA, the fossil was preserved in an iron-rich mineral known as siderite that seemingly prevented its body from rotting away.

When it was first described, it was thought to be a cirrate octopus, a group which are still found in the depths of the ocean today. Their deep-sea lifestyle means they lack some of the usual features of octopuses, such as the ink sac and a feeding structure known as the radula, which would explain why these weren’t found in Pohlsepia.

Other scientists were more sceptical. They suggested that the reason these features couldn’t be found was that the fossil didn’t represent an octopus at all. However, there wasn’t an easy way to test this at the time.

A quarter of a century later, technology has now caught up with the ambitions of the scientific community and allowed Pohlsepia to be reinvestigated.

“We wanted to apply a range of modern analytical methods to this fossil to see what we could find out about it,” adds Imran. “We used scanning electron microscopy, micro-CT scanning and a synchrotron – a type of particle accelerator that produces intense X-rays – to analyse the fossil.”

“These methods allowed us to peer beneath the surface of the fossil for the first time and work out what this animal actually is.”

A decayed fossil

The scans revealed that Pohlsepia can’t have been a cirrate octopus, as it didn’t have even the most basic features of an octopus. There’s no siphon, while the fossil lacks any distinct arms or tentacles.

Instead, the team found a radula that hadn’t been detected in the initial study. The radula is used by molluscs to grind down food using rows of teeth, and the number of teeth in each row varies between different groups. Octopuses have seven or nine, but Pohlsepia has 11, meaning it can’t be an octopus.

As Pohlsepia’s radula bears a striking resemblance to that of Paleocadmus pohli, a fossil nautiloid also found in the Mazon Creek, the team think they might be the same species. It seems that Pohlsepia’s body partly rotted before fossilising, which disguised its nautiloid identity.

This revelation rewrites the octopus family tree, suggesting that they didn’t split from squid and their other relatives until more than 100 million years later. Instead, octopuses probably evolved at some point during the Jurassic Period while dinosaurs stalked the Earth.

Even if it’s not the first octopus, the fossil is still important as it represents the oldest known nautiloid soft tissue. It beats the previous record by around 220 million years, offering a much-needed glimpse of how these animals might have looked.

“We typically only find the hard parts of nautiloids in the fossil record, so this provides new details about the body plan of these animals,” Imran says. “As our analytical techniques improve further, it’s possible that we might be able to find out even more from this exceptional fossil in the future.”

“But what we really need now are more fossils, as this is the only known Paleocadmus where the body is still somewhat intact. Additional specimens will bring us more data to help understand how these ancient animals evolved.”