300-Million-Year-Old Fossil Reclassified as Nautilus Relative

Scientists have reclassified a renowned 300-million-year-old fossil long celebrated as the world's oldest octopus, determining it actually belongs to a relative of the modern nautilus, a shelled cephalopod. The specimen, known as Pohlsepia mazonensis, was discovered at the Mazon Creek fossil site in Illinois and first described in 2000, pushing back estimates of octopus origins by more than 150 million years and earning it a place in Guinness World Records.

The reinterpretation, detailed in a study published Wednesday in the journal Proceedings of the Royal Society B, resolves decades of debate among paleontologists. Lead author Thomas Clements, a lecturer in invertebrate zoology at the University of Reading in England, explained that the fossil's deceptive appearance stemmed from weeks of decomposition before burial. "It turns out the world's most famous octopus fossil was never an octopus at all," Clements said. "It was a nautilus relative that had been decomposing for weeks before it became buried and later preserved in rock, and that decomposition is what made it look so convincingly octopus-like."

Originally, researchers identified features such as eight arm-like...