Research Reclassifies 300-Million-Year-Old Fossil as Nautilus Relative, Not Octopus

LONDON -- Researchers from the University of Reading have concluded that Pohlsepia mazonensis, a 300-million-year-old fossil from the Mazon Creek area in Illinois, is not an octopus but a relative of the nautilus. The fossil, discovered in a region about 50 miles southwest of Chicago and rich in pre-dinosaur fossils, measures about the size of a human hand.

It was named after its discoverer, James Pohl, and is held in the collection of the Field Museum in Chicago.

In 2000, paleontologists identified the fossil as an octopus, which suggested that cephalopods with eight tentacles appeared earlier in evolutionary history than previously estimated. This identification created a significant gap, as the next oldest confirmed octopus fossil dates to about 90 million years ago.

The new research, led by zoologist Thomas Clements, addresses ongoing scientific debate about the fossil's classification.

his team used a synchrotron, a device that generates high-intensity light beams using fast-moving electrons, to examine the fossil embedded in rock.

The analysis revealed a radula, a ribbon-like structure with teeth common to mollusks, containing rows of 11 teeth. Octopuses typically have radulas with seven or nine teeth, indicating that Pohlsepia mazonensis does not match octopus anatomy. The radula's structure aligns with that of Paleocadmus pohli, another nautiloid fossil from the same Mazon Creek location.

Researchers noted that the fossil's appearance may have been misleading due to decomposition, which likely caused the loss of its shell before fossilization. This soft tissue preservation makes it significant for studying ancient cephalopods.

The findings were published this week in the journal Proceedings of the Royal Society B.