300-Million-Year-Old 'Octopus' Fossil Misidentified, Scientists Say

New analysis reveals the ancient specimen is actually a relative of the modern nautilus.

Apr. 9, 2026 at 3:25pm

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A striking black-and-white photograph featuring a complex, abstract pattern of intersecting geometric shapes and wave-like forms, conveying the intricate hidden structures and evolutionary forces that shaped an ancient marine creature.Advanced imaging techniques reveal the true nature of a 300-million-year-old fossil, challenging previous assumptions about the evolutionary history of marine life.Chicago Today

A 300-million-year-old fossil that was previously thought to be the world's oldest octopus has been reclassified by scientists as a relative of the modern nautilus. Researchers used advanced imaging techniques, including synchrotron scanning, to uncover hidden anatomical details that showed the fossil did not belong to an octopus.

Why it matters

The misidentification of this ancient fossil highlights the importance of using the latest scientific tools and techniques to accurately classify prehistoric specimens. As paleontology becomes more technologically advanced, researchers are making important discoveries that revise our understanding of the evolutionary history of various species.

The details

The fossil, named Pohlsepia mazonensis, was found at the Mazon Creek site in Illinois and had long puzzled paleontologists. Its age of 300 million years made it far older than the next oldest known octopus fossil, which dates back about 90 million years. However, certain features of the fossil, such as the length and shape of its arms, did not quite match what would be expected of an octopus. Using new analytical methods, including scanning electron microscopy and geochemical analysis, the research team was finally able to determine that the fossil belonged to an animal related to the modern nautilus, which has tentacles and an external shell.

  • The fossil was originally discovered at the Mazon Creek site in Illinois.
  • The first analysis of the fossil was published in 2000, identifying it as the world's oldest octopus.
  • In 2026, a new study was published in the journal Proceedings of the Royal Society B, reclassifying the fossil as a relative of the nautilus.

The players

Thomas Clements

A lecturer in invertebrate zoology at the University of Reading, England, and the lead author of the study that reclassified the fossil.

Pohlsepia mazonensis

The name given to the 300-million-year-old fossil that was originally misidentified as the world's oldest octopus.

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What they’re saying

“We basically used a wide selection of new analytical techniques to discover hidden anatomical characteristics within the rock.”

— Thomas Clements, Lead study author

“It revealed anatomical characters that were hidden just below the surface of the rock, so you can't visually see it when you're looking at the fossil.”

— Thomas Clements, Lead study author

What’s next

The research team plans to continue using advanced imaging techniques to re-examine other ancient fossils that may have been misclassified in the past.

The takeaway

This case highlights the importance of embracing new technologies in paleontology to ensure accurate classification of prehistoric specimens. As scientific tools become more sophisticated, researchers are able to uncover hidden details that can significantly revise our understanding of evolutionary history.