Considered for 20 years as the oldest octopus in the world, a 300-million-year-old fossil found near Chicago has just been reclassified as nautilus after X-rays revealed hidden teeth in the rock.

Written by Douglas Avila

Published on 15/04/2026 at 23:04

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The fossil that was considered the oldest octopus in the world for over 20 years has just been reclassified as an extinct nautilus after analysis with the most powerful X-rays on the planet revealed hidden teeth in the rock

For more than two decades, a 300-million-year-old fossil found near Chicago was considered the oldest octopus ever discovered. Now, that classification has been overturned.

A study published on April 8, 2026, in the journal Proceedings of the Royal Society B revealed that the animal was not an octopus. In fact, it is a decomposed nautiloid, an extinct relative of modern nautiluses.

The discovery was made possible thanks to micro-CT scan technology with X-rays. Researchers used what they describe as “the most powerful X-rays in the world” to see inside the rock.

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Tiny teeth hidden in the rock for 300 million years overturned a 20-year classification

“Basically, we used a wide range of new analytical techniques to uncover hidden anatomical features within the rock,” Clements stated.

The technique revealed a radula — a feeding structure — with at least 11 teeth per row. Modern octopuses have only 7 or 9 teeth per row.

“It was these tiny teeth that we found that allowed us to identify that it was not an octopus”, the researcher told CNN International on April 9.

“And we were able to determine that it is not an octopus, but actually a very decomposed nautiloid, which is a relative of modern nautiluses,” Clements added.

The fossil is called Pohlsepia mazonensis. It was found in Mazon Creek, Illinois, a geological formation known for its exceptional fossil preservation.

A 210-million-year gap in the history of octopuses

The reclassification has important implications for science. The Pohlsepia was the supposed oldest octopus by a huge margin: the next fossil octopus record is only 90 million years old.

Living nautilus swimming in the deep ocean

This means that there was a 210-million-year gap between the Pohlsepia and the second oldest octopus. This gap has always intrigued paleontologists.

Now, the gap makes sense. The animal was never an octopus — it was a nautiloid with tentacles and an external shell that decomposed misleadingly.

“The research demonstrates the power of new technologies to advance our scientific understanding,” Clements concluded.

Mazon Creek: the paleontological site in Illinois that still holds surprises

Paleontological site of Mazon Creek in Illinois

Mazon Creek is an iconic paleontological site in the United States. Fossils preserved in concretions hold extraordinary details of organisms from the Carboniferous period.

As reported by R7, the previous classification as an octopus was based on superficial visual evidence. The new non-invasive imaging techniques have changed the game.

The reclassification does not invalidate all previous science. However, it corrects an interpretation that has been held for over 20 years. The case shows that ancient fossils can still surprise when reexamined with modern technology.

Considered for 20 years as the oldest octopus in the world, a 300-million-year-old fossil found near Chicago has just been reclassified as nautilus after X-rays revealed hidden teeth in the rock.

The fossil that was considered the oldest octopus in the world for over 20 years has just been reclassified as an extinct nautilus after analysis with the most powerful X-rays on the planet revealed hidden teeth in the rock

Tiny teeth hidden in the rock for 300 million years overturned a 20-year classification

A 210-million-year gap in the history of octopuses

Mazon Creek: the paleontological site in Illinois that still holds surprises

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Considered for 20 years as the oldest octopus in the world, a 300-million-year-old fossil found near Chicago has just been reclassified as nautilus after X-rays revealed hidden teeth in the rock.

The fossil that was considered the oldest octopus in the world for over 20 years has just been reclassified as an extinct nautilus after analysis with the most powerful X-rays on the planet revealed hidden teeth in the rock

Tiny teeth hidden in the rock for 300 million years overturned a 20-year classification

A 210-million-year gap in the history of octopuses

Mazon Creek: the paleontological site in Illinois that still holds surprises

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China is building an artificial island in the middle of the sea, covering 20 km², to construct the largest offshore airport in the world, the Dalian Jinzhouwan. This $4.3 billion project is anchored on more than 3,000 pillars embedded in the rock beneath the ocean and is scheduled to open in 2035.

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