Uncovering the Truth: Oldest 'Octopus' Fossil Revealed to Be a Nautiloid

Pohlsepia mazonensis, a visually unassuming fossil from Illinois, has fundamentally challenged our understanding of cephalopod evolution. Described in 2000 and touted as the oldest known octopus in the fossil record, the specimen dated back to the late Carboniferous period, approximately 311 to 306 million years ago. Pohlsepia was an outlier—all other fossil evidence strongly suggested that crown coleoids, the group containing octopuses, squid, and cuttlefish, diverged much later, during the Jurassic period.

To solve this puzzle, Thomas Clements, a paleontologist at the University of Leicester, and his team put this supposed oldest octopus fossil through a series of high-tech imaging tests. Their findings revealed that Pohlsepia was not an octopus at all, but rather a decomposed, squashed nautiloid.

A Rorschach Test of the Fossil Record

The reason a nautiloid managed to masquerade as an octopus for almost a quarter of a century was due to the unique way that fossils from the Mazon Creek Lagerstätte formed. Around 300 million years ago, this area was a brackish, tidal marine basin that was periodically inundated by massive amounts of iron-rich river mud. When organisms died and were buried in this sediment fan, the high iron content triggered the precipitation of the mineral siderite around their decaying bodies, locking them inside hard geological nodules.

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This process created a Rorschach test of the fossil record, where the distorted, compressed shapes of the organisms trapped within the nodules could be interpreted in multiple ways. In the case of Pohlsepia, the paleontologists who first examined the fossil were convinced that its eight-armed appearance was evidence of an ancient octopus. However, the new high-resolution imaging techniques employed by Clements and his team revealed the true nature of the specimen.

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The researchers found that Pohlsepia possessed a series of tentacle-like structures that were actually the remnants of a nautiloid's feeding tentacles, rather than the distinct arms of an octopus. Furthermore, the team discovered that the specimen lacked the characteristic beak and radula (tongue-like organ) found in modern coleoids, solidifying the conclusion that it was not an early octopus at all.

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This discovery not only challenges our understanding of cephalopod evolution but also highlights the importance of using advanced imaging techniques to re-examine and verify the nature of fossil specimens, especially those that seem to contradict the broader paleontological evidence. As Clements noted, "Pohlsepia is a cautionary tale about the pitfalls of relying too heavily on the interpretation of distorted fossil material."