Oldest Octopus Fossil Reclassified as Nautiloid

Pohlsepia mazonensis, once celebrated as the oldest known octopus fossil, has been reclassified as a nautiloid, challenging previous understandings of cephalopod evolution. This fossil, discovered in Illinois and dating back to the late Carboniferous period, was initially thought to be an octopus due to its superficial resemblance to deep-water octopuses. However, high-tech imaging techniques revealed that it lacked key octopus features, such as suckers and an internal shell vestige.

The fossil's misidentification stemmed from the peculiar preservation conditions of the Mazon Creek Lagerstätte, where organisms were encased in siderite nodules, preserving them as flat, two-dimensional stains. This made interpreting the fossil akin to reading a Rorschach test, leading to initial misinterpretations of its anatomy.

Doubts about Pohlsepia's classification arose when researchers failed to find melanosomes in its supposed eyespots, a feature common in other Mazon Creek fossils. To resolve these uncertainties, Thomas Clements and his team employed advanced imaging techniques, including synchrotron X-ray beams, to analyze the fossil's chemical composition.

These analyses revealed that the features thought to be octopus-like were merely part of the general body outline, with no evidence of eight distinct arms or specialized tentacles. Furthermore, the supposed ink sac and eyespots were identified as mineral deposits, not biological structures.

The definitive evidence came from the discovery of a radula, a toothed structure used by mollusks, confirming Pohlsepia as a nautiloid. This finding not only redefined the fossil but also marked it as the oldest known evidence of nautiloid soft tissues in the Paleozoic fossil record.

Clements' study highlights the potential of advanced imaging techniques to reassess other fossils from Mazon Creek, suggesting that many could reveal new insights into ancient life forms.