Pohlsepia mazonensis, from the late Carboniferous Mazon Creek Lagerstätte (311 - 306 Ma), has been suggested to be the oldest fossil octopus. This single specimen provides a key calibration point for molecular clock studies which estimated a Palaeozoic origin for octobrachians, pushing the fossil record of crown octopuses back 150 million years. Despite being a textbook ‘phylogenetic fuse’ example, the systematic position of Pohlsepia is controversial and, consequently, our understanding of cephalopod evolution remains in flux. We present the first comprehensive reassessment of this enigmatic fossil, alongside multiple new specimens, using a suite of advanced analytical techniques. Synchrotron micro-X-ray fluorescence elemental mapping reveals a radula obscured by the matrix, definitively confirming a molluscan affinity. Crucially, radular tooth count and morphology indicate that Pohlsepia is not an early octobrachian but rather a decomposed nautiloid. This reinterpretation refutes a Palaeozoic origin for octobrachians and provides the only unequivocal evidence of nautiloid soft tissue in the Palaeozoic fossil record. Our findings demonstrate how analytical methods can be used to resolve the interpretive challenges posed by the exceptional but often ambiguous soft tissue preservation in the Mazon Creek Lagerstätte, offering great potential for the re-evaluation of other fossil specimens from this iconic site.