A fossil once celebrated as the world’s oldest octopus may be no octopus at all. New synchrotron scans of the roughly 300-million-year-old Pohlsepia mazonensis specimen led researchers to conclude it is a decomposed nautiloid rather than an ancient ancestor of modern eight-armed cephalopods. The finding, led by Thomas Clements of the University of Reading and published in Proceedings of the Royal Society B, would remove the specimen’s long-held distinction and could prompt researchers to revisit when octopuses appear in the fossil record.
What is verified so far
Pohlsepia mazonensis was first described in 2000 from the Mazon Creek fossil deposit in Illinois, a site known for its exceptional preservation of soft tissues. The specimen, catalogued as PE51727 A/B, quickly gained fame. It was treated as the earliest known octopus, a status that was widely repeated in coverage and press materials, including references to a Guinness World Records entry in reports such as ScienceDaily’s write-up. That classification shaped how researchers understood the deep evolutionary history of octobrachians, the group that includes modern octopuses.
The new study used synchrotron-based imaging and elemental mapping to look inside the fossil at a level of detail that was not possible when the specimen was originally described. These high-energy X-ray scans detected a structure that earlier analyses had missed entirely: a radula, a feeding organ made up of rows of tiny teeth. Octopuses do possess radulae, but the tooth arrangement found in Pohlsepia matches the pattern seen in nautiloid specimens rather than octopuses. That single diagnostic feature, combined with other nautiloid-consistent characters visible in the synchrotron data, led the research team to reclassify the specimen as a decomposed nautiloid.
Thomas Clements and his colleagues have also deposited the raw CT slice images and scan metadata, including projections, frames, and exposure settings, in the Dryad repository, allowing other scientists to independently verify the radula and other internal features. That level of transparency is significant because the original 2000 description relied on surface-level observation of a compressed, soft-bodied specimen, a method that left room for ambiguity and subjective interpretation.
The Mazon Creek deposit, housed in part at the Smithsonian collections, preserves a wide range of Carboniferous organisms in ironstone concretions. While that preservation is remarkable, it can also obscure internal anatomy by flattening and mineralizing soft tissues. In Pohlsepia’s case, what looked like a simple soft-bodied octopus on the surface concealed tooth rows and other hard structures that only synchrotron technology could resolve, underscoring how new imaging tools can overturn long-standing identifications.
What remains uncertain
Doubts about Pohlsepia’s identity did not begin with this study. A 2022 paper in Nature Communications on early vampyropod evolution explicitly described Pohlsepia as controversial and unreliable, arguing it should not be used in studies of cephalopod interrelationships until it was formally redescribed. That earlier warning, however, stopped short of offering a positive alternative identification. The 2026 synchrotron study is the first to provide one, but the reclassification as a nautiloid still needs to survive peer scrutiny and potential replication by independent teams using the open dataset.
One gap in the current evidence is the absence of any public response from the researchers who originally described Pohlsepia as an octopus in 2000. Their interpretation stood for over two decades and influenced phylogenetic models of cephalopod evolution. Whether they accept the nautiloid reclassification, contest the radula identification, or offer a third interpretation remains unknown based on available sources, leaving an important part of the scientific dialogue undocumented.
The Guinness World Records listing is another loose thread. While the University of Reading release and secondary outlets reference the record, no official statement from Guinness confirming its removal or revision has surfaced. The practical effect is the same, since the scientific basis for the record has been dismantled, but the formal status of the listing is unconfirmed and may lag behind the updated consensus.
There is also an open question about what this reclassification means for the broader timeline of octopus origins. If Pohlsepia is removed from the octopus lineage, the oldest reliable octopus fossils shift forward by tens of millions of years, likely into the Mesozoic. In the 2026 synchrotron paper, the authors argue the result undercuts a Palaeozoic origin for octobrachians, but molecular clock estimates from genetic studies sometimes place the divergence of octopuses earlier than the fossil record suggests. Reconciling those two lines of evidence will require additional fossil discoveries, careful reassessment of other problematic specimens, and improved models that integrate both molecular and paleontological data.
How to read the evidence
The strongest piece of evidence in this story is the peer-reviewed paper itself, published in Proceedings of the Royal Society B. It presents direct physical data: synchrotron images and elemental maps of a specific specimen with a specific catalogue number. The radula tooth rows are the key diagnostic feature, and because the raw CT data are publicly available, any researcher with the right software can examine those structures independently. That combination of primary evidence and open data puts this reclassification on firmer ground than a typical reinterpretation based solely on visual morphology or artistic reconstructions.
The institutional press releases from the University of Reading provide useful context and direct quotes from the lead author but should be read as summaries rather than independent lines of evidence. They highlight the technological leap represented by synchrotron imaging and frame the work as an example of how new tools can reshape established narratives in evolutionary biology.
Secondary coverage, such as the report carried by science news outlets, largely echoes the university communications, emphasizing the loss of the “oldest octopus” title and the broader implications for cephalopod evolution. These stories help translate technical findings for a wider audience, but they inevitably compress nuance and may underplay remaining uncertainties, particularly the unresolved tension between fossil dates and molecular divergence estimates.
Readers who want to dig deeper can start with the peer-reviewed paper and the open dataset in Dryad, which allow independent inspection of the scan evidence behind the reclassification.
For now, the Pohlsepia case stands as a cautionary tale about the limits of interpreting flattened soft-bodied fossils without access to internal anatomy. It also showcases how advances in imaging technology can overturn long-accepted identifications and, in doing so, redraw key branches of the tree of life. Whether future work confirms every detail of the nautiloid interpretation or introduces new complexities, the central message is clear: even iconic fossils can change identity when seen in a new light, and the timeline of octopus evolution remains very much a live scientific question.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
