World's Oldest 'Octopus' Fossil Reclassified as Nautiloid by UK Scans

In a stunning revelation that has sent ripples through the paleontology community, advanced imaging techniques have upended long-held beliefs about the origins of octopuses. What was once celebrated as the world's oldest octopus fossil, Pohlsepia mazonensis, has been reclassified as a nautiloid cephalopod—a shelled relative of modern nautiluses—thanks to meticulous work led by researchers from the University of Reading in the UK.55117

This 300-million-year-old specimen, discovered in the renowned Mazon Creek Lagerstätte in Illinois, USA, was initially described in 2000 and even earned a spot in the Guinness Book of World Records. Its apparent eight arms, eyes, and ink sac-like structure convinced early researchers it represented an ancient octopod. However, doubts lingered, prompting Dr. Thomas Clements and an international team to revisit it with cutting-edge synchrotron X-ray technology.

🔬 The Hidden Radula: A Game-Changing Discovery

The breakthrough came from synchrotron micro-X-ray fluorescence (µXRF) imaging conducted at the SOLEIL facility in France. This non-destructive method, using an ultra-bright X-ray beam focused to micrometer precision, pierced the fossil's enclosing rock matrix to reveal structures invisible to the naked eye or traditional micro-CT scans.117

Buried beneath the surface lay a radula—a chitinous, tooth-covered ribbon used by mollusks for feeding. The fossil's radula boasted at least 11, possibly 13, denticles: a unicuspid central tooth flanked by wide triangular laterals and crescent-shaped marginals. This configuration perfectly matches the nautiloid Paleocadmus pohli from the same deposit, while octopuses typically have seven or nine denticles with distinct shapes.

"A row of tiny hidden teeth, concealed for 300 million years, have fundamentally changed what we know about octopus evolution," explained Dr. Clements, a paleobiologist specializing in taphonomy—the study of decay and fossilization—at the University of Reading.55

From Octopus Icon to Nautiloid Impostor

Pohlsepia mazonensis, housed at the Field Museum of Natural History in Chicago (specimen PE51727), hails from the late Carboniferous period, approximately 308 million years ago. The Mazon Creek site is famed for its exceptional preservation within iron-rich siderite concretions, capturing soft-bodied organisms like jellyfish, worms, and rare cephalopods that rarely fossilize.58

Early analysis misinterpreted decay artifacts as octopus traits: fins and arms resulted from weeks of decomposition before burial, dispersing the shell and mimicking a boneless coleoid body plan. No melanosomes—pigment granules indicative of an ink sac—were found, further debunking the octopus label.44

This reclassification resolves a phylogenetic puzzle. Previously, Pohlsepia anchored molecular clock estimates suggesting Paleozoic origins for octobrachians (octopuses and allies), conflicting with Jurassic body fossils. Now, the timeline aligns: nautiloids and coleoids diverged around 416 million years ago in the mid-Paleozoic, but soft-bodied octopuses emerged in the Mesozoic, around 165 million years ago with Proteroctopus ribeti.66112

Advanced Imaging: Revolutionizing Paleontological Research

Synchrotron radiation, generated by particle accelerators, delivers X-rays billions of times brighter than medical scanners. In paleontology, it maps elemental distributions—iron in beaks, copper in radulae—revealing hidden anatomies without damaging irreplaceable specimens.117

UK institutions lead this field. The Natural History Museum London and Diamond Light Source have imaged Jurassic mammals' tooth cementum for life history data and Devonian lizard origins.77 Co-authors Imran Rahman (NHM, Oxford) and Alan Spencer (NHM, Imperial College) exemplify interdisciplinary collaboration driving these advances.

Read the full study in Proceedings of the Royal Society B, published April 8, 2026.117

Cephalopod Family Tree Redrawn

Cephalopods—head-footed mollusks including squids, cuttlefish, octopuses, and nautiluses—trace to the late Cambrian (~500 Ma). Nautiloids retained external shells; coleoids internalized or lost them, evolving boneless bodies for agility.66

• Nautiloids: Dominant Paleozoic group, pearly shells, numerous arms with rigid suckers, radula for rasping.
• Coleoids: Decabrachians (10-armed squids/cuttlefish) vs. octobrachians (8-armed octopuses/vampire squids); diverged Permian-Mesozoic.

Pohlsepia's demotion pushes nautiloid soft-tissue record back 220 million years, offering unprecedented decay insights from Mazon Creek's brackish estuary.

Mazon Creek: A Window into Ancient Ecosystems

This Illinois lagerstätte (German for 'storage place') preserves over 200 species across terrestrial, freshwater, and marine habitats. Cephalopods here include goniatites and rare soft-bodied forms, but taphonomic biases—rapid concretion formation amid decay—often distort anatomies.56

Dr. Clements' taphonomy expertise, honed through decay experiments, explains Pohlsepia's masquerade: shell loss, tissue softening, and arm-like appendages from mantle unraveling.

University of Reading's Paleobiology Excellence

Dr. Clements, Lecturer in Invertebrate Zoology, bridges University of Reading and Leicester, focusing on exceptional preservation. His work on Mazon Creek spans years, cited 400+ times.98 Reading's School of Biological Sciences fosters such innovation, training PhD students in synchrotron techniques and fieldwork.

This study highlights higher education's role in resolving evolutionary debates, with implications for molecular biology and biodiversity research.

Implications for Future Fossil Reexaminations

With Pohlsepia synonymized as Paleocadmus pohli, similar Mazon Creek 'coleoids' warrant scrutiny. Candidates like Stewartionekes may follow. Globally, synchrotrons could unlock dozens of ambiguous fossils.117

Recent X (formerly Twitter) buzz underscores public fascination, with posts from science communicators amplifying the story.87

Careers in Paleontology and Higher Ed Research

This discovery exemplifies opportunities in university research. Roles in taphonomy, imaging, and systematics abound at institutions like Reading, NHM, and Field Museum. Aspiring researchers can pursue postdocs, lectureships, or curatorial positions blending fieldwork, labs, and computation.Field Museum careers page lists openings; UK unis seek paleobiologists amid synchrotron expansions.

Photo by Bruno Martins on Unsplash

Broader Impacts on Science and Education

Beyond timelines, the study stresses taphonomy's centrality: fossils reflect decay as much as life. In classrooms, it teaches critical evaluation—question icons, embrace tech. For conservation, understanding ancient cephalopod resilience informs modern threats to nautiluses and coleoids.

Read more on ScienceAlert's coverage.