A nautiloid fossil misinterpreted as the oldest octopus for 26 years

A fossil from the Mazon Creek Lagerstätte in Illinois, USA—Pohlsepia mazonensis—was once regarded as the oldest known octopus, and even pushed the origin of the entire octopod lineage back by more than 150 million years. However, with the advancement of analytical techniques, this conclusion has gradually come into question.

Specimen of Pohlsepia mazonensis PE51727, part and counterpart（Image source：Clements T et al. (2026)， CC BY 4.0 ）

This fossil was discovered within a siderite concretion from Late Carboniferous strata, dating to approximately 311–306 million years ago. At the time, researchers described it as an organism with an octopus-like appearance, featuring a sac-like fused head and mantle, symmetrical fin-like structures, paired "eyespots," and appendages interpreted as arms and tentacles. It also appeared to lack any internal or external shell, closely resembling modern octopuses. These features naturally suggested a primitive deep-water octopod, and the fossil was rapidly incorporated into the evolutionary framework of Octopoda, serving as a key calibration point for molecular clock studies.

Yet this interpretation was questionable from the outset. Many of the so-called "octopus-like" features were based on interpretations of indistinct outlines. Fossils from the Mazon Creek Lagerstätte are typically preserved as two-dimensional soft tissue stains within concretions, often lacking clear morphology and easily influenced by taphonomic processes. Subsequent studies noted the absence of defining octopod characteristics, such as sucker arrangements and detailed arm structures. Even the regions interpreted as eyes showed no evidence of melanosome-related microstructures, which are commonly preserved in other cephalopod fossils.

The decisive turning point came with the application of more advanced analytical techniques. In this study, the research team employed synchrotron micro-X-ray fluorescence (µXRF), scanning electron microscopy, micro-computed tomography (micro-CT), and multispectral imaging to re-examine the fossil in detail. These approaches allow researchers to penetrate beneath the surface and reveal structures and elemental distributions invisible to the naked eye, enabling a far more refined interpretation of previously ambiguous features.

Virtual reconstruction based on micro-CT, with the mouthparts highlighted in purple（Image source：Clements T et al. (2026)， CC BY 4.0 ）

Synchrotron micro-X-ray fluorescence elemental mapping showing copper (red), arsenic (yellow), and iron (blue)（Image source：Clements T et al. (2026)， CC BY 4.0 ）

Most importantly, a radula was identified in the buccal region. The radula is a band-like structure bearing rows of teeth and is a defining characteristic of molluscs. High-resolution elemental mapping revealed that the fossil preserves at least 11 radular elements, including a central rachidian tooth, lateral teeth, and marginal teeth. This discovery is critical, as radular morphology provides strong taxonomic resolution among cephalopods.

Radula（Image source：Clements T et al. (2026)， CC BY 4.0 ）

When these radular features were compared with those of both extant and fossil cephalopods, the arrangement did not match that of octopuses or other modern coleoids such as squids and cuttlefish. Instead, it closely resembled that of nautiloids. Nautiloids typically possess a greater number of radular elements, and the missing components in this fossil are likely the result of incomplete preservation. Consequently, the specimen is best interpreted as a decayed nautiloid rather than an early octopus.

This reinterpretation fundamentally alters its evolutionary significance. The idea that octopuses originated in the Palaeozoic relied heavily on this single fossil. Once removed, the timeline returns to one consistent with other lines of evidence, indicating that octopods likely originated in the Jurassic. This alignment strengthens molecular clock estimates and eliminates misleading scenarios in which a single fossil artificially extends the evolutionary history of an entire lineage.

The external appearance of a fossil does not always faithfully reflect the original anatomy of the organism. Many structures in this specimen that were interpreted as arms, fins, or eyes may instead represent products of decay. For instance, the fin-like features could be remnants of decomposed cephalic retractor muscles or digestive gland lobes, while the eyespots may simply be depressions formed by sediment infilling. Such misinterpretations arise from visual impressions of shape and coloration rather than robust anatomical evidence.

The absence of a shell in this fossil also provides insight into post-mortem processes. Nautiloids possess chambered shells that may remain buoyant after death. As soft tissues decay, they can detach and eventually sink to the seafloor. Thus, the preservation of soft tissues without the associated shell is entirely plausible, yet it can easily lead to misidentification as a shell-less organism.

Reconstruction of the palaeoenvironment at the time（Image source：Franz Anthony， CC BY 4.0 ）

The exceptional preservation of soft tissues at Mazon Creek is attributed to rapid burial by iron-rich sediments, which subsequently formed siderite concretions around decaying organisms. This mode of preservation differs from the more common phosphatization and allows certain delicate structures to be retained. However, preservation quality varies depending on burial rate and geochemical conditions, meaning that fossils of the same organism can appear dramatically different.

Scientific understanding is not static. Even a fossil once considered a textbook example can be reinterpreted in light of new evidence and improved methodologies. The transformation of Pohlsepia mazonensis from "the oldest octopus" to a decayed nautiloid represents a significant recalibration of cephalopod evolutionary history, bringing our interpretation closer to the underlying biological reality.

Author: Shui-Ye You

Reference:

Clements T et al. (2026). Synchrotron data reveal nautiloid characters in Pohlsepia mazonensis, refuting a Palaeozoic origin for octobrachians. Proc Biol Sci.