A significant fossil, previously considered foundational to understanding cephalopod evolution, has been definitively identified as an impostor.
The renowned 300-million-year-old specimen, designated as Pohlsepia mazonensis, which was unearthed in 2000 and subsequently recognized by the Guinness Book of World Records as the earliest documented octopus species, has undergone reclassification. It is now understood to be a nautiloid – a shelled cephalopod closely affiliated with the extant nautiluses that inhabit contemporary oceans.
This reevaluation necessitates a substantial revision of the cephalopod evolutionary lineage. The revised understanding extends the known fossil record of nautiloid soft tissue by approximately 220 million years, while concurrently advancing the earliest evidence of octopuses by roughly 150 million years.
“It has come to light that the world’s most celebrated octopus fossil was, in fact, never an octopus. It represented a relative of the nautilus that had undergone extensive decomposition before its entombment and subsequent fossilization in rock; this very decomposition imparted the convincing octopus-like appearance,” stated paleobiologist Thomas Clements from the University of Reading in the UK.
“For a quarter of a century, scientists classified Pohlsepia as an octopus. However, the application of contemporary investigative techniques has illuminated what lay concealed beneath the rock’s surface, thereby definitively resolving the matter. We now possess the most ancient recorded instance of nautiloid soft tissue preservation alongside a much clearer elucidation of the actual timeline for the initial emergence of octopuses on Earth.”
Pohlsepia is documented through a solitary fossil specimen discovered within the Mazon Creek Lagerstätte in Illinois. At the time of its preservation in the soft sediment 300 million years ago, its body had significantly deteriorated, presenting a considerable interpretive challenge to the paleontologists examining the flattened remains.
Notwithstanding these difficulties, investigators identified characteristics suggestive of eight appendages, a pair of ocular organs, and an ink sac – all features considered indicative of an octopodean classification. Over the subsequent years, while some scientific discourse questioned this interpretation, no conclusive findings were formally published.
However, in the decades following the initial establishment of Pohlsepia‘s identity, technological advancements have emerged, enabling scientists to scrutinize fossils internally without necessitating their destruction. Clements and his research cohort leveraged state-of-the-art synchrotron imaging, which employs X-rays generated by particle accelerators to penetrate dense materials.
This X-radiation possesses an intensity many orders of magnitude greater than typical medical X-ray equipment, facilitating the discernment of intricate, high-resolution details that might be overlooked by alternative imaging modalities, such as computed tomography.
Consequently, when the researchers subjected Pohlsepia to this advanced diagnostic method, they detected an anatomical structure previously unobserved: eleven minute, tooth-like formations arranged in linear succession. This formation, they posited, is most likely a radula – a lingual ribbon adorned with denticles, a structure exclusive to mollusks.
The precise number and morphology of these denticle structures served as the decisive evidence indicating that Pohlsepia is not an octopus, according to the researchers. Octopuses typically exhibit seven or nine elements per row on their radula, whereas nautiloids possess thirteen. The count observed in Pohlsepia falls between these figures, but its form aligns more closely with that of nautiloids than octopuses.
Furthermore, the investigation revealed minimal evidence to support the interpretation of a previously identified structure as an ink sac; specifically, there was an absence of the melanosome pigment typically associated with such organs.
Upon comparing their findings with other cephalopod fossils from the Mazon Creek Lagerstätte, the researchers encountered an additional revelation. The newly identified radula bore a striking resemblance to that of Paleocadmus pohli, an extinct nautiloid previously known from the same geological formation.
This discovery, in turn, implies that the established evolutionary tree of octopuses requires revision. This exemplifies the dynamic nature of scientific inquiry. In 2000, researchers conducted their analyses based on the available evidence. Today, advancements in technology and methodology provide new insights that propel our understanding of life’s history on Earth forward.
“Periodically, the re-examination of contentious fossil discoveries utilizing novel techniques unearths subtle clues that precipitate remarkably significant findings,” commented Clements.
“It is profoundly remarkable to contemplate how a sequence of minute, concealed teeth, preserved within rock for 300 million years, has fundamentally reshaped our comprehension of the timing and mechanisms of octopus evolution.”
