An artist’s depiction of Pohlsepia decaying on the seafloor

Much of the American Midwest, including parts of Illinois, were covered in shallow seas 300 million years ago during the Carboniferous Period. During that time, the ancient waters were home creatures that would seem exotic and bizarre in the Midwest today, though many of these creatures have living relatives today.

The Mazon Creek fossil beds in northern Illinois preserve many of strange creatures from this time. The Mazon Creek fossil beds are what paleontologists call a lagerstätte - an area that has excellent fossil preservation, including for soft-bodied organisms. Other famous lagerstätte locations include the Burgess Shale in British Columbia and the Solnhofen Limestone in Bavaria.

One enigmatic creature from the Mazon Creek fossil beds is known as Pohlsepia mazonensis, known from a single fossil and traditionally interpreted as an early octopus. If indeed an octopus, Pohlsepia would be the oldest octopus in the world, predating other fossil octopus specimens known from the Jurassic Period by about 150 million years.

However, a number of paleontologists have long been skeptical of Pohlsepia. Now, a study published this month by the Royal Publishing Society has shown that Pohlsepia is not an octopus at all, but rather a nautiloid. The study, titled, Synchrotron data reveal nautiloid characters in Pohlsepia mazonensis, refuting a Palaeozoic origin for octobrachians, is available online with current open access. Thomas Clements, Ph.D., Lecturer in Invertebrate Zoology at the U.K.’s University of Reading, is the paper’s lead author.

Clements and his co-authors re-examined the Pohlsepia fossil using a scanning electron microscope, micro-CT scanning and synchrotron micro-X-ray fluorescence to determine as much as they could from the ambiguous fossil.

Ultimately, the synchrotron mapping revealed a preserved radula in the fossil. In cephalopods, radulae are specialized “teeth” used for feeding. Clements determined that the shape and fine details of the radula in Pohlsepia indicates the fossil creature is a nautiloid, and not a close relative of modern octopi. It seems that at some point in the decay process, the Pohlsepia fossil lost its shell, leading to paleontologists mistaking it for an octopus a few hundred million years later.

Other creatures from the Mazon Creek fossil beds

Paleocadmus

Nautilus in Berlin Zoo Aquarium, 2007. J. Baecker, public domain, via Wikimedia Commons

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Fossilized radulae have been found in the Mazon Creek fossil beds and are assigned to the genus Paleocadmus. Analysis of these radulae suggest that Paleocadmus was a nautiloid and a relative of the modern chambered nautilus.

After determining Pohlsepia to be a nautiloid, Clements suggested that the species is a junior synonym of Paleocadmus pohli. In scientific naming, if two differnet species turn out to be really the same species, scientists will refer to the species by which ever name is older. Although only from radulae, Paleocadmus was described by scientists in 1979, whereas Pohlsepia was only named in 2000.

Tully monster

The Mazon Creek fossil beds are also the exclusive home of Illinois’ state fossil, the Tully monster (Tullimonstrum gregarium).

The Tully monster is a bizarre creature that has left paleontologists confounded for decades. It appears to be have certain features similar to cephalopods, worms and jawless vertebrates, yet, at the same time, be none of those things.

Clements was the lead author on a 2016 study, The eyes of Tullimonstrum reveal a vertebrate affinity that argued that Tully monster was a vertebrate. Other scientists, however, have disputed this.