For a quarter of a century that fossil had an almost untouchable aura. Pohlsepia mazonensisfound in Illinois and described in 2000, had become a famous case of paleontology: the supposedly oldest octopus in the world, a find about 300 million years old, bulky enough to even end up in the Guinness Book of Records. Now that label is off. The new study published on Proceedings of the Royal Society B moves the find out of the octopus line and brings it closer to the nautiloids, the group to which the modern one belongs Nautilusmarine mollusk with an external shell still alive today.
The decisive point lies in what the rock held beneath the surface. The researchers used the synchrotronan imaging technique that uses extremely intense beams of light, described as brighter than the Sun, to read structures invisible to the naked eye. The image is almost like a forensic laboratory: a find over 300 million years old put back under examination with tools that were simply missing twenty-five years ago. Doubts about his identity had been circulating for some time, but there was no right way to really verify those doubts.
From the outside, that fossil seemed to tell another story. Arms, fins, profile of a soft cephalopod: enough to make it part of the studies on the evolution of octopuses and anticipate their origin by about 150 million years. The problem was upstream, long before fossilization. The animal had been decomposing for weeks before being buried, and that degradation had altered the body to the point of making it believably “octopus-like.” Even the absence of the shell, today, is read within this same process of decomposition.
Then came the tiny details, the ones that blow up entire castles. One emerged from the rock radulathe ribbon-like food structure with rows of teeth typical of molluscs. In each row the researchers saw at least 11 tooth-like elements. It is here that the fossil changes the box: octopuses generally show 7 or 9 teeth per row, nautiloids reach 13. The shape and number of teeth instead match Paleocadmus pohlia previously known nautiloid fossil from the same site in Mazon Creek, Illinois.
Those tiny teeth move the fossil among the nautiloids
The biggest consequence concerns the evolutionary calendar. Having removed this finding from the history of octopuses, the origin of the group returns later and realigns itself to a picture that had caused discussion for years. Data supports an appearance of octopuses in Jurassictherefore inside the era Mesozoicand also place the separation between octopuses and their ten-armed relatives, such as squid, in that period. That old fossil out of place, which for decades disturbed the general picture, thus stops forcing the chronology towards the Paleozoic.
There is also a second effect, less noticeable and very important. The remains of Paleocadmus preserved at Mazon Creek now become the oldest known evidence of soft tissues of a nautiloid in the fossil record, with an advantage of approx 220 million years on the previous record. In paleontology it often happens like this: one record falls, another takes shape at the same time, only in a completely different direction from the one imagined at the beginning.
The most beautiful part, here, remains the disproportion. For years the fossil seemed enormous in its fame and solid in its label. What put everything back in order was a row of tiny teeth, which remained closed in the rock for more than 300 million years.
