We are one step closer to solving the mystery of an unusual hexagonal fossil that has appeared on Earth over the past 500 million years. A new study describes the creature responsible for this trace fossil—known as Paleodictyon—and identifies two likely candidates. The findings have been published in the journal Earth-Science Reviews.
Paleodictyon has baffled scientists for centuries, representing one of paleontology’s most enduring unsolved mysteries. Italian paleontologist Giuseppe Meneghini named the fossil in 1850 after discovering it in Eocene-era deposits.
The challenge was that he was describing not an animal itself, but a strange imprint left behind by an unidentified creature.
Since then, Paleodictyon—the name given to this type of trace fossil—has been found both as ancient fossils on mountain ridges and as fresh traces on the ocean floor.
Whatever animal created these structures has inhabited Earth for half a billion years and remains alive today. So, what kind of animal is it?
A team of scientists led by paleontologist Andrea Baucon from the University of Genoa has proposed that an arthropod is responsible. Arthropods are invertebrates characterized by segmented bodies and hard exoskeletons; millions of species have been described across terrestrial and marine ecosystems.
To get to the bottom of the mystery, the team conducted an extensive literature review for existing examples of Paleodictyon and undertook fieldwork to find new fossil specimens. This involved days of trekking through the Apennines and the Alps in search of these minute traces. While this might seem like an odd place to look for something known to inhabit the sea, these landscapes were once an ancient ocean floor.
Here, we find ancient specimens of Paleodictyon, yet underwater vehicles have captured images of modern versions on the seafloor—not fossils, but complex burrows dug by organisms that are still alive today.
Both the fossilized and modern structures consist of straight tunnels meeting at sharp angles. This structural pattern is associated with arthropods, as their rigid exoskeletons and jointed limbs dictate their range of movement; the team has identified two key candidates.
“Our work suggests that the most likely creators of Paleodictyon were arthropods,” said Baucon. “Among arthropods, the prime candidates are deep-sea isopods and amphipods. Their terrestrial and shallow-water marine relatives are surprisingly familiar: isopods—or woodlice—can be found under flowerpots; they are small crustaceans that curl into a ball when disturbed. You may have also seen amphipods hopping across the sand on beaches (some are known as sandhoppers or sand fleas).”
We are beginning to form a mental picture of the creature that creates Paleodictyon, but the exact purpose of the structure remains a mystery.
Previous theories suggested it might be a system for cultivating bacteria, but new research indicates it could be a communication network for a colony of animals.
These hexagonal tunnel systems could facilitate the transmission of chemical, hydraulic, or acoustic signals. It is difficult to say for certain without seeing the process in action, but scientists are working on that as well. “The next step in our research will be to establish underwater observatories on the ocean floor to observe Paleodictyon (and its creator) in its natural habitat,” said Baucon. “The opportunity to observe the creator in real time as it forms these structures will be like time travel—it will give us the chance to see firsthand how one of the most mysterious patterns in the fossil record is formed.”
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