An international team of scientists led by Andrea Baucon has partly solved a palaeontology cold case. By studying 50 fossils across 530 million years, researchers identified a small burrowing crustacean as the maker of Paleodictyon, a mysterious hexagonal honeycomb network found in marine sediments worldwide.
Half a Billion Years of Unsolved Geometry
For more than half a billion years, a repeating, honeycomb-like pattern has left scientists searching for its author. Known as Paleodictyon, this hexagonal network of burrows spans geological history all the way back to the Cambrian period, which began approximately 539 million years ago and lasted for about 52 to 55 million years. Its distribution is remarkably vast, appearing in locations ranging from 3,500-meter depths on the Mid-Atlantic Ridge to the high mountain tops of the Italian Apennines, according to findings published in the journal Earth-Science Reviews
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The enduring mystery has attracted keen observers across the centuries. Leonardo da Vinci drew the pattern around 1500 without knowing what created it, and Giuseppe Meneghini first described and formally named it in 1850. Yet, despite centuries of curiosity, the organism responsible remained entirely unknown, earning the phenomenon a reputation as one of the longest-running cold cases of palaeontology
according to Andrea Baucon, a palaeontologist from the University of Cagliari, Italy, who led the international team of scientists.
Cracking the Case Across Ancient Portuguese and Italian Rocks
To solve the mystery, the research team began with a working hypothesis that the hexagonal structure was created by a marine arthropod. Arthropods are well known for producing straight tunnel segments due to their jointed appendages and rigid exoskeletons, traits that naturally result in sharp directional changes. To test this, the scientists investigated 50 fossils spanning 530 million years, stretching from Cambrian rocks in Portugal to formations in Poland and the Italian Apennines. The oldest fossil utilized in the study was discovered in a quarry located in northern Portugal.
Fieldwork carried out in the Piacenza Apennines by researchers including Filippo Guerrini and Girolamo Lo Russo provided crucial material. Michelle Piazza, a co-author of the study, noted that anyone walking in the Apennines is essentially walking on an ancient ocean floor uplifted during mountain-building processes. Those rocks were deposited in the exact same kind of environment where underwater vehicles now film Paleodictyon on muddy ocean floors today. In those mountainous locations, researchers examined burrows recently excavated by living organisms rather than fossilized remains.
Six Lines of Evidence and Advanced Arthropod Mechanics
To confirm their hypothesis, the research team worked methodically through six distinct lines of evidence: morphology, stratigraphic distribution, environmental distribution, taphonomy, tracemaker attributes, and inferred function. Girolamo Lo Russo described the fossilized network as a signature without an author,
explaining that since we cannot see the animal, we decided to let the geometry of the Paleodictyon speak.
“Those burrows were produced 460 million years before T. rex,” adds Carlos Neto de Carvalho, co-author of the study. “And something is still producing them today, thousands of metres deep, in total darkness.”
Carlos Neto de Carvalho, co-author of the study
The research team measured 1,314 points on fossil burrows housed at the Natural History Museum of Piacenza. Morphometric analysis carried out at the University of Cagliari revealed that within a single hexagonal cell of Paleodictyon, the six sides differ by half a millimetre on average—a geometry remarkably close to a regular hexagon. Rather than spreading errors between cells, the animal responsible corrected them locally. This local correction points to advanced orientation and measurement capabilities compatible with an arthropod brain, though not exclusively so. Furthermore, the first appearance of Paleodictyon coincides directly with the earliest fossil record of arthropods, lending further weight to the hypothesis.
Remaining Suspects and Deep-Sea Mysteries
While the findings offer robust support for arthropods as the most plausible creators of the pattern, the research does not completely close the inquiry. Arthropoda represents the largest animal phylum, encompassing species that account for over 80 percent of all known living animal species. Consequently, the team points toward specific likely suspects within the phylum, highlighting peracarid crustaceans such as amphipods and isopods—deep-sea cousins of the woodlouse—particularly for occurrences appearing past the Ordovician period, which spans from about 485 to 443 million years ago.
