An international team of scientists, led by paleontologist Andrea Baucon of the University of Cagliari, Italy, has partly solved the mystery of a strange natural pattern called Paleodictyon found around the world. The mystery at hand: what animal made (and continues to make) these unusual markings? The findings are published in the journal Earth-Science Reviews. Paleodictyon is a hexagonal network of honeycomb burrows found in marine sediments dating back to the Cambrian, a geological period that began approximately 539 million years ago and lasted between 52 and 55 million years. This pattern is as widespread as it is mysterious and is found everywhere from 3,500 m deep in the Mid-Atlantic Ridge to the mountain tops of the Italian Apennines. Leonardo da Vinci drew it around 1500 without even knowing what it was, and it was first described and named in 1850 by Giuseppe Meneghini. It has persisted for more than 500 million years, yet its creator remained unknown for centuries. Geopaleontological fieldwork in the Piacenza Apennines. From left to right: Andrea Baucon, Filippo Guerrini, Girolamo Lo Russo. Credit: Baucon et al. By investigating 50 fossils spanning 530 million years, from Cambrian rocks in Portugal to the Italian Apennines, scientists have identified the prime suspect: a small burrowing crustacean, a relative of the deep-sea mealybug. “Paleodictyon is one of the longest-standing unsolved cases in paleontology,” says Andrea Baucon. “The fossils come from the Apennines, Portugal and Poland, while morphometric analysis was carried out at the University of Cagliari. This approach allowed us to use the geometry of the footprints to infer information about the organism that produced them.” The oldest fossil used in the study was found in a quarry in northern Portugal. “These burrows were built 460 million years before T. rex,” adds Carlos Neto de Carvalho, co-author of the study. “And something still produces them today, thousands of meters deep, in total darkness.” Solving the case The researchers started from a hypothesis: the hexagonal structure was created by a marine arthropod. Arthropods are known for creating straight tunnel segments, thanks to their articulated appendages and rigid exoskeletons, which also result in abrupt directional changes. To confirm this hypothesis, the team worked through six lines of evidence: morphology (shape), stratigraphic distribution (when it appears in the rock record), environmental distribution (where it is found), taphonomy (how it is preserved), tracer attributes (what type of animal might make it), and inferred function (what it was used for). “It is a signature without an author,” says Girolamo Lo Russo, co-author of the study. “Since we can’t see the animal, we decided to let the geometry of the Paleodictyon do the talking.” The researchers measured 1,314 points in fossil burrows located at the Natural History Museum of Piacenza. They also went out into the countryside in search of new fossil specimens, spending days on their knees, searching the Apennines and the Alps for any trace of the mysterious hexagon. “Anyone who walks in the Apennines walks on an ancient ocean floor, elevated in the formation of that mountain range,” says Michelle Piazza, another co-author. “These rocks were deposited in the same environment where underwater vehicles now film Paleodictyon on muddy seabeds. But in this case they are not fossils, but burrows recently excavated by living organisms.” The researchers found that within a single hexagonal Paleodictyon cell, the six sides differ by half a millimeter on average, a geometry very close to that of a regular hexagon. Instead of propagating errors between cells, the animal that created the pattern corrected them locally, suggesting advanced targeting and measurement capabilities. These abilities are compatible with the abilities of the arthropod brain, although not exclusively. The first appearance of Paleodictyon also coincides with the oldest fossil record of arthropods, further supporting the hypothesis. Paleodictyon, Piacenza Apennines. Late Eocene (35 million years ago). Credit: Baucon et al. Paleodictyon, Piacenza Apennines. Late Eocene (35 million years ago). Credit: Baucon et al. Paleodictyon, Piacenza Apennines. Late Eocene (35 million years ago). Credit: Baucon et al. The Remaining Mystery These new findings are by no means the end of the Paleodictyon mystery: The research provides strong evidence supporting arthropods as the most plausible producers of the pattern, but Arthropoda is the largest animal phylum, with several species accounting for more than 80 percent of all known living animal species. The researchers point out some possible suspects within the phylum: peracarid crustaceans such as amphipods and isopods, particularly in cases after the Ordovician period (about 485-443 million years ago). And another question remains: what is Paleodictyon for? Whatever animal believes it, why does it do it? Until now, the main hypothesis was that it could be used to grow bacteria inside burrows. This study proposes another. “The hexagonal mesh could function as a colony’s communication network, transporting chemical, hydraulic or acoustic signals,” explains Andrea Barucci, co-author of the study. The scientists note that more research is needed to address the gaps that still exist in our understanding of this mysterious hexagonal pattern. Photogrammetric video of the Paleodictyon. Credit: Baucon et al. Read the full findings here. Cover image: Paleodictyon, Piacenza Apennines. Late Eocene (35 million years ago). Credit: Baucon et al.
