Elusive builders of nearly perfect hexagonal tunnel fossils were probably small crustaceans—not worms

For over 170 years, scientists have puzzled over the origin of the perfectly hexagonal tunnels found in fossils known as paleodictyon. A new study published in Earth Science Reviews suggests these tunnels were likely created by small crustaceans, challenging the popular theory that worms were responsible. Paleodictyon fossils, dating back over 500 million years, have been discovered worldwide in marine rocks from various depths.

The tunnels consist of a network of hexagonal meshes with vertical outlets, ranging from a few millimeters to less than 1 millimeter in diameter. Scientists know the builders are small, but the identity of the creature that made them remains a mystery. Leonardo da Vinci may have been among the first to examine paleodictyon, as evidenced by a drawing attributed to him that potentially illustrates the structure.

However, it is uncertain if he specifically applied the idea of trace fossils to paleodictyon. Previous hypotheses, including worms, single-celled organisms, sponges, and crustaceans, have been proposed. The latest study examined fossil specimens from Italy, Portugal, and Poland and found that the builders were likely arthropods, which include crustaceans, bugs, spiders, and centipedes.

High-resolution photography and 3D models were used to measure the geometry of the fossil tunnel networks and compare them with burrows made by living arthropods. The straight tunnels and sharp corners of paleodictyon fit arthropods better than soft-bodied worms, which tend to create tortuous burrows with rounded corners. The team also noted that the fossil networks showed consistent tunnel widths, lengths, and angles close to the 120 degrees of a regular hexagon, suggesting the builder had strong navigation and distance-sensing abilities.

The timeline of paleodictyon aligns with the appearance of the first arthropod trace fossils in the Cambrian and modern arthropod habitats. While the evidence supporting arthropods is strong, other possibilities, such as worms, inorganic processes, or even communication networks, cannot be fully ruled out. Further evidence, such as video of a modern paleodictyon maker or a fossilized maker, would help settle the debate.