Paleodictyon, a hexagonal network of fossilized burrows found in ancient seafloor rocks in Italy's Apennine mountains, has puzzled paleontologists for decades. Modern examples exist on the ocean floor, but no one has caught the creature responsible. A new study in Earth-Science Reviews argues that the most likely maker is an unidentified marine arthropod.
Researchers led by Andrea Baucon measured more than 1,000 interior angles, side lengths, and burrow widths from museum specimens and newly found outcrops. The hexagons were strikingly precise: interior angles averaged only 8.5 degrees from a perfect 120 degrees, and side lengths varied by about half a millimeter. Straight, sharp-angled tunnels are more consistent with arthropods' jointed appendages and hard exoskeletons than with soft-bodied worms, which tend to follow paths of least resistance.
Other evidence supports the idea. Paleodictyon appears in the fossil record in the early Cambrian, when arthropods were diversifying. The apparent ability to correct small errors in the network suggests a level of cognition suited to building such structures in darkness and near-freezing water, and the pattern resembles honeycombs built by bees. Still, the case is not settled: the trace maker may have evolved over 500 million years, and multiple creatures could have produced similar patterns. The study's detailed statistical approach makes it a notable contribution, but the identity of the architect remains, for now, unconfirmed.