Sharp corners and 120-degree angles point paleodictyon's builder toward crustaceans

The argument turns on how the two candidates dig. An arthropod excavates by scraping the wall at the front of its burrow with its appendages and shifting the loosened sediment elsewhere, and the authors say that method fits paleodictyon's precise straight-line burrowing and sediment removal ^[8]. A worm takes the path of least resistance through sediment, and worm burrows come out tortuous, with rounded corners ^[9]. Straight tunnels and sharp corners, the team says, suit animals with rigid exoskeletons and jointed limbs better than soft-bodied worms ^[7].

A second line of evidence is regularity. Measured widths, lengths and angles were consistent across the fossil networks, with angles close to the 120 degrees of a regular hexagon, and the team takes that as a sign the builder corrected small construction errors as it went, using navigation and distance-sensing abilities that arthropods have ^[10]. The timing agrees: the oldest paleodictyon appears at about the same point in the Cambrian as the first arthropod trace fossils ^[11].

For anything alive, the study narrows the search to a depth band. The authors wrote that the modern occurrences of paleodictyon put the bathymetric range of its tracemakers between ~1300 and ~5000 m, and that numerous infaunal arthropods with body widths suitable for paleodictyon openings live within that range, among them isopods, amphipods, copepods and decapods ^[12]. That window is 3,700 m thick ^[13]. The fossils themselves occur worldwide in marine rocks going back to the early Cambrian, more than 500 million years ago, and at varying sea depths, so the band describes where to look today ^[3].

The interpretation being challenged has long roots. The authors note a drawing attributed to Leonardo da Vinci that may illustrate paleodictyon, and credit him with the idea that trace fossils are the products of worm-like organisms ^[15]. "The hypothesis that paleodictyon represents a fossil burrow produced by such worm-like organisms gained considerable support during the 1970s and remains a well-established hypothesis today," they wrote ^[14].

The study compares shapes and habitat overlap; it does not put an animal inside a burrow. The identification is therefore a compatibility argument, and the test that would settle it is at sea: sample a fresh lattice somewhere between 1300 and 5000 m, sieve the sediment, and see whether an isopod or an amphipod comes up. Function would still be open. Grazing trail, trap, bacterial farming and brooding are all on the list of speculative interpretations, and the authors say each could work for an arthropod tracemaker ^[16]. Earlier proposals for the maker also included single-celled organisms and sponges ^[17].