Can arthropods produce Paleodictyon?

Abstract

Paleodictyon is a common structure documented in marine sediments from the Cambrian to the present times. Despite its pervasive distribution and distinctive hexagonal geometry, the tracemaker remains unidentified. This study investigates the hypothesis that a marine arthropod could be responsible for its production. We assess this hypothesis by reviewing six key characteristics of Paleodictyon: morphology, stratigraphic distribution, envi¬ ronmental distribution, taphonomy, tracemaker attributes, and inferred function. Arthropods are well-known for creating straight tunnel segments, a capability afforded by their jointed appendages and rigid exoskeletons, which also facilitate sharp directional changes. These attributes are consistent with the straight burrow segments of Paleodictyon, which converge at 120◦ angles. In contrast, worm-like organisms, characterised by their cylin¬ drical body plan and hydrostatic skeletons, tend to create more tortuous burrows because burrowing proceeds preferentially along paths of least resistance. A review of published evidence suggests that the tracemaker was likely highly motile and, if solitary, an exceptionally rapid burrower. Morphometric analysis of 1314 landmarks shows that interior angles depart from 120◦ by only 8.5◦ and side lengths vary within each cell by 0.5 mm. Rather than propagating errors between cells, the tracemaker corrected them locally, indicating advanced ca¬ pabilities in orientation and odometry. These cognitive skills are compatible (though not exclusive) with the sophisticated navigation skills of the arthropod brain. The first appearance of Paleodictyon in the early Cambrian coincides with the earliest fossil record of arthropods, further supporting the hypothesis. Although environ¬ mental distribution and inferred function provide less definitive evidence, they do not exclude arthropod tracemakers. Taken together, these observations suggest that arthropods are the most likely producers of Pale¬ odictyon, challenging the hypotheses of a vermiform or protist producer. Isopods and amphipods emerge as the best potential candidates for producing Paleodictyon. However, only direct neoichnological observation of burrow construction, or the discovery of a tracemaker preserved within a fossil burrow, can definitively resolve the enduring mystery of its tracemaker.