Neoichnology of amphibious arthropods: effects of subaqueous and subaerial substrate conditions on trace morphology

Neoichnology, the study of the traces of extant organisms, provides a vital tool for better understanding trace fossils. We conducted neoichnological experiments to test hypotheses regarding producers and the effects of substrate conditions on trace fossils produced by aquatic to amphibious arthropods. Our experiments comprised two protocols: subaerial and subaqueous substrates; and we utilised five arthropods: fully aquatic ostracods (Ostracoda indet.), to amphibious horseshoe crabs (Limulus polyphemus), shore crabs (Carcinus maenas), and scarlet hermit crabs (Paguristes cadenati), and the largely terrestrial sea slaters (Ligia oceanica). The different arthropods were observed performing locomotory, resting/stationary, and feeding behaviors, which all resulted in different traces influenced by the substrate conditions and their preference for living in and out of water. In general, trace depth increased with arthropod mass and, for each individual arthropod except the scarlet hermit crab, trace depth was also greater in exposed compared to subaqueous substrates. In the majority of cases, comparisons with selected trace fossils supported previous hypotheses as to their producers. The traces of horseshoe crabs, shore crabs, sea slaters and ostracods resembled the ichnotaxa Kouphichnium, Laterigradus, Pterichnus and Mermia respectively. Other experimental work has shown hermit crabs produce traces similar to Coenobichnus and our results further increase the range of trace morphologies that can be attributed to hermit crabs. The results of this research have bearing on debates in ichnology where the interpretation of producers and substrate conditions at the time of trace formation are critical, such as the trace fossil evidence for the colonization of land.