Interstitial Terrestrialization in Arthropoda

It has long been hypothesized that some arthropod lineages transitioned to land by following an interstitial pathway through the spaces between sand grains. In recent years, various molecular phylogenetic analyses suggest a greater number of terrestrialization events within Arthropoda than previously hypothesized. The relative importance of an interstitial route to land is likely to have been underestimated because of biases in the fossil record and the choice of techniques used for collecting extant arthropods from sands and other types of mineral regolith (sediment with low organic content). A number of early-branching taxa are microarthropods that are common in mineral regolith, providing phyloecological evidence for an interstitial pathway onto land. Following interstitial terrestrialization, hexapods and early-branching arachnids may have remained minute and soft-bodied within mineral regolith until the Early Devonian, when organically rich soils developed on much of the land surface, resulting in increased food resources but also increased rates of predation. This led to defensive modifications and increases in surface abundance and body size, which would have all elevated the probability of fossilization.