Early Terrestrial Arthropods: A Fragmentary Record

W. D. I. Rolfe

Abstract

The earliest unequivocal terrestrial fossils are uppermost Silurian (Pridoli) myriapods, presumed to be pioneer decomposers. Descendants of their marine ancestors may be discernible in the Cambrian lobopod Aysheaia (recently challenged). Known euthycarcinoids are too late except as survivors from such a lobopod ancestry. Unique arthropods are also known from Cambrian and Ordovician lake deposits, but shed no light on origin of terrestrial forms. Among relevant arthropods, only scorpions have a continuous record from Silurian aquatic to Devonian terrestrial records (eurypterids are dealt with elsewhere in this volume). Respiratory organs are unknown in Silurian forms, but at least three types of gills may have existed by the Devonian, arguing for great diversification by then, and possible multiple colonization of the land. Trace fossils suggest some Devonian scorpions were amphibious, while development of a pre-oral tube indicates they were adapted for feeding out of water. Morphology of Silurian forms implies they were aquatic, solid feeders. The presence of Silurian fungivorous microarthropods is indicated by Ludlow faecal pellets containing fungal hyphae. The terrestrial decomposer niche was occupied, and soils therefore in existence, by then. Constant humid microclimates created by early land plants were important in enabling land colonization by arthropods otherwise susceptible to desiccation. Mamayev's gravitational hypothesis provides an explanation for evolution of hexapody from uniramians clambering up and over early land plants. Plants of the Siegenian Rhynie Chert show pathological features that may indicate terrestrial plant-animal interaction by that time, as do other lines of circumstantial evidence. Energy costs incurred by plants producing anti-herbivore and anti-detritivore defences have therefore probably been significant in ecosystems since the Devonian. Recent major backwards extensions in time of terrestrial arthropods prove that the terrestrial geological record is still poorly sampled. Givetian Gilboa shows that, as with early plants, much can be hoped for from study of cuticle fragments. New finds should be sought in pre-Devonian equivalents of the Carboniferous Mazon Creek delta plain facies.