The origin of terrestrial isopods (Crustacea: Isopoda: Oniscidea)

Living isopods of the suborder Oniscidea (commonly called woodlice) are the only group of Crustacea almost entirely composed of terrestrial forms. Furthermore, woodlice are completely independent from the aquatic environment from which they originally arose. From marine ancestors, woodlice are a key taxon to study the conquest of the land among arthropods because of their interesting gradation of morphological, physiological and behavioral adaptations for terrestriality. However, the origin and evolution of this model group are still poorly known. Herein, we provide a synthesis of the oniscidean fossil record to replace this group in a deep-time context. Because members of the Oniscidea are difficult to fossilize, their fossil record alone is undoubtedly fragmentary and not representative of their complete evolutionary history, but it maintains an important relevance by providing reference points. To date, the first attested occurrences of Oniscidea are recorded from the Early Cretaceous. At this time, woodlice were already widely distributed (from Western Europe to Eastern Asia) with several species. By evaluating phylogenetic studies, palaeobiogeographic context of fossil specimens and current biological considerations, we discuss and support a pre-Pangaean origin of the Oniscidea, in the Late Paleozoic-most likely during the Carboniferous.