A molecular palaeobiological exploration of arthropod terrestrialisation

Understanding animal terrestrialisation, the process through which animals colonised the land, is crucial to understanding extant biodiversity and biological adaptation. Arthropoda (insects, spiders, centipedes and their allies) represent the largest majority of terrestrial biodiversity. Here we implemented a molecular palaeobiological approach, merging molecular and fossil evidence, to elucidate the deepest history of the terrestrial arthropods. We focused on the three, independent, Palaeozoic arthropod terrestrialisation events (those of Myriapoda, Hexapoda and Arachnida) and showed that a marine route to the colonisation of land is the most likely scenario. Molecular clock analyses confirmed an origin for the three terrestrial lineages bracketed between the Cambrian and the Silurian. While molecular divergence times for Arachnida are consistent with the fossil record, Myriapoda are inferred to have colonised land earlier, substantially predating trace or body fossil evidence. An estimated origin of myriapods by the early Cambrian precedes the appearance of embryophytes and perhaps even terrestrial fungi, raising the possibility that terrestrialisation had independent origins in crown-group myriapod lineages, consistent with morphological arguments for convergence in tracheal systems.