A REFINED TIMESCALE FOR CLITELLATE ANNELID EVOLUTION

Annnelid worms can be split into two distinctive morphogroups, the primarily marine polychaetes and the clitellates. While polychaetes are now recognised as paraphyletic, clitellates are a monophyletic comprising earthworms, leeches and other closely related lineages. Extant clitellates have diversified in freshwater habitats, with both earthworms and leeches have made the transition to the terrestrial realm. Earthworms have long been heralded as one of Earth’s most important terrestrial organisms, acting as crucial ecosystem engineers through extensive modification of soils. Despite this, we have limited confidence on the timescale over which earthworms made the transition to land and consequently their contribution to the assembly of terrestrial ecosystems is effectively unknown. Here we employ a new molecular dataset including 40 new transcriptomes from earthworms and close outgroups which we combine with 15 fossil calibrations derived from the marine annelid fossil record in relaxed molecular clock analyses. Our results recover successive branching events within the clitellate crown from the Ordovician onwards, with the earthworm crown group dated to the Carboniferous-Permian boundary. This late Palaeozoic radiation postdates the origin of other soil invertebrate groups and is coincident with large scale restructuring of terrestrial ecosystems.