Molecular Timetrees Reveal a Cambrian Colonization of Land and a New Scenario for Ecdysozoan Evolution

SummaryEcdysozoans have been key components of ecosystems since the early Cambrian, when trilobites and soft-bodied Burgess Shale-type ecdysozoans dominated marine animal communities [1]. Even today, the most abundant animals on Earth are either nematode worms or plankton-forming crustaceans, whereas the most diverse are the insects [2]. Throughout geological time, several ecdysozoan lineages independently colonized land [3], shaping both marine and terrestrial ecosystems and providing an adequate environment for successive animal terrestrialization. The timing of these events is largely uncertain [4, 5] and has been investigated only partially using molecular data [6–10]. Here we present a timescale of ecdysozoan evolution based on multiple molecular data sets, the most complete set of fossil calibrations to date, and a thorough series of validation analyses. Results converge on an Ediacaran origin of all major ecdysozoan lineages (∼587–543 million years ago [mya]), followed by a fast Cambrian radiation of the pancrustaceans (∼539–511 mya), a Cambro-Ordovician colonization of land of different arthropod lineages (∼510–471 mya), and a relatively recent radiation of extant nematodes, onychophorans, and tardigrades (∼442 mya). Arthropods colonized land nearly synchronously with land plants. Further diversification within flying insects, nematodes and onychophorans might be related to the evolution of vascular plants and forests