Aquatic stem group myriapods close a gap between molecular divergence dates and the terrestrial fossil record

Significance Most arthropod diversity is now found on land, with hexapods (insects), arachnids, and myriapods being major terrestrial radiations. Molecular dating consistently predicts that these groups have earlier evolutionary origins than are recorded by fossils. A reason for this difference between molecular- and fossil-based age estimates is that few candidates for marine or freshwater stem groups of these terrestrial lineages have been identified. A Devonian euthycarcinoid arthropod preserves details of the head shared by myriapods, adding support to the theory that they are each other’s closest relative. The fossil record of euthycarcinoids in the Cambrian, including trackways made on tidal flats, clarifies the marine-to-terrestrial transition in the myriapod lineage and reconciles molecular and fossil-based estimates for the timing of myriapod origins.