Fossil calibrations for the arthropod Tree of Life (original) (raw)

The interrelationships of major arthropod clades have long been contentious, but refinements in molecular phylogenetics underpin an emerging consensus. Striking topologies from these studies include the position of hexapods (insects) within a paraphyletic Crustacea, and a putative myriapod/chelicerate alliance. Although some morphological characters support these groupings, they have not been recovered in large-scale phylogenetic analyses based on morphology. An analysis herein of 753 morphological characters for 309 fossil and Recent panarthropods shows enhanced congruence with molecular phylogenies. We resolve hexapods within Crustacea, with remipedes their closest extant relatives. Selective taxon and character removal exposes the traditional myriapod/hexapod linkage as an artefact of convergent character acquisition during terrestrialisation. Fossils break up such long branches, their intermediate morphologies linking clades that have otherwise had their ancestral conditions overprinted. Pycnogonids are an exemplar, grouping with euchelicerates when fossils are included, and as sister-taxon to all other extant arthropods when fossils are removed.