400 million years on six legs: on the origin and early evolution of Hexapoda

Identifying the unambiguous sister group to the hexapods has been elusive. Traditional concepts include the Myriapoda (the Tracheata/Atelocerata hypothesis), but recent molecular studies consistently indicate it is the Crustacea, either in part or entirety (the Pancrustacea/Tetraconata hypothesis). The morphological evidence in support of Tracheata is reviewed, and most features are found to be ambiguous (i.e., losses, poorly known and surveyed structures, and probable convergences), though some appear to be synapomorphic, such as tentorial structure and the presence of styli and eversible vesicles. Other morphological features, particularly the structure of the eyes and nervous system, support Pancrustacea, as does consistent molecular evidence (which is reviewed and critiqued). Suggestions are made regarding hexapod-crustacean limb homologies. Relationships among basal (apterygote) hexapods are reviewed, and critical Paleozoic fossils are discussed. Despite the scarceness of Devonian hexapods, major lineages like Collembola and even dicondylic Insecta appeared in the Early Devonian; stem-group and putative Archaeognatha are known from the Carboniferous through Permian and the Late Devonian, respectively. Thus, the earliest divergences of hexapods were perhaps Late Silurian, which is considerably younger than several estimates made using molecular data.