Ecdysozoa: The Relationship between Cycloneuralia and Panarthropoda

The hypothesis that molting protostomes such as nematodes and arthropods form a monophyletic group known as Ecdysozoa is directly opposed to Articulata, in which some segmented protostomes such as annelids and arthropods form a monophyletic taxon. Ultrastructural and cladistic studies have led to the widely accepted hypothesis that nematodes belong among the protostomes. While early molecular studies suggested that nematodes were basal triploblasts, more recent molecular evidence suggests that this was an artifact of ‘long branch attraction’ and 18S rRNA gene, total evidence and hox gene studies all support the placement of nematodes within Ecdysozoa. The branching pattern within Ecdysozoa has been difficult to elucidate, but it now appears that priapulids and kinorhynchs form the earliest branching clade, followed by nematodes + nematomorphs, and finally the panarthropods. This suggests that Cycloneuralia is paraphyletic and that arthropods are the most derived of the ecdysozoans.