James R. Garey, Department of Biology, University of South Florida, Tampa, FL 33613

ABSTRACT:   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.

Figure 6.  The Neighbor-Joining topology of Ecdysozoa using only slowly evolving ecdysozoan sequences. Branches are drawn to scale and bootstrap values are shown at each node.  Note that all the deep nodes among the ecdysozoan phyla are supported with bootstrap values ranging from 77 to 96 percent.