The Ecdysozoa: Artifact or monophylum?

The molecular support for the recently postulated hypothesis `Ecdysozoa' is re-examined. Some ISS rDNA-alignments clearly contain a pattern of charact er states that cause tree building methods to identify a clade comprised of arthropods and Cycloneuralia. However, examination of these alignments sho ws that the supporting positions are very noisy, with the consequence that this weakens the hierarchical structure of these data sets. This noisy feat ure of the data is reflected in lowered bootstrap values on reconstructed t rees, the shape of relative apparent synapomorphy analyses (RASA) plots as weil as in PHYSID and Hadamard spectra which show strong contradictory sign als. The unexplained nature of the contradictions in the 18S rDNA leads us to be cautious in accepting the Ecdysozoa hypothesis, particularly since a strong argument for the alternative hypothesis Articulata can be rr,ade fro m morphological data.