Animal evolution - the end of the intermediate taxa?

Contrary to general belief, there has not been a reliable, global phylogeny of animals at hand within the past few decades. Recent progress in molecul ar phylogeny is rapidly changing the situation and has provided trees that constitute a reference frame for discussing the still controversial evoluti on of body plans. These trees, once purged of their possible artefacts, hav e already yielded confirmation of traditional, anatomically based, phylogen ies as well as several new and quite significant results. Of these, one of the most striking is the disappearance of two superphyla (acoelomates such as flatworms, pseudocoelomates such as nematodes) previously thought to rep resent grades of intermediate complexity between diploblasts (organisms wit h two germ layers) and triploblasts (organisms with three germ layers). The overall image now emerging is of a fairly simple global tree of metazoans, comprising only a small number of major branches,The topology nicely accou nts for the striking conservation of developmental genes in all bilaterians and suggests a new interpretation of the 'Cambrian explosion' of animal di versity.