USING DNA-SEQUENCES TO UNRAVEL THE CAMBRIAN RADIATION OF THE ANIMAL PHYLA

Most animal phyla appeared in the Cambrian radiation, and many have a rich fossil record. The phylogenetic relationships among phyla are, ho wever, still poorly understood. Although systematics based on morpholo gical characters has been revitalized by cladistic methods, incongruen t results have been obtained for most relationships. Gene sequence dat a offer the potential for adding substantial new characters for evalua ting phylum level relationships. Molecular studies of the Cambrian rad iation are just beginning, and only a few of the potential characters available from genes have been examined. The achievements and controve rsies between molecular studies are reviewed and compared to correspon ding studies based on morphological data. Because molecular systematic s is intensely dependent upon the methods used to infer trees, importa nt methodological considerations that affect the studies of phylum-lev el molecular systematics are discussed. The analysis of available gene sequence data has resolved conflicts between morphological interpreta tions. However, molecular systematics has not yet produced phylogeneti c trees of broad phylum relationships more robust than those based on morphology.