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, however, still poorly understood. Although systematics based on morphological characters has been revitalized by cladistic methods, incongruent results have been obtained for most relationships. Gene sequence data offer the potential for adding substantial new characters for evaluating phylum level relationships. Molecular studies of the Cambrian radiation are just beginning, and only a few of the potential characters available from genes have been examined. The achievements and controversies between molecular studies are reviewed and compared to corresponding studies based on morphological data. Because molecular systematics is intensely dependent upon the methods used to infer trees, important methodological considerations that affect the studies of phylum-level molecular systematics are discussed. The analysis of available gene sequence data has resolved conflicts between morphological interpretations. However, molecular systematics has not yet produced phylogenetic trees of broad phylum relationships more robust than those based on morphology.