THE PHYLOGENETIC INTERRELATIONSHIPS AND TEMPORAL ORIGINATIONS OF METAZOAN PHYLA: INSIGHTS INTO THE ECOLOGY OF THE CAMBRIAN RADIATION

The advent of molecular systematics has given paleontologists tremendous insights into the metazoan radiation. One question in particular lends itself particularly well to phylogenetic and molecular clock analysis: the nature and timing of the evolutionary and ecological linkages between the pelagos and the benthos. 18S rDNA studies strongly indicate that "Porifera" is paraphyletic. This suggests that the last common ancestor of all living animals, estimated by molecular clocks to have evolved ~ 650 mya, was benthic and suspension fed, primarily upon bacteria. By ~ 610 mya the last common ancestor of cnidarians and bilaterians evolved, still a benthic population of animals but with an unknown feeding mode. Molecular studies further suggest that acoelomorph flatworms are basal bilaterians, and they evolved between 610-570 mya. Their basal position indicates that the earliest bilaterians were direct developing micrometazoans, and were most likely grazers upon bacterial mats. The last common ancestor of the remaining bilaterians (called Nephrozoa) evolved ~ 570 mya, and despite its developmental genetic complexity, the absence of trace fossils until 555 mya suggests that these animals were still micrometazoans with internal fertilization and direct development. Over the next 20 my Nephrozoa split into its three primary taxonomic constituents Deuterostomia, Lophotrochozoa, and Ecdysozoa. Within each of these three great clades many metazoan taxa grew much larger, leaving numerous body and trace fossils by ~ 530 mya. The evolution of large size, which occurred independently in numerous clades, was most likely driven by predation on the benthos, and eventually allowed for the evolution of external fertilization and ultimately planktotrophic development. Thus, the establishment of pelagic organisms, including marine larvae, most likely occurred in the latest Neoproterozoic or Earliest Cambrian. This is consistent with a molecular clock date of 545 mya for the evolution of the last common ancestor of all living cnidarians, a population whose life cycle lacked a medusa stage and was entirely benthic. The relatively late evolution of pelagic metazoans is consistent with both the fossil record of Neoproterozoic algae, and the dynamics of the Neoproterozoic carbon cycle, and provides an ecological impetus for the Cambrian explosion itself.