The Diversity of Animal Life: Introduction to Early Emerging Metazoans

Abstract

“superphyla” (Fig. 3.1 )—Ecdysozoa (the “moulting” animals, within which the arthropods and nematodes are the major component phyla), Lophotrochozoa (crest or wheel animals; mollusks, annelids, and platyhelminths are the major groups), and Deuterostomia (echinoderms, hemichordates, urochordates, and chordates). The overwhelming majority of extant animals have two obvious body axes— anterior/posterior and dorsal/ventral—and hence the phyla to which they belong are known as the Bilateria (or bilaterally symmetric animals), the “higher” animals. That the Bilateria have a single origin (i.e., are a monophyletic group) is most con- vincingly shown by the fact that the molecular mechanisms involved in patterning along these two axes are conserved even between such different kinds of animals as vertebrates and insects. There are, however, at least four extant phyla whose origins predate those of the Bilateria: these are the Cnidaria (jellyfi sh, sea anemones, corals, hydras), Porifera (sponges), Ctenophora (comb jellies), and Placozoa (no common name). Cnidaria and Porifera are large and diverse phyla, each containing many thousands of spe- cies, whereas the Ctenophora is a small group and the Placozoa a phylum of one. Whole-genome sequences are available for representatives of each of these, but their relationships are still equivocal and hotly debated (see also below). Perhaps even more contentious are the interrelated issues of the timing of the emergence of the Metazoa and the relationship between extant animals are those that are known only from Precambrian fossils.