The early history of animals, a shotgun approach

The primary focus of this dissertation is the evaluation of alternative phylogenetic hypotheses concerning cladogenetic events that occurred early in the history of Metazoa. Alternative hypotheses are evaluated using complete sequences of the gene that codes for the small subunit of the ribosome. Concerning Bilateria, these data suggest that: (1) Bilateria is not the sister group of Radiata (Ctenophora and Cnidaria) or Ctenophora; (2) Bilateria did not give rise to Cnidaria or Ctenophora; and (3) Bilateria, Placozoa, and Cnidaria form a clade. From these three conclusions, a few inferences can be drawn. Mesoderm is not a synapomorphy of Bilateria and Ctenophora. The simplicity of placozoan body architecture is secondarily derived from a more complex ancestor. Placozoans should have a gastrulation stage during development. Bilateria and Placozoa may share a more recent common ancestor than either does to Cnidaria. Concerning the timing of bilaterian origins, there is no unequivocal physical evidence for bilaterians prior to 540 million years before present (MYBP). Simple fossil trails are often used to infer the existence of relatively complex bilaterians beginning at about 570 MYBP. This hypothesis is falsified by investigations of the trace-making capabilities of relatively simple bilaterians and non-bilaterians, some of which are quite capable of making trails as complex as those found in rocks dated between 570 and 540 MYBP. Concerning Hydrozoa, the molecular data presented herein indicate that: (1) Hydrozoa is monophyletic; (2) Hydrozoa is composed of two clades, Trachylina (Limnomedusae, Trachymedusae, and Narcomedusae) and Hydroidolina (Capitata, Filifera, Hydridae, Leptomedusae, and Siphonophora). Concerning sponges, molecular data suggest that: (1) Sponges do not form a clade; Calcarea shares a more recent common ancestor with non-sponge animals than it does with Demospongia and Hexactinellida; and (2) Demospongia and Hexactinellida form the monophyletic group Silicea. Sponges are a grade of organization out of which other animals evolved; an adult sponge is in our ancestry.