ABSTRACT
Understanding key transitions in animal evolution is dependent in on an accurate evolutionary framework or phylogeny. Molecular phylogenetics greatly informed the study of animal evolutionary relationships during the late 1980s and 1990s and brought about several major changes to the animal tree of life (e.g. Lophotrochozoa, Ecdysozoa). More recently, our understanding of higher-level animal evolutionary relationships has been further resolved, although many questions remain unanswered. Major recent changes to our view of the animal tree of life include the placement of Acoela and Nemertodermatida at the base of Bilateria, the inclusion of sipunculans within Annelida, the placement of Onychophora as the sister taxon of Arthropoda, and the surprising fi nd of a clade composed of Craniata and Tunicata to the exclusion of Cephalochordata. Despite these advances, a number of issues remain unresolved; including which of the basal metazoan phyla (Porifera, Placozoa, Cnidaria, or Ctenophora) is basal-most, the branching order of the three major lineages of Arthropoda, the relationships within Lophotrochozoa, and the placement of several socalled minor groups such as Chaetognatha. Here, we review the current understanding of animal phylogeny with emphasis on the most recent work and illustrate remaining gaps in our knowledge.
