E

Panarthropoda

Lophotrochozoa

Priapulida

Kinorhyncha

Loricifera

Nematoda

Nematomorpha

Tardigrada

Arthropoda

Onychophora

Figure 5 Phylogeny of extant amniotes, with emphasis on basic reptile relationships. Among living amniotes the mammals and reptiles are sister clades. Within the living reptiles, Lepidosauria and Archosauria are likely sister taxa. The tuataras and squamates (snakes and lizards) comprise the Lepidosauria, while birds and crocodilians comprise the Archosauria. The phylogenetic position of the turtles remains unresolved. See text for further discussion.

often typified as diploblasts, these earliest diverging nonbilaterian metazoans (Figures 1 and 2) could hardly be said to be characterized by the possession of a common body plan, nor could they be considered as members of a monophyletic clade. I therefore prefer to refer to them simply as nonbila-terians. Although the precise evolutionary branching sequence of these groups is still contentious (Rokas et al., 2003a), they likely form a grade of organization (paraphyletic group) basal to the Bilateria. The morphological disparity between

Figure 7 Protostome phylogeny with emphasis on ecdysozoan relationships. The three main ecdysozoan clades are Scalidophora, Nematoida, and Panarthropoda. The scalidophor-ans comprise the loriciferans, penis worms (priapulids), and mud dragons (kinorhynchs). Nematoida comprises the horsehair worms (nematomorphs), and roundworms (nematodes). Panarthropoda comprises the arthropods, water bears (tardigrades), and velvet worms (onychophorans). See text for further discussion.

these phyla spans an enormous range of body architectures, with the maximally simple placozoans at opposite extremes, with just four differentiated somatic cell types, and the cnidarians, of which some of the most complex forms, such as cubozo-ans, possess differentiated muscle and nervous systems, and some remarkably complex sensory organs. The elucidation of the precise sequence of divergences of these taxa is therefore vital to understanding the assembly of more complex body plans at the base of the Bilateria. However, all these groups are traditionally considered to lack bilateral symmetry.

Gnathifera Platyzoa

Eutrochozoa

Ectoprocta Gastrotricha Chaetognatha Micrognathozoa Syndermata Gnathostomulida Platyhelminthes Entoprocta Cycliophora Annelida Echiura Siboglinidae Mollusca Sipuncula Nemertea Myzostomida Phoronida Brechiopoda

Figure 8 Phylogeny of the Lophotrochozoa. Only several clades can currently be recognized. Phoronids and brachiopods form a clade. The eutrochozoans form a clade characterized by a trocho-phore larva (also found in entoprocts). The Platyzoa is a putative clade of noncoelomate lophotrochozoans, and the clade Gnathifera is supported by characteristic features of their cuticularized jaws. The relationships of the remaining members of the Lophotrochozoa remain unresolved. See text for further discussion.

Surprisingly, recent investigations have added another group to the base of the Bilateria. The enigmatic, microscopical, and parasitic myxozoans were until very recently considered to be protozoans. However, recent advances in molecular phyloge-netics and ultrastructural research have established their metazoan affinities, and their possible relationships to either the cnidarians or the Bilateria (Okamura and Canning, 2003; Zrzavy and Hypsa, 2003).

The earliest diverging unambiguously bilaterally symmetrical organisms appear to be the Acoelomorpha, comprising two taxa: Acoela and Nemertodermatida (Figures 1 and 3). Previously considered to be basal flatworms (Platyhelminthes), molecular data and a reinterpretation of available morphological evidence instead suggest that these relatively simple flatworm-like organisms are the most basally branching crown group bilaterians (Baguna and Riutort, 2004). Consequently, the acoelomorphs are considered to be the sister group to the remaining bilaterians, the three main clades of which will be introduced below.

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