Methods

All widely used phylogenetic methods have been used to reconstruct metazoan phylogeny, including distance methods, parsimony analysis, likelihood analysis, and, most recently, Bayesian analysis. Currently, molecular sequence data have been analyzed with all these methods, and morphological evidence has been studied with both parsimony and Bayesian analysis.

1.02.2 Overview of Major Metazoan Clades and Grades

Comprehensive molecular and morphological phy-logenetic analyses have generated abundant support for the monophyly of the Metazoa. Among the morphological features shared by most metazo-ans are the possession of an extracellular matrix, intercellular junctions such as septate junctions, spot desmosomes, and spermatozoa. On the molecular level the evolution of at least one Hox gene seemed to have accompanied the origin of the Metazoa. The multicellularity characteristic of plants and fungi has evolved independently of metazoan multicellularity.

Among the unicellular eukaryotes the choanofla-gellates are likely the closest living relatives of the animals (Lang et al., 2002). Morphological and functional similarities between choanoflagellate cells and the choanocytes found in sponges (Porifera) have long been interpreted as providing support for a close relationship between animals and choanoflagellates (Maldonado, 2004). Increasing research efforts are now under way to elucidate the genomic makeup of the choanoflagel-lates, which could yield insights into the nature of the first metazoan genomes.

Recent phylogenetic analyses of SSU and LSU sequences have suggested another protistan candidate as the closest relative of metazoans. These studies found the mesomycetozoans (also know as ichthyosporeans) to be the closest relative of either the Metazoa or the choanoflagellates. Nevertheless, because the mesomycetozoans are mostly specialized tissue parasites, the free-living choanoflagellates may better serve as less-modified models of the metazoan ancestor.

The remainder of this section presents a brief overview of the main branches on the metazoan phylogeny (Figure 1). This is followed by a more detailed overview of the precise composition and phylogenetic relationships within the main animal clades (Figures 2-8).

1.02.2.1 Nonbilaterians and Acoelomorpha

Although the Porifera (sponges), Placozoa (Trichoplax adhaerens), Cnidaria (e.g., jellyfish, sea anemones), and Ctenophora (comb jellies) are

Deuterostome And Acoela

Nonbilaterians

Acoelomorpha Deuterostomia Ecdysozoa Lophotrochozoa

Figure 1 Concise phylogeny of the Metazoa, indicating the main grades and clades. A grade of nonbilaterians comprises Porifera, Placozoa, Cnidaria, Ctenophora, and Myxozoa. The Bilateria comprises the Acoelomorpha, which unites Acoela and Nemertodermatida. The remaining bilaterians are called Nephrozoa because of the widespread occurrence of nephridia. The three main clades are Deuterostomia, Ecdysozoa, and Lophotrochozoa. See text for further discussion.

Bilateria

Nephrozoa

Protostomia

Nonbilaterians

Acoelomorpha Deuterostomia Ecdysozoa Lophotrochozoa

Figure 1 Concise phylogeny of the Metazoa, indicating the main grades and clades. A grade of nonbilaterians comprises Porifera, Placozoa, Cnidaria, Ctenophora, and Myxozoa. The Bilateria comprises the Acoelomorpha, which unites Acoela and Nemertodermatida. The remaining bilaterians are called Nephrozoa because of the widespread occurrence of nephridia. The three main clades are Deuterostomia, Ecdysozoa, and Lophotrochozoa. See text for further discussion.

Metazoa-

Eumetazoa/Epitheliozoa

■Hexactinellida ■Demospongiae ■Calcarea ■Ctenophora Placozoa Bilateria Myxozoa Cnidaria

Figure 2 Basal metazoan relationships. The poriferans form a basal grade, with calcarean sponges more closely related to the other metazoans. The branching sequence of the nonbilaterians is not well understood. Cnidaria, and possibly Myxozoa, are probably most closely related to the Bilateria. See text for further discussion.

Bilateria

Nephrozoa

Ambula-craria

Deuterostomia

Chordata

Ambula-craria

Deuterostomia

Chordata

Acoela

Nemertodermatida

Protostomia

Echinodermata

Hemichordata

Xenoturbella

Cephalochordata

Craniata

Urochordata

Figure 3 Overview phylogeny of the Bilateria, with an emphasis on deuterostome phylogeny. The acoels and nemerto-dermatids are the earliest diverging extant bilaterians. The Protostomia and Deuterostomia are likely sister taxa. Within the deuterostomes there are two sister clades. The clade Ambulacraria unites echinoderms and hemichordates, and probably Xenoturbella. The clade Chordata unites the urochordates, cephalochordates, and the craniates. See text for further discussion.

Echinoderm Hemichordate

Figure 4 Craniate phylogeny. The extant agnathans (hagfish and lamprey) form two successive sister groups to the gnathos-tomes. Within the Gnathostomata there is a basal division between bony fishes (Osteichthyes) and cartilaginous fishes (Chondrichthyes). Within the clade of bony fishes there is a division between lobe-finned fishes (Sarcopterygii) and ray-finned fishes (Actinopterygii). Coelacanths (Actinistia) and lungfishes (Dipnoi) are successive sister taxa to the Tetrapoda, which include all terrestrial vertebrates. See text for further discussion.

Lepidosaurs Archosaurs

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