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THE FISHES

VERTEBRATA—VERTEBRATES

The Vertebrata is one of the most successful lineages of animals, dominating both aquatic and terrestrial habitats around the globe. This diverse group, with well over 60,000 species, is characterized by the presence of ossifications surrounding and often occluding the notochord (in most living species), a well-developed brain, a notochord that is restricted posterior to the brain, a chambered heart, and a host of other features (Forey, 1995; Nelson, 2006). Aquatic representatives number well over 30,000 species and include the jawless fishes (Agnatha), cartilaginous fishes (Chondrichthyes), and ray-finned fishes (Actinopterygii). Terrestrial habitats are largely the domain of the Tetrapoda, the dominant clade of the Sarcopterygii, which also includes a handful of aquatic lung fishes and the coelacanths. Relationships among these major lineages of vertebrates have been discussed for decades and a consensus has been reached (Meyer and Zardoya, 2003). The lobe-finned and ray-finned fishes form a monophyletic group (called the Osteichthyes or “bony fishes”); they together with the cartilaginous fishes make up the “jawed vertebrates” or the Gnathostomata. The jawless fishes are the sister group of all other extant vertebrates. This book covers all major lineages of the Vertebrata with the exception of the Tetrapoda.

Hypothesized phylogenetic relationships of the major lineages of vertebrates.