Basic Information
Vertebrate animals belong to the chordate phylum. All chordates share one characteristic: a backbone, also known as a vertebral column or spine. Most species of chordates belong to one of five major classes, each of which contains more than a thousand species. Although the numbers are not certain and new species are still sometimes discovered, there are about 10,000 bird species, 9,000 reptile species, 6,000 amphibian species, and 5,700 mammal species. All of these classes are outnumbered by the ray-finned fish class, which has more than 30,000 species. The five major chordate classes are listed below: