The vertebrates are the largest and ecologically most important subphylum of chordates, and indeed of all animals.
Although they do not include the largest number of species or largest number of individuals per species, they include the largest animals, whales, and the largest average-sized species. Only the largest trees exceed the mass of whales. Vertebrates are characterized by the possession of a vertebral column, a braincase, medial fins, a skeleton of cartilage and/or bone, a true skin (epidermis plus dermis), a kidney, liver, and pancreas, a specialized heart and blood cells, and a neural crest. They arose with the invasion of freshwaters by the protochordates, and the core evolution of vertebrates to the tetrapods was a radiation within the freshwaters. Marine groups of vertebrates have invaded the oceans secondarily and are dead-end lineages. Higher vertebrates (such as gnathostomes) possess jaws with teeth, paired fins with an internal skeleton articulating with limb girdles, bony scales, and lungs. The tetrapods have limbs evolved from fins, digits, a neck, specialization Of the segments of the vertebral column, and horny scales, hairs, or feathers.
The vertebrates are divided into 7 classes plus 1 extinct class, the Placodermi. These classes are arranged into several groupings, some of which are not natural taxa. One set is the Agnatha, or jawless fishes, versus the Gnathostomata, or jawed vertebrates. The second is the Tetrapoda. or terrestrial tetrapods, in contrast to the Pisces, or fishes in the broadest sense. And the last is the Amniota, which include the reptiles, birds, and mammals based on the possession of a complex set of extraembryonic membranes, as opposed to the Anamniota, the lower classes which have only the trilaminate yolk sac.
Walter J. Bock