Vertebrates are animals classified in the subphylum Vertebrata, phylum Chordata. Vertebrates share a number of features. They all have an internal skeleton of bone and/or cartilage, which includes a bony cranium surrounding the brain and a bony vertebral column enclosing the spinal cord. Vertebrates are all covered by a skin composed of dermal and superficial epidermal layers of scales, feathers or fur, a ventral heart, formed red and white blood cells, a liver, pancreas, kidney, and a number of other internal organs. The most advanced vertebrates also have jaws, teeth, limbs or fins, and an internal skeletal structure with pelvic and pectoral girdles, and thoracic lungs.
Eight classes of vertebrates are recognized. These are listed below, in the order of their first appearance in the fossil record:
- The class Agnatha is a group of jawless, fish-like animals with poorly developed fins, which first appeared more than 500 million years ago, during the late Cambrian. The 75 surviving species include the jawless lampreys and hagfishes.
- The class Placodermi is an extinct group of bony-plated aquatic animals. The placoderms were primitive, jawless, fish-like creatures, whose head was heavily armored by an external shield of bony plates. These creatures were most abundant during the Devonian period, some 413-365 million years ago.
- The class Chondrichthyes includes about 800 living species of sharks, rays, and rat fishes, all of which have a cartilaginous skeleton, true jaws, and a number of other distinctive characters.
- The class Osteichthyes includes some 20,000 species of true fishes, with a bony skeleton, a sutured skull, teeth fused to the jaws, lobed or rayed fins, and a number of other distinguishing features.
- The class Amphibia includes some 3,500 living species of frogs, toads, salamanders, newts, and caecilians, all of which have four limbs (making them tetrapods), a moist glandular skin, external fertilization, and a complex life cycle.
- The class Reptilia are four-legged, tailed animals, with dermal scales, internal fertilization, amniotic eggs, and direct development. Living reptiles include about 6,200 species of crocodiles, turtles, lizards, snakes, and tuataras. Important extinct groups of reptiles include the dinosaurs, hadrosaurs, ichthyosaurs, pterosaurs, and plesiosaurs.
- The class Aves, the birds, is a diverse group of about 8,800 species of warm-blooded (or homoiothermic) tetrapods whose forelimbs are specialized for flight (although some species are secondarily flightless). Birds have a characteristic covering of feathers, a beak which lacks teeth, and reproduce by laying eggs.
- The class Mammalia includes more than 4,000 species of homoiothermic tetrapods, with epidermal hair and female mammary glands for suckling the young. All give birth to young, although a very few, primitive species reproduce by laying eggs.
Vertebrates are the most complex of Earth's animal life forms. The earliest vertebrates were marine, jawless, fish-like creatures that probably fed on algae, small animals, and decaying organic matter. The evolution of jaws allowed a more complex exploitation of ecological opportunities, including the pursuit of a predatory life style. The evolution of limbs and the complex life cycle of amphibians allowed the adults to exploit moist terrestrial habitats as well as aquatic habits. The subsequent evolution of internal fertilization and the self-contained, amniotic eggs of reptiles, birds, and mammals allowed reproduction on land, and led to fully terrestrial forms. Birds and mammals further advanced vertebrate adaptations to terrestrial environments through their complex anatomical, physiological, and behavioral adaptations, and this has allowed them to extensively exploit all of Earth's habitable environments.