Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. About 57,739 species of vertebrates have been described. Vertebrates started to evolve about 530 million years ago during the Cambrian explosion, which is part of the Cambrian period (first known vertebrate is Myllokunmingia). Their name derives from the bones of the spinal column (or vertebral column), the vertebrae.
Vertebrata is the largest subphylum of chordates, and contains most animals with which people are generally familiar (except insects). Fish (including lampreys, but traditionally not hagfish, though this is now disputed), amphibians, reptiles, birds, and mammals (including humans) are vertebrates. Characteristics of the subphylum are a muscular system that mostly consists of paired masses, as well as a central nervous system which is partly located inside the backbone (if one is present).
Usually, the defining characteristic of a vertebrate is considered the backbone or spinal cord, a brain case, and an internal skeleton, but the former do not hold true for lampreys, and the latter is arguably present in some other chordates. Rather, all vertebrates are most easily distinguished from all other chordates by having an unequivocal head, that is, sensory organs - especially eyes are concentrated at the fore end of the body and there is pronounced cephalization. Compare the lancelets which have a mouth but no true head, and "see" with their entire back.
The internal skeleton which defines vertebrates consists of cartilage or bone, or in some cases both. An outer skeleton in form of a bony armour was the first bony substance the vertebrates evolved. It is possible its primary function was as a phosphate reservoir, excreted as calcium phosphate and stored around the body, offering protection at the same time.
The skeleton provides support to the organism during the period of growth. For this reason vertebrates can achieve larger sizes than invertebrates, and on average vertebrates are in fact larger. The skeleton of most vertebrates, that is excluding the most primitive ones, consists of a skull, the vertebral column and two pairs of limbs. In some forms of vertebrates, one or both of these pairs of limbs may be absent, such as in snakes or whales. These limbs have been lost in the course of evolution.
- Agnatha - Agnatha (Greek, "no jaws") is a paraphyletic superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. There are two extant groups of jawless fish (sometimes called cyclostomes), the lampreys and the hagfish, with about 60 species between them.
- Amphibians - Amphibians (class Amphibia) are a taxon of animals that include all tetrapods, four-legged vertebrates, that do not have amniotic eggs. Amphibians (from Greek αμφις "both" and βιος "life") are ectotherms, and generally spend part of their time on land, but most do not have the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes).
- Birds - Birds are bipedal, warm-blooded, oviparous vertebrate animals characterized primarily by feathers, forelimbs modified as wings, and hollow bones.
- Chondrichthyes (Sharks) - The Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage.
- Mammals - The mammals are the class of vertebrate animals characterized by the presence of mammary glands, which in females produce milk for the nourishment of young; the presence of hair or fur; and endothermic or "warm-blooded" bodies.
- Osteichthyes (Fish) - Osteichthyes are a taxonomic superclass of fish, also called bony fish that includes the ray-finned fish (Actinopterygii) and lobe finned fish (Sarcopterygii).
- Reptiles - Reptiles are tetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane. Today they are represented by four surviving orders:
- Crocodilia (crocodiles, caimans and alligators): 23 species
- Rhynchocephalia (tuataras from New Zealand): 2 species
- Squamata (lizards, snakes and amphisbaenids): 7,600 species
- Testudines (turtles): approximately 300 species