The largest natural group (clade, monophyletic group) of chordate animals. Like all chordates, vertebrates have a notochord, but it is largely replaced by vertebrae (initially neural arches that surround the nerve chord) during development. Together with the living hagfishes (Hyperotreti; Myxiniiformes) and many fossil groups, vertebrates are members of the Craniata. One major theme of craniate evolution is the increasing elaboration of the brain, its cranial nerves, and many sensory organs, such as the organs of smell (olfactory), hearing (inner ear), and feel (sensory organs of the lateral line system). A unique innovation is the evolution of neural crest cells, cells that begin development along the neural crest of the embryo and which then migrate to many parts of the body, giving rise to or contributing to the formation of such structures as the cranium and the branchial arches (a remnant of which remains in humans as parts of the larynx). Vertebrates have a long fossil history. Earliest fossils of jawless vertebrates are known from the late Cambrian (490 million years before present, MYBP). Jawed vertebrate fossils are known from the Ordovician (450 MYBP) and this group began to predominate in the Devonian (about 360 MYBP).  See also: Chordata; Nervous system (vertebrate); Vertebra; Vertebrate brain (evolution)