The Vertebrata, or vertebrates, constitute a diverse group of animals that includes fish, amphibians, reptiles, mammals, and birds. These organisms all possess a vertebral column, or backbone, that runs from head to tail and surrounds and protects the main nerve cord. Perhaps the most important vertebrate feature is the presence of embryonic neural crest cells, which give rise to many important nerves as well as to head and facial features. Other features that separate vertebrates from the larger parent group of chordates include a cranium that encloses a well-developed brain, paired complex eyes, a muscularized mouth and pharynx, and a heart and circulatory system. The first vertebrates appeared during the Cambrian, more than 500 million years ago, and lacked jaws (as do the living lampreys). The jawed vertebrates (the majority of vertebrates living today) did not appear until 100 million years later, in the Silurian.