A superclass of the subphylum Vertebrata (Craniata). Gnathostomes are animals with jaws, involving a vertical biting that developed from modified gill arches. They are further characterized by a notochord (an elongated dorsal cord of cells that is the primitive axial skeleton in all chordates) that is present in the ontogeny of all lineages but replaced by vertebral centra (the main bodies of vertebrae) in most taxa; early fossil forms with a bony exoskeleton that was lost in the higher lineages; limb girdles (secondarily lost in some forms) supporting paired appendages in all but the most primitive taxa; myelinated neurons; an adaptive immune system; intrinsic eye muscles; a sperm duct linked to the urinary system; and a distinct cerebellum. Some of the characters were carried over from the primitive superclass Agnatha, but jaws and paired appendages are unique to Gnathostomata.