Gnathostomes are characterized by:
Pelvic fins. These are the paired fins or limbs situated just in front of the anus. Interventrals and basiventrals in the backbone. These are the elements of the backbone which lie under the notochord, and match the basidorsals and interdorsals respectively. Gill arches which lie internally to the gills and branchial blood vessels, contrary to the gill arches of all jawless craniates, which are external to the gills and blood vessels. A horizontal semicircular canal in the inner ear. Paired nasal sacs which are independent from the hypophysial tube. In all extant and fossil jawless craniates, the nasal sacs, which contain the olfactory organs, open into a median duct, the nasohypophysial duct, which takes part to the formation of the pituitary gland and either leads postriorly to the pharynx (e.g. in hagfish and galeaspids) or ends as a blind pouch (e.g. in lampreys and osteostracans). In the gnathostomes, this pouch remains as a thin canal in the palate, the buccohypophysial canal, whereas the nasal sacs open separately to the exterior by external nostrils.
- A vertically biting device called jaws, and which is primitively made up by two endoskeletal elements, the palatoquadrate and Meckelian cartilage, and a number of dermal elements called teeth, sometimes attached to large dermal bones.
The skull of a gnathostome, or jawed vertebrates (here a shark), are characterized by vertically biting jaws (red) consisting of the palatoquadrate dorsally and the Meckelian cartilage ventrally. The gill arches (green) are situated internally to the gill filaments, and the nasal capsules (blue) open to the exterior by means of paired nostrils. (After Janvier 1996.)
There are numerous other characteristics of the soft anatomy and physiology (e.g. myelinated nerve fibres, sperms passing through urinary ducts, etc.), which are unique to the gnathostomes among extant craniates, but cannot by observed in fossils.