Gnathostomata

Gnathostomata Temporal range: Late Ordovician - Recent
Gnathostomata are jawed vertebrates
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata Gegenbaur, 1874
Subgroups
Placodermi † Chondrichthyes Acanthodii † Actinopterygii Sarcopterygii

Gnathostomata (Template:Pron-en) is the group of vertebrates with jaws. The term derives from Greek γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth". In addition to opposing jaws, living gnathostomes also have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with many other distinguishing characteristics.

The group is traditionally a superclass, broken into three top-level groupings: Chondrichthyes, or the cartilaginous fish; Placodermi, an extinct clade of armored fish; and Teleostomi, which includes the familiar classes of bony fish, birds, mammals, reptiles, and amphibians. Some classification systems have used the term Amphirhina. It is a sister group of the jawless craniates Agnatha.

New fossil finds suggests thelodonts as the closest relatives of the Gnathostomata.

It is believed that the jaws evolved from anterior gill support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively — the buccal pump mechanism. The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would with time become stronger and tougher, being transformed into real jaws.

Placoderms used sharp bony plates as teeth instead, and newer research indicates the jaws in placoderms evolved independently of those in the other Gnathostomata.

Other distinguishing characteristics of living gnathostomates are the myelin sheathes of neurons. Another is an adaptive immune system that uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the Variable lymphocyte receptor gene.

The Gnathostomata first appeared in the Ordovician period and became common in the Devonian period.

Taxonomy and phylogeny

Subphylum Vertebrata
├─(unranked) Gnathostomatomorpha
└─Infraphylum Gnathostomata
      ├─Class Placodermi — extinct (armored gnathostomes)
      └Microphylum Eugnathostomata (true jawed vertebrates)
         ├─Class Chondrichthyes (cartilaginous fish)
         └─(unranked) Teleostomi (Acanthodii & Osteichthyes)
             ├─Class Acanthodii — extinct ("spiny sharks")
             └Superclass Osteichthyes (bony fish)
                ├─Class Actinopterygii (ray-finned fish)
                └─Class Sarcopterygii (lobe-finned fish)
                     └Superclass Tetrapoda
                         ├─Class Amphibia (amphibians)
                         └(unranked) Amniota (amniotic egg)
                             ├─Class Sauropsida (reptiles or sauropsids)
                             │  └─Class Aves (birds)
                             └─Class Synapsida
                                 └─Class Mammalia (mammals)
 
Note: lines show evolutionary relationships.

External links

• Tree of Life discussion of Gnathostomata
• The Gill Arches: Meckel's Cartilage