Gnathostomata

Jawed vertebrates Temporal range: Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N (Possible Late Ordovician record, 444 Ma)[1]
Example of jawed vertebrates: Tetrapoda ).
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Olfactores
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata Gegenbauer, 1874
Subgroups
† paraphyletic ) Eugnathostomata Total group Chondrichthyes †Acanthodii (paraphyletic) Chondrichthyes (cartilaginous fishes) Osteichthyes (bony fish, including tetrapods)

Gnathostomata (

Jaw development in vertebrates is likely a product of the supporting gill arches. This development would help push water into the mouth by the movement of the jaw, so that it would pass over the gills for gas exchange. The repetitive use of the newly formed jaw bones would eventually lead to the ability to bite in some gnathostomes.[10]

Newer research suggests that a branch of Placoderms was most likely the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm named Entelognathus had a bony skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in Chondrichthyes is a derived trait.^[11] The fossil findings of primitive bony fishes such as Guiyu oneiros and Psarolepis, which lived contemporaneously with Entelognathus and had pelvic girdles more in common with placoderms than with other bony fish, show that it was a relative rather than a direct ancestor of the extant gnathostomes.^[12] It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes.^[11] Fossil findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making them impossible to replace or regrow as they broke or wore down as they grew older, proves the common ancestor of all gnathostomes had teeth and place the origin of teeth along with, or soon after, the evolution of jaws.^[13][14]

Late

.

Vertebrata	Gnathostomata     † Placodermi      Eugnathostomata       Acanthodians, incl. Chondrichthyes (cartilaginous fishes)   Euteleostomi / Osteichthyes     Actinopterygii     Sarcopterygii      Tetrapoda       Amphibia      Amniota      Sauropsida     Sauria     Synapsida      Mammalia

Subgroups of jawed vertebrates
Subgroup		Common name	Example	Comments
Placodermi (extinct)		Armoured fish	Coccosteus	† prehistoric fish, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. A 380-million-year-old fossil of one species represents the oldest known example of live birth.[17] The first identifiable placoderms evolved in the late Silurian; they began a dramatic decline during the Late Devonian extinctions , and the class was entirely extinct by the end of the Devonian.
Chondrichthyes		Cartilaginous fishes	Great white shark	nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class). Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates, the extant members of which all fall into Teleostomi .
† Acanthodii (extinct)		Spiny sharks	Acanthodes bronni	† Fanjingshania is the tooth-based acanthodian species Qianodus duplicis[19] that represents the oldest unequivocal toothed vertebrate.
Osteichthyes		Bony fishes	Blue runner	Osteichthyes (bone-fish) or bony fishes are a taxonomic group of fish that have bone, as opposed to cartilaginous skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of 45 orders, with over 435 families and 28,000 species.[20] It is the largest class of vertebrates in existence today. Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). The oldest known fossils of bony fish are about 420 million years ago, which are also transitional fossils, showing a tooth pattern that is in between the tooth rows of sharks and bony fishes.[21]
Tetrapoda		Tetrapods	Fire salamander	palaeontologists at present.

Evolution

Vertebrate classes

spiny sharks

.

The appearance of the early vertebrate jaw has been described as "a crucial innovation"

The ancestor of all jawed vertebrates have gone through two rounds of whole genome duplication. The first happened before the gnathostome and cyclostome split, and appears to have been an autopolyploidy event (happened within the same species). The second occurred after the split, and was an allopolyploidy event (the result of hybridization between two lineages).[29]

The customary view is that jaws are