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Gnathostomata // are the jawed vertebrates. The term derives from Greek γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes also have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as 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.
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.
Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians or "shark-like fishes," may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the early Silurian, and become abundant by the start of the Devonian.
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.
|Subgroups of jawed vertebrates|
|Armoured fish|| 140px||†Placodermi (plate-skinned) is an extinct class of armoured 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. 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||140px<center>Great white shark||Chondrichthyes (cartilage-fish) or cartilaginous fishes are jawed fish with paired fins, paired 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.|
|Spiny sharks|| 140px||†Acanthodii, or spiny sharks are a class of extinct fishes, sharing features with both bony and cartilaginous fishes. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins). They may have been an independent phylogenetic branch of fishes, which had evolved from little-specialized forms close to Recent Chondrichthyes. Acanthodians did, in fact, have a cartilaginous skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine. They are distinguished in two respects: they were the earliest known jawed vertebrates, and they had stout spines supporting their fins, fixed in place and non-movable (like a shark's dorsal fin). The acanthodians' jaws are presumed to have evolved from the first gill arch of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage. The common name "spiny sharks" is really a misnomer for these early jawed fishes. The name was coined because they were superficially shark-shaped, with a streamlined body, paired fins, and a strongly upturned tail; stout bony spines supported all the fins except the tail - hence, "spiny sharks".|
|Osteichthyes||Bony fishes|| 140px||Osteichthyes 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, and over 435 families and 28,000 species. 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.|
|Tetrapoda||Tetrapods|| 140px||Tetrapoda (four-feet) or tetrapods are the group of all four-limbed vertebrates, including living and extinct amphibians, reptiles, birds, and mammals. Amphibians today generally remain semi-aquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans have lost some or all of their limbs. And many tetrapods have returned to partially aquatic or (in the case of cetaceans and sirenians) fully aquatic lives, throughout the history of the group. The tetrapods evolved from the lobe-finned fishes about 395 million years ago in the Devonian. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.|
- Subphylum Vertebrata
- 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)
- Superclass Tetrapoda
- ↑ Cooper MD, Alder MN (February 2006). The evolution of adaptive immune systems. Cell 124 (4): 815–22.
- ↑ Mark V. H. Wilson & Michael W. Caldwell. New Silurian and Devonian fork-tailed 'thelodonts' are jawless vertebrates with stomachs and deep bodies. nature. URL accessed on 2007-08-22.
- ↑ Susan Turner and Randall F. Miller (2005). New Ideas About Old Sharks. American Scientist. URL accessed on 2007-08-22. [dead link]
- ↑ Hanke, Gavin F., and Mark VH Wilson. "New teleostome fishes and acanthodian systematics." Recent advances in the origin and early radiation of vertebrates (2004): 189-216. ("... record of acanthodian fishes is limited to microremains from the latest Ordovician (JANVIER 1996)")
- ↑ Sansom, Ivan J., Moya M. Smith, and M. Paul Smith. "Scales of thelodont and shark-like fishes from the Ordovician of Colorado." (1996): 628-630.
- ↑ Fossil reveals oldest live birth. BBC. URL accessed on May 30, 2008.
- ↑ Bony fishes SeaWorld. Retrieved 2 February 2013.
- ↑ Jaws, Teeth of Earliest Bony Fish Discovered
- ↑ Clack 2012
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