- Gnathostomata
Taxobox
name = Gnathostomata
fossil_range = fossilrange|462|0Late Ordovician - Recent
image_width= 200px
image_caption = Gnathostomata are jawed vertebrates
regnum =Animal ia
phylum = Chordata
subphylum = Vertebrata
infraphylum = Gnathostomata
subdivision_ranks = Subgroups
subdivision =Placodermi †Chondrichthyes Acanthodii †Actinopterygii Sarcopterygii Gnathostomata is the group of
vertebrate s withjaw s.The group is traditionally a superclass, including the familiar classes of
fish ,bird s,mammal s,reptile s, andamphibian s, and a sister group of the jawless vertebratesAgnatha . However, recent genetic studies are causing a reassessment of Gnathostomata as a grouping.New fossil founds suggests thelodonts as the closest relatives of the Gnathostomata. [cite web | url=http://www.nature.com/nature/journal/v361/n6411/abs/361442a0.html | title=New Silurian and Devonian fork-tailed 'thelodonts' are jawless vertebrates with stomachs and deep bodies | author=Mark V. H. Wilson & Michael W. Caldwell | publisher=nature | date=1993-02-04 | accessdate=2007-08-22]
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 - thebuccal 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.Placoderm s used sharp bony plates as teeth instead, and newer research indicates the jaws in placoderms evolved independently of the rest of the remaining gnathostomates. [cite web | url=http://www.americanscientist.org/template/AssetDetail/assetid/42395/page/2;jsessionid=baa9... | title=New Ideas About Old Sharks | author=Susan Turner and Randall F. Miller | publisher=American Scientist | DOI=10.1511/2005.3.244 | month=June | year=2005 | accessdate=2007-08-22]Other distinguishing characteristics of living gnathostomates are the
myelin sheathes ofneuron s, and an adaptiveimmune system .The Gnathostomata first appeared in the Ordovician period and became common in the Devonian period.
Taxonomy and phylogeny
Subphylum
Vertebrata ├─(unranked)Gnathostomatomorpha └─Infraphylum Gnathostomata ├─ClassPlacodermi - "extinct" (armored gnathostomes) └MicrophylumEugnathostomata (true jawed vertebrates) ├─ClassChondrichthyes (cartilaginous fish) └─(unranked)Teleostomi (Acanthodii & Osteichthyes) ├─ClassAcanthodii - "extinct" ("spiny sharks") └SuperclassOsteichthyes (bony fish) ├─ClassActinopterygii (ray-finned fish) └─ClassSarcopterygii (lobe-finned fish) └SuperclassTetrapoda ├─ClassAmphibia (amphibia ns) └(unranked)Amniota (amniotic egg) ├─Class Sauropsida (reptiles or sauropsids) │ └─Class Aves (birds) └─ClassSynapsida └─ClassMammal ia (mammals) Note: lines show evolutionary relationships.References
External links
* [http://tolweb.org/tree?group=Gnathostomata&contgroup=Vertebrata Tree of Life discussion of Gnathostomata]
* [http://www.palaeos.com/Vertebrates/Bones/Gill_Arches/Meckelian.html The Gill Arches: Meckel's Cartilage]
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