Gnathostomes

Cross references:  Agnatha    Sharks & Rays   Osteichthyes (Bony Fish) 

Gnathostomata (Wiki) 
    "Gnathostomata ... is the group of vertebrates with jaws. The term derives from Greek γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth". 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.
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The Gnathostomata first appeared in the Ordovician period and became common in the Devonian period.
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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.
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