Development of true jaws

Gnathostomes are a group of organisms which have true jaw structures, rather than a simple mouth. It is believed that these jaw structures developed from modified gill arches in fish. The first arch, the premandibular arch, is thought to have moved upward, forming the beginnings of the braincase as a plate underneath the brain itself. The second gill arch, the Mandibular arch, bends forward into two seperate sections. The top, which would eventually form the upper jaw, is called palatoquadrate cartilage. The bottom, forming the lower jaw, is mandibular cartilage. Eventually these cartilaginous structures would ossify in some organisms, giving them boney jaws. The third gill arch, the hyoid arch, forms a structure which supports the jaws themselves, providing a foundation to rest or directly connecting to the jaws. This also serves the purpose of anchoring the jaw to the braincase. Of course this anchoring method differs, or is entirely absent depending on the form of jaw suspension that develops in a particular organism.