Jaws developed in the common ancestor of all gnathostomes. Jaws afford anchorage for powerful muscles and the teeth. They not only provide the ability to bite, but assist in food capture. Joints and ligaments may connect the jaws with the braincase (as in a shark, for example), or the upper jaw may be fused to the braincase (as in the human skull).

Dunkleosteus
Xiphactinus