The mandible is the only movable bone in the skull it is made up of two parts, a left side and a right side that are held together at the front with soft tissue. In some animals the two sections of bone fuse together at the front as the animal matures
Picture courtesy of Kris Byrant
Lion (Panthera leo)
The mandible attaches to the skull by a hindged joint, this allows the animal to open and close its mouth. Some animals, like the sheep, can move their jaws side to side. This movement is necessary to grind down grass.