Evolution of the mammalian jaw

    The ancestral synapsid (mammal-like reptile) jaw has four elements, with the articulation formed between the Articular and Quadrate bones, which is therefore referred to as a Quadro-Articular jaw.  In therapsids (the ancestors of mammals), the joint has been simplified to an articulation between the Dentary and Squamosal bones, which is called a Dentary-Squamosal jaw.  The Articular and Quadrate bones have moved into the middle ear as the Malleus and Incus, respectively. The squamosal also contributes to the zygomatic arch, which is the mammalian cheek bone.