In true mammals, one jaw joint is formed by the squared bone of the skull and the dentary bone of the lower jaw.
Behind the dentary a small bone, the articular, forms a joint with the quadrate bone near the rear of the skull.
In many cases, however, the anterior teeth of the premaxilla and the dentary are incisiform and thus must have been used for grasping (as such teeth are in the living porgies).
The lower jaw was dominated by the dentary bone; the other lower-jaw elements, characteristic of reptiles, were relatively reduced, as in mammals and their near relatives.
To simplify definitions and to allow the strict delimitation of the Mammalia, some authors have suggested basing the boundary on a single characteristic, the articulation of the jaw between the dentary and squamosal bones and the attendant movement of accessory jawbones to the middle ear as auditory ossicles.
Reptiles have a number of bones in the lower jaw, only one of which, the dentary, bears teeth.
In most lizards, teeth are present along the jaw margin (on the maxilla, premaxilla, and dentary bones).
In jaw construction and articulation tritylodonts were not mammalian; the lower jaw retained components from earlier amniotes rather than the single bone, the dentary, that is characteristic of the mammals.
In contrast, the lower jaw of a mammal is made up of a single bone, the dentary; the articular and quadrate have become part of the chain of little bones in the middle ear.
A single bone, the dentary, dwarfs the other lower jawbones, a trend toward the mammalian condition of only one bone, the dentary.
There were prominent upper and lower tusklike teeth at the front of the mouth (the upper set in the premaxillary bones, the lower on the dentary bones).
