251-201 million years ago


In addition to the ancestral reptilian jaw joint, cynodonts evolved a second jaw joint--the one shared by all modern mammals. Over time the reptilian jaw joint became less prominent and the mammalian joint became moreso.

During the evolution of the cynodonts, the ancestral jaw joint between the quadrate and articular bones was supplemented by, and eventually replaced by, the jaw joint used by modern mammals between the dentary and squamosal (of the temporal bone). Over millions of years of evolution in this group which possessed two functioning jaw joints, the dentary/squamosal joint (the mammalian jaw joint) became more and more prominent while the quadrate/articular jaw joint (the reptilian jaw joint) became further reduced. By the evolution of the intermediate cynodonts, the dentary had become the predominant bone of the lower jaw and the postdentary bones were reduced. The quadrate was also reduced in size. The dentary was larger with a high coronoid process and a deep fossa for masseter muscle. The angular was reflected and supported the tympanum.

In the most advanced cynodonts, the quadrate and articular had become tiny bones, which the first mammals would incorporate into the middle ear where they function in hearing.