Articular

The articular bone is in the lower jaw of most tetrapods, including amphibians, sauropsids ("reptiles"), birds and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular; and it forms the jaw joint by articulating with the quadrate bone of the skull.

But in mammals the articular and quadrate bones have migrated to the middle ear and are known as the malleus and incus. In fact paleontologists regard this modification as the defining characteristic of mammals.