Quadrate bone

The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint (the other part is the articular bone at the rear end of the lower jaw).

In snakes, the quadrate bone has become elongated and very mobile, and contributes greatly to their ability to swallow very large prey items.

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 mammalian hearing structures.