However, it would take millions of years before the stapes would take on its role as a part of the terrestrially adapted middle ear. During the Devonian, the hyomandibula shrank in size and got lodged in a hole in the braincase, where it became the stapes. The stapes evolved from a fish a bone known as the hyomandibula responsible for orchestrating feeding and breathing movements in fish. Even when they were a functional jaw articulation, the articular and quadrate probably passed sound vibrations received by the lower jaw to the stapes. Therapsid fossils show an obvious trend through time for an increase in size of the dentary, including formation of a coronoid process, and reduction of the postdentary elements of the lower jaw.
Hammer anvil and stirrup series#
Indeed, in therapsids and other vertebrates with jaws, the lower jaw is made up of a number of bones (dentary and a series of additional bones) rather than just of one jawbone like in modern mammals. The malleus is formed from the embryonic prearticular and articular, and the incus is derived from the quadrate bone. In their embryonic stage, mammals have their ear bones still attached to the lower jaw implying that the middle ear bones evolved from bones of the lower jaw. The malleus, incus, and stapes (hammer, anvil, and stirrup) are the middle ear bones responsible for magnifying and conducting sound received from the outer to the inner ear in mammals. The Beginning of the Mammals The Characteristics of the Mammals
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