Comparative Anatomy
The middle ear of tetrapods is homologous with the spiracle of fishes, an opening from the pharynx to the side of the head in front of the main gill slits. In fish embryos, the spiracle forms as a pouch in the pharynx, which grows outward and breaches the skin to form an opening; in most tetrapods, this breach is never quite completed, and the final vestige of tissue separating it from the outside world becomes the eardrum. The inner part of the spiracle, still connected to the pharynx, forms the eustachian tube.
In reptiles, birds, and early fossil tetrapods, there is only a single auditory ossicle, the stapes. This runs directly from the eardrum to the fenestra ovalis.
The structure of the middle ear in living amphibians varies considerably, and is often degenerate. In most frogs and toads, it is similar to that of reptiles, but in other amphibians, the middle ear cavity is often absent. In these cases, the stapes either is also missing or, in the absence of an eardrum, connects to the quadrate bone in the skull, although, it is presumed, it still has some ability to transmit vibrations to the inner ear. In many amphibians, there is also a second auditory ossicle, the operculum (not to be confused with the structure of the same name in fishes). This is a flat, plate-like bone, overlying the fenestra ovalis, and connecting it either to the stapes or, via a special muscle, to the scapula. It is not found in any other vertebrates.
Mammals are unique in having three ear bones, which allow for finer detection of sound. The malleus has evolved from the articular bone of the lower jaw, and the incus from the quadrate. In other vertebrates, these bones form the joint of the jaw, but the expansion of the dentary bone in mammals has allowed those animals to develop an entirely new jaw joint, freeing up the old joint to become part of the ear. In many mammals, the middle ear also becomes protected by a bony sheath, the auditory bulla, not found in other vertebrates. This is often a separate structure, but, in humans, it is part of the temporal bone.
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