In reptiles, birds, and early fossil tetrapods, there is only a single auditory ossicle, the stapes.

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).

In modern amniotes (including mammals), the middle ear collects airborne sounds through an ear drum and transmits the vibrations to the inner ear via thin cartilaginous and ossified structures, which usually include the stapes (a stirrup-shaped auditory ossicle).