The lesson is dual function . If you are needing a sound amplifier, you can use the stapes to vibrate against the ear capsule, while it is also serving as a jaw support. The stapes is a hearing bone, an auditory ossicle, present in all derived tetrapods. The three bones make up a chain, located within the middle ear. The stapes retains its old position of butting against a window in the inner ear capsule, but now the malleus connects to a new membrane, the tympanium or ear drum, and the incus is in between the other two.
Modern mammals develop the ear bones from cartilage rudiments in a von Baerian style.
When the elements first develop they form a jaw-like assemblage, and then they move into the middle ear during later development. Using dissection of a developmental series of pig embryos he was able to follow Meckel's cartilage through the transition to the Malleus. This was simply amazing science for its day. Mutation of Hoxa3 in the mouse converts the stapes into the incus. The brain can control the vibrations of the auditory ossicles by contracting muscles attached to these bones, in order to dampen their vibrations.