The petrosal, also know as the periotic, is one of five bones that make up the ear system in mammals, the others being the tympanic bone, the malleus, the incus, and the stapes (you might remember the last three from middle-school biology as the hammer, anvil, and stirrup). These basic parts are found in all mammals, but they are modified in whales to allow better underwater hearing. In whales incoming sound vibrates the tympanic bone, which in turn vibrates the malleus, incus, and stapes. The stapes inserts into a hole in the petrosal that leads to the fluid-filled cochlea. The moving stapes causes pressure waves in the fluid inside the cochlea, which vibrates tiny hairs in the cochlea. The moving hair cells make electrical impulses that travel along the auditory nerve to the brain. A long process to detect a sound!
This diagram shows the hearing process in humans, although all the bones aren’t indicated. This article in Pharyngula talks about some of the research on how whale ears have been modified over the course of their evolution.
Whale ear bones are dense, and preserve well as fossils; petrosals and tympanic bones are quite common, although usually damaged. The other three bones are not as commonly found as fossils. Even though they are quite dense, the incus, malleus, and stapes are tiny, usually only a few millimeters long, and so they’re easily overlooked.
Which brings us back to the Carmel Church whale. In preparing the petrosal I noticed this:
The small bone indicated by the arrow is the stapes, still in place against the petrosal. It’s only about 2 mm across. Unfortunately we didn’t find the malleus or the incus.This is the first example of a middle ear bone identified at Carmel Church.