Mammalian Explosion 299the jaw joint into the middle ear. For example, snakes cannot hear when they rear up to face
the snake charmer because their jaw is not in contact with the ground and so cannot pick up
vibrations. They are responding to the body motions of the snake charmer, but the sound
of the flute is just for the tourists—the snake cannot hear a thing. Sure enough, once the
quadrate and articular became tiny and disconnected from the jaw joint function, they did
not vanish—they are in your middle ear right now (fig. 13.7)! The quadrate bone turned into
the incus or “anvil” bone, which transmits sound to the stirrup or stapes. The articular bone
turned into the malleus or “hammer” bone, which transmits sound from the eardrum to the
incus. So as you listen to sounds, they are being transmitted through bones that started out
as part of the jaw and skull articulation. If that story seems too incredible, just think of this:
when you were an embryo, your ear bones were represented by cartilage in the lower jaw
and skull, and through embryonic development they shift until they reached the middle ear,
retracing the path they took during evolution!
The most amazing clinching fossil, however, is Yanoconodon, described by my friend Luo
Zhexi and his colleagues (Luo et al. 2007) from the Lower Cretaceous Yixian Formation of
Hebei Province. It is equivalent in age to the beds in the Liaoning Province that produced
all the birds described in the last chapter. It is a beautiful complete specimen (fig. 13.8) with
all the bones articulated in a death pose as it died and was preserved in the delicate lake
FIGURE 13.7. The ear region also undergoes a dramatic transformation, as the articular bone of the lower jaw
hinge and the quadrate bone of the jaw hinge in the skull shift to the middle ear and become the incus and the
malleus (“anvil” and “hammer”). This same transformation can be seen not only in fossils but also during the
embryology of a mammal. When you were an embryo, your middle ear bones started out in your jaw.
Reptile StirrupEardrum
QuadrateArticularAngularStirrupEardrumAnvil
HammerEctotympanicMammal