From Jaw to Ear: Transition Fossil Reveals Ear Evolution in Action

The mammal ear is a very precise system for hearing—enabling everything from human appreciation of music to the echolocation of bats. Three tiny bones known as ossicles—the hammer (malleus), anvil (incus) and stirrup (stapes)—work together to propagate sound from the outside world to the tympanic membrane, otherwise known as the eardrum. From there, the sound is transmitted to the brain and informs the listener about pitch, intensity and even location.

But it has been a mystery how this delicate system evolved from the cruder listening organs of our reptilian ancestors. Paleontologists have scoured fossil records in search of signs of how the jawbones of reptiles migrated and became the middle ear of mammals. Now Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh and his colleagues have found one: Yanoconodon allini, an intermediate between modern mammals and their distant ancestors. "It helps to show a transit