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 transitional structure in the long process of evolution of mammal ears," Luo says.

The Luo team found the new tiny mammal—just five inches (12.7 centimeters) long—in the Yan Mountains of Hebei Province in China. Similar rocks in other formations date to the Mesozoic era 125 million years ago when dinosaurs roamed Earth and early mammals are thought to have been relegated to scurrying through the undergrowth. Yanoconodon sports three cusps on its molars for feeding on insects and worms as well as a long body compared with its stubby limbs, ideal for scrabbling in the dirt for dinner. "This particular mammal has a very long body but relatively short limbs," Luo says. "By looking at the claw structure, hand bones and foot bones, our general interpretation is that it is a mammal that lived on the ground surface or perhaps was capable of digging."

More importantly, the nearly complete fossil shows a separation between the jawbones and the inner-ear bones, but one that is incomplete. Yanoconodon's stirrup, anvil and hammer bones are still connected to the jaw by another bone—gone from adult modern mammals. In fact, they display the same layout as mammal embryos do today, before the cartilage precursors of the jaw and ear bones separate during gestation. "Reptiles have [a] jaw full of ear bones from mammals and mammals have an ear full of jawbones of reptiles," Luo notes. "Proportion of the ear bones [is] already like those of modern mammals [in this animal] but the reptilian connection to the jaw is retained."

This means Yanoconodon not only picked up the high frequencies associated with modern mammal hearing but also the vibrations transmitted through the ground. "It has not completely lost this ability to sensitively detect ground vibrations through the jaw but has gained some of the modern mammal ability to hear airborne sounds," Luo adds.

The extinct early mammal had some other unusual features, including more vertebrae than any terrestrial mammal alive today. This means that in this feature it closely resembled monotremes (egg-laying mammals like the platypus), whereas other features brought it closer to marsupials and placental mammals. Regardless, it represents a key middle step in evolving the exquisitely sensitive modern mammal ear.