Yanoconodon's stirrup, anvil and hammer bones are still connected to the jaw by another bone-gone from adult modern mammals. More importantly, the nearly complete fossil shows a separation between the jawbones and the inner-ear bones, but one that is incomplete. "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." "This particular mammal has a very long body but relatively short limbs," Luo says. 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. 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. The Luo team found the new tiny mammal-just five inches (12.7 centimeters) long-in the Yan Mountains of Hebei Province in China. "It helps to show a transitional structure in the long process of evolution of mammal ears," Luo says. 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. Paleontologists have scoured fossil records in search of signs of how the jawbones of reptiles migrated and became the middle ear of mammals. From there, the sound is transmitted to the brain and informs the listener about pitch, intensity and even location.īut it has been a mystery how this delicate system evolved from the cruder listening organs of our reptilian ancestors. 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. The mammal ear is a very precise system for hearing-enabling everything from human appreciation of music to the echolocation of bats.
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