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Fossil shows how mammals developed sensitive hearing

| Friday, Oct. 9, 2009

Though it weighed 2 ounces and was 5 inches long, the chipmunk-like mammal that lived 123 million years ago had something its dinosaur predators didn't: middle ear bones partially independent of its jaw bone.

That evolutionary development helped Maotherium asiaticus have more sensitive hearing.

"This made it possible for mammals to be active in the night," said Dr. Zhe-Xi Luo, the Carnegie Museum of Natural History associate director of science and curator of vertebrate paleontology, in an e-mail from Germany.

"In the Mesozoic (Era), when dinosaurs dominated the world, this is a key adaptation for the survival of mammals," he said.

Luo and a team of paleontologists discovered the existence of the previously unknown mammal when it recovered a three-dimensionally preserved fossil in Liaoning Province in northeastern China among the famous beds of the Yixian Formation.

Finding a fossil preserved in three dimensions and not flattened "like prehistoric roadkill" is unusual, Luo said.

Luo and team members report the discovery in today's issue of the journal Science. The article's primary authors are Luo, Qiang Ji of the Chinese Academy of Geological Sciences in Beijing and Xinliang Zhang of the Henan Provincial Geological Museum.

The find is significant because it provides clues to how mammals' ears evolved from reptiles' ears, Luo said.

Professor Guillermo Rougier, a paleontologist at the University of Louisville not involved in the research, explained why the work by Luo and his team is critical to science.

"For mammalian evolution, the ear is crucial, really," Rougier said. "It's one of the key features that distinguish mammals from other animals today."

Other features include hair and complex teeth, Rougier said.

The evolutionary detachment and movement of ear bones enabled mammals to chew food without the noise of that chewing overwhelming all other sound.

"This is good because we (mammals) chew our food more intensively than other animals," Rougier said. "Old mammals had a loud and noisy system."

Because many mammalian middle ear bones are tiny — human ear bones are smaller than a pea — finding fossilized specimens is difficult, Rougier said, "so this is an extremely exciting find."

Mammals are more sensitive than nonmammalian vertebrates in the range of audible sounds, Luo said. Mammals are especially better at hearing higher-frequency sounds. This is imperative for mammal feeding on insects and to have great sensory perception in the darkness.

"The champions of mammalian hearing are whales and bats," Luo said. "Humans have very poor hearing by comparison to other mammals. Of course, if you've listened to too much loud rock 'n' roll, you won't be very sensitive by comparison to other healthy humans."

A peasant in Liaoning Province came upon the fossil, Luo said. Officials from Henan Provincial Geological Museum acquired it.

By 2007, when Luo was examining the piece, "we first started to realize that this mammal has preserved an interesting feature in its middle ear," he said. "But it took several years for us to sort through its scientific ramifications."

Dinosaurs common to the time of Maotherium asiaticus include the feathered Microraptor and Beipiaosaurus.

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