Let's hear it for the new Mesozoic mammal!

Yanoconodon allini: Artist's representation of Yanocondon, a Mesozoic mammal fossil found in China.  Courtesy Nicolle Rager Fuller, National Science Foundation.
Yanoconodon allini: Artist's representation of Yanocondon, a Mesozoic mammal fossil found in China. Courtesy Nicolle Rager Fuller, National Science Foundation.
A newly discovered fossil dating back to the time of the dinosaurs presents fresh evidence of the evolution of the mammalian ear.

The small squirrel-like mammal, named Yanoconodon allini, measured about five inches in length, with short limbs and claws, and, in life, would have weighed less than an ounce.

The remains, found in the Yan Mountains of the Hebei Province in China by an international team of American and Chinese scientists, present a mammal unlike any known before. The new creature, which belongs to the group of Mesozoic mammals known as triconodonts, contains an unusually large number (26) of thoracic and lumbar vertebrae for such a small animal. Similar creatures have only 19 or 20 vertebrae. It’s the first Mesozoic mammal found in the fossil-rich deposits located about 180 miles from Beijing.

But the new mammal’s ear structure is what interests scientists most.

Mammals far surpass other vertebrates in their ability to hear. Small bones in the middle the mammalian ear structure, (the hammer, the anvil, the stirrup, and the bony ring for the eardrum), are responsible for this aural sophistication. And while scientists have long recognized that these bony structures evolved from detached segments of the jaw hinge of reptilian ancestors, the new fossil’s well-preserved ear structure provides an intermediate phase in that evolution.

"Now we have a definitive piece of evidence, in a beautifully preserved fossil split on two rock slabs," said Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburg, Pennsylvania. "Yanoconodon clearly shows an intermediate condition in the evolutionary process of how modern mammals acquired their middle ear structure."

Luo and his Chinese collaborators published their discovery in a recent issue of the journal Nature.

"This new fossil offers a rare insight in the evolutionary origin of the mammalian ear structure," added Luo. "Evolution of the ear is important for understanding the origins of key mammalian adaptations."

An acute sense of hearing would have certainly come in handy to the pip-squeak Yanoconodon scurrying among the gigantic dinosaurs that ruled its Mesozoic world 125 million years ago.

"This early mammalian ear from China is a Rosetta-stone type of discovery which reinforces the idea that development of complex body parts can be explained by evolution, using exquisitely preserved fossils," said H. Richard Lane, program director in National Science Foundation’s Division of Earth Sciences, which helped fund the discovery.

National Science Foundation

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