October 9, 2009

Mesozoic Mammal Could Be Key To Evolution Of Hearing

The fossil of a previously unknown chipmunk-sized mammal was discovered by researchers in north eastern China, who believe it could lead to a better understanding of how human hearing evolved.

The team of paleontologists found the 123-million-year-old creature, which is just five inches long, in fossil-rich Liaoning Province, close to where China borders North Korea.

"What is most surprising, and thus scientifically interesting, is the animal's inner ear," said Zhe-Xi Luo, a curator at the Carnegie Museum of Natural History in Pittsburgh and one of the study's authors.

The report is published in the October 9 issue of the prestigious journal Science.

The little animal was discovered in "remarkably well preserved" condition, and the three dimensional fossil has given an international team of researchers the ability to reconstruct how the middle ear was connected to the jaw of the creature.

The find could give way to an explanation for how the three bones of the mammalian middle ear ultimately detached from the jaw hinge, where the reptilian ear is found, to form a complex and highly-performing auditory system.

"Mammals have highly sensitive hearing, far better than the hearing capacity of all other vertebrates, and hearing is fundamental to the mammalian way of life," said Luo.

Because of the intricacy of the middle ear structure, mammals have more sensitive hearing, and are able to distinguish a broader range of sounds than other vertebrates. 

The ear's development is considered to be crucial to understanding survival techniques that allowed mammals, including human ancestors, to navigate through the dinosaur-infested Mesozoic period around 250 to 66 million years ago.

"The mammalian ear evolution is important for understanding the origins of key mammalian adaptations," he said.

It is still unclear as to where the Maotherim Asiatic us creature belongs in the evolutionary chain, and the new ear connection may only be a simple adaptation caused by changes in development, rather than an evolutionary link.


Image 2: The new Cretaceous mammal Maotherium is a chipmunk-sized nocturnal mammal. It lived 123 million years ago. It had terrestrial habits and scampered around on the ground. From its skeleton it is estimated to have weighed about 70-80 grams (2 ounces), and was about 15 cm (5 inches) in length. Maotherium is a generalized ground-living mammal. Because it is related to the common ancestor of marsupials and placentals, its tooth and skeletal structures show the ancestral condition from which marsupials and placentals could have evolved. Credit: Mark A. Klingler/Carnegie Museum of Natural History

Image 3: Maotherium is related to such modern mammals as Didelphis. But unlike marsupials and placentals, in which the middle ear is separated from the mandible, in Maotherium the middle ear is still attached to the mandible. Recent developmental biology studies have shown that the connection of middle ear to the mandible can "re-appear" as the genes control their development can change, and such genetic and developmental changes can impact evolution. Maotherium provides a strong case of how development has impacted fossil evolution in the deep history of Earth. Credit: Zhe-Xi Luo/Carnegie Museum of Natural History


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