October 9, 2009

UNDER STRICT EMBARGO
Until Thursday, October 8, 2009
2 p.m. (14:00) EDT/18:00 GMT

Chinese and American paleontologists discover a new Mesozoic mammal;
Ear structure shows how mammalian ear evolution occurred
while dinosaurs dominated the world

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Image 1: New Mesozoic mammal Maotherium asiaticus. Skeletal and Life restorations by Mark A. Klingler/Carnegie Museum of Natural History

The original fossil (type specimen) is preserved in the siltstone. The fossil belongs to the Henan Provincial Geological Museum (41H-III-0321). Collected by Henan Provincial Geological Museum (Zhengzhou, Henan Province, China), and jointly studied by Chinese and American scientists.

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.

Etymology: “Mao” means “hairs” in Chinese. “Therion” means “beast” in Greek. The first specimen of Maotherium sinensis was found to have fur impression, and was named the “furry beast.” Maotherium belongs to the fossil mammal group of symmetrodonts, characterized by three cusps arranged in a symmetric triangle on molar teeth (“symmetro” – symmetry; “don” tooth; “symmetrodont” – mammal with symmetric-cusped tooth).

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Image 2: Discovery site of Maotherium asiaticus.

The fossil site of Maotherium asiaticus is in Beipiao City of Liaoning Province, China, northeast of the Chinese capital of Beijing. The fossil site was dated at 123 million years of age.

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Image 3.  Comparison of the middle ear between the modern mammal Opossum (Didelphis) and the Cretaceous fossil mammal Maotherium. Illustration: Zhe-Xi Luo/Carnegie Museum of Natural History. For background, see http://en.wikipedia.org/wiki/Ear#Middle_ear

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.

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Image 4: Relationship of Maotherium to other mammals and new evidence on evolution of mammalian ear and jaw from Maotherium. Illustration: Zhe-Xi Luo/Carnegie Museum of Natural History

All modern mammals (platypus, opossum and human) have a middle ear separated from the lower jaw (see example from living opossum). This jaw-ear separation is an important evolutionary innovation. It becomes possible for mammals to have a delicate and highly sensitive ear structure for better hearing, and to have a more robust lower jaw and jaw hinge for better feeding. Also, the jaw and ear are not interfering with each other.  By comparison, the middle ear bones are a part of the lower jaw and form the jaw hinge in pre-mammalian relatives (see the example of Morganucodon).

The ear bones in Maotherium are partly separated from the jaw, and more similar to those in modern mammals than to mammaliaforms, but still retain the pre-mammalian condition in which the jaw and the ear are connected to each other.  Moreover, the connected jaw-ear structure of Maotherium is similar to the ear structure of modern mammals at embryonic and fetal growth stages. This phenomenon is known as “paedomorphosis” (“paedo:” child-like; “morphosis:” similarity) and is caused by timing change in growth. The analysis of the new fossil suggests that the evolutionary pattern of the mammalian ear is directly related to timing changes in growth, as well as in changes in genes for mammalian development.

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Dr. Zhe-Xi Luo

Photo: Mindy McNaugher/Carnegie Museum of Natural History