PITTSBURGH, PA.- International teams of paleontologists have discovered a new species of mammal that lived 125 million years ago during the Mesozoic Era in what is now the Hebei Province in China. This new mammal, documented in the March 15 issue of the prestigious British journal Nature, provides first-hand evidence of early evolution of mammalian middle ear – one of the most important features for all modern mammals.

Named Yanoconodon after the Yan Mountains in Hebei, this remarkable fossil was unearthed in the fossil-rich beds of the Yixian Formation and is the first Mesozoic mammal recovered from Hebei. The fossil site is about 300km outside of Beijing.

The researchers discovered that the skull of Yanoconodon revealed a middle ear structure that is somewhere between those of modern mammals and those of near relatives of mammals, also known as mammaliaforms.

“This new fossil offers a rare insight in the evolutionary origin of the mammalian ear structure,” said Dr. Luo “Evolution of the ear is not only important for understanding the origins of key mammalian adaptations, but also a prominent case study to document a new and complex structure can transform through evolution, but not by intelligent design."

Mammals have highly sensitive hearing, far better than the hearing capacity of all other vertebrates, and hearing is fundamental to mammalian way of life. Consequently, paleontologists and evolutionary biologists have been searching for more than a century for clues on the evolutionary origins of mammal ear structure.

Mammalian hearing adaptation is made possible by a sophisticated middle ear of three tiny bones, known as the hammer (malleus), the anvil (incus) and the stirrup (stapes), plus a bony ring for the eardrum (tympanic membrane) (background: http://en.wikipedia.org/wiki/Ear#Middle_ear).

The mammal middle ear bones evolved from the bones of the jaw hinge in their reptilian relatives. However, paleontologists have long attempted to understand the evolutionary pathway via which these precursor jaw bones became separated from the jaw and moved into the middle ear of modern mammals.

“Now we have a definitive piece of evidence, in a beautifully preserved fossil split on two rock slabs. Yanoconodon clearly shows an intermediate condition in the evolutionary process of how modern mammals acquired their middle ear structure,” said Dr. Luo. (See graphics for the intermediate stage of mammalian ear evolution, as represented by Yanoconodon).

Yanoconodon is about 5 inches (or 15 cm) long (Photo Images) and estimated to weigh about 30 grams. Its teeth are notable for the three cusps in a straight line on molars (thus known as a triconodont) for feeding on insects and worms. It has a long body, short and sprawling limbs and claws that were ideal for either digging or living on the ground.

In addition to its unique ear structure, Yanoconodon also has a surprisingly high number of 26 thoracic (“chest”) and lumbar (“waist”) vertebrae, unlike most living and extinct terrestrial mammals that commonly have 19 or 20 thoracic and lumbar vertebrae. The extra vertebrae give Yanoconodon a more elongated body form, in contrast to its relatively shorter and very primitive limb and foot structures. The new mammal also has lumbar ribs, a rare feature among modern mammals.

By studying all features in this exquisitely preserved fossil, researchers believe Yanoconodon to be more closely related to marsupials and placentals than to monotremes. However, the vertebral features are more similar to monotremes than to marsupials and placentals.

“It is unusual that Yanoconodon has turned out to have these lumbar ribs, in sharp contrast to the absence of these ribs in its close relatives. But this is not inexplicable, especially in light of recent advances in developmental biology,” said Dr. Luo.

Modern developmental biology has shown that developmental genes (Homeobox genes) can trigger the development of unusual vertebral structures, such as “reappearance” of lumbar ribs and shifting the identities of vertebral segments. The scientific team studying the fossil suggests that the unusual vertebral patterns in Yanoconodon, such as shifting vertebral identities and redevelopment of lumbar ribs, are actually the manifestation of developmental gene mutations in the deep times of Mesozoic mammal evolution.

“The discoveries of exquisitely preserved Mesozoic mammals from China have finally built the evidence up to a point where biologists and paleontologists are able to make sense of how developmental mechanism has impacted the morphological evolution of the earliest mammals,” said Dr. Luo.

The discovery of the new fossil of Yanoconodon and its scientific study is published in the 15 March 2007 issue of Nature. The article is authored by Dr. Luo and his collaborators, Drs. Peiji Chen and Gang Li of Nanjing Institute of Geology and Palaeontology, China, and graduate student Meng Chen of Nanjing University.

The researchers received support from National Science Foundation (USA), National Natural Science Foundation (China), Ministry of Science and Technology (China), and National Geographic Society.