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 A new Mesozoic mammal with mid-ear from Western Liaoning, China Minimize

A new Mesozoic mammal with mid-ear from Western Liaoning, China

- The ear structure of this animal shows how mammalian ear evolution occurred
 in the deep time of Mesozoic when dinosaurs dominated the world
Chinese and American paleontologists have discovered a new species of mammal that lived 123 million years ago, which was unearthed from the Yixian Formation in Beipiao, Liaoning. This new fossil, reported in the 9 October issue of the prestigious journal SCIENCE, offers an important clue of how the mammalian middle ear evolved.
Fig.1. Holotype of Maotherium asiaticus nov.species.
The hearing is an important sensory function of human, whether for hear an interesting conversation, or beautiful music.For mammals as a whole, hearing is important evolutionary adaptation that is vital for survival.With a tiny and intricate structure of their middle ear, mammals have a more sensitive hearing of wider range of sounds than other vertebrates. The more sensitive hearing was crucial for mammals to develop nocturnal adaptation and to survive in the dinosaur-dominated Mesozoic.
Named as Maotherium asiaticus nov.species, the newly discovered fossil is very well preserved fossil.It is from famous fossil-rich beds of the Yixian Formation.This mammal has a kind of teeth with symmetrically arranged cusps on teeth (therefore known as a symmetrodont) specialized for feeding on insects and worms.It was a ground-living animal with a body of 15 cm (5 inches) long and weighed approximately 70 to 80 grams. By studying all features in this exquisitely preserved fossil, researchers believe Maotherium to be more closely related to marsupials and placentals than to monotremes.
Fig.2. Reconstruction of the new Cretaceous mammal Maotherium asiaticus nov. species.
But what is most surprising, therefore scientifically interesting, is this animal’s ear. Mammals have highly sensitive hearing, far better than the hearing capacity of all other vertebrates, and hearing is fundamental to mammalian way of life. The mammalian ear evolution is important for understanding the origins of key mammalian adaptations.It is also a prominent case study to document a new and complex structure can transform through evolution.  
According to Prof. JI Qiang and others who studied this new mammal, the middle ear bones of Maotherium is partly similar to those of modern mammals. But Maotherium’s middle ear has an unusual connection to the lower jaw that is unlike the adult modern mammals.This connection, also known as the ossified Meckel’s cartilage, resembles the embryonic condition of living mammals and the primitive middle ear of pre-mammalian ancestors. This new fossil offers a rare insight in the evolutionary origin of the mammalian 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).
These 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.
Maotherium asiaticus nov. species is a beautifully preserved fossil in three-dimension. This made it possible for paleontologists to reconstruct how the middle ear would be attached to the jaw. The new mammal represents either an intermediate condition in the evolutionary process of how modern mammals acquired their middle ear structure, or it can be interpreted as having a secondarily reversal to the ancestral condition.
Modern developmental biology has shown that developmental genes and their gene network can trigger the development of unusual middle ear structures, such as “re-appearance” of the Meckel’s cartilage in modern mice. The middle ear morphology in fossil mammal Maotherium of the Cretaceous is very similar to the mutant morphology in the middle ear of the mice with mutant genes. The scientific team studying the fossil suggests that the unusual middle ear structure, such as the ossified Meckel’s cartilage, are actually the manifestation of developmental gene mutations in the deep times of Mesozoic mammal evolution.
The discoveries of exquisite Mesozoic mammals from China have finally built the evidence up to a point where developmental biologists and paleontologists are able to make sense of how developmental mechanism has impacted the morphological evolution of the earliest mammals.
The discovery of the new fossil of Maotherium asiaticus nov. species and its scientific study are made by Drs. Qiang Ji and Chongxi Yuan of Chinese Academy of Geological Sciences (Beijing), Zhexi Luo (Carnegie Museum), Xingliao Zhang and Li Xu(Henan Provincial Geological Museum).
The researchers received support from Ministry of Science and Technology (China, 973), China Geological Survey (China), National Science Foundation (USA), and National Geographic Society (USA).
Fig.3. Evolution of the mammal’s mid-ear.
Fig4. Researcher Ji Qiang working in the field


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