The insectivore Yanoconodon allini lived under the Cretaceous dinosaurs.
Reading The bones of a 30-gram insectivore, which lived 125 million years ago in today's China, gives paleontologists great enthusiasm. For the first time, the fossil Yanoconodon allini shows how powerful mammalian hearing developed from a typical reptilian jaw, researchers report Zhe-Xi Luo from the Carnegie Museum in Pittsburgh. Mammals can hear better than all other vertebrates. They owe this to the complicated construction of the middle ear: the sound waves from the air are transmitted through the eardrum to the ossicles hammer, anvil and stirrup, which transmit the vibrations to the fluid in the inner ear. For a long time evolutionary scientists had suspected that the three bones in an ancestor of today's mammals still belonged to the jaw. In the embryos of some mammal groups, this transition is still observed.

The tender bones of Yanoconodon now prove for the first time that the theory is correct. Although the shape of the hammer and anvil are similar to those of a modern platypus, they are still connected to the lower jaw by ossified cartilage. "Yanocodon is clearly a transitional stage in the evolution of the mammalian middle ear, " says Luo, "this wonderfully preserved fossil provides the evidence."

It remains puzzling, however, why platypuses have a modern ear, although their lineage earlier diverged from the family tree of the mammals as the family to which Yanoconodon belongs. The researchers have two possible explanations for this phenomenon: Either the fine mammalian hearing developed twice independently, or it was lost again in Yanocodon and his relatives.

The fossil also has other unusual features: The small, insect and worm-eating animal had 26 vertebrae. Nearly all extinct and living mammals, however, have 19 or 20 vertebrae. Even in the lumbar region Yanoconodon still had ribs, which is also extremely rare in modern mammals. The animal probably crawled on the ground, its short legs and primitive feet good for crawling and digging. The fossil comes from the layers of the Yixian Formation, which gives a unique view of the Middle Cretaceous world: In addition to the famous feathered dinosaurs were found in it several other unusual mammals of the Mesozoic. display

Zhe-Xi Luo (Carnegie Museum of Natural History, Pittsburgh, USA): Nature Vol. 446, p. 288 Ute Kehse

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