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