The discovery of a complete fossil Liaoconodon hui with the three ossicles of the mammalian ear attached to the lower jaw by cartilage near the groove in the lower jaw that had long been something of a puzzle, represents a clear intermediary form between early and modern mammals. This find in Mesozoic rock in China was predicted by evolutionary theory. It helps to demonstrate a more gradual transition from reptilian jaw to mammalian middle ear.Liaoconodon lived about 125 Mya and would have been about 14" (36 cm) long. The ossicles were only used for hearing. They were still closer to the lower mandible than are the bones of the middle ear in modern mammals.
An early mammal, Morganucodon, that lived 200 million years ago (Mya) had bones that were more reptilian. Morganucodon was a small shrew-like animal, only about four inches long. The bones that became the ossicles in later mammals were smaller in Morganucodon than in reptiles. They still functioned as part of the jaw even though they also were used for hearing.
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