Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. 2011. J. Meng, et al. Nature 472: 181-185.
Paleontologists have announced the discovery of Liaoconodon hui, a complete fossil mammal from the Mesozoic found in China that includes the long-sought transitional middle ear.
The specimen shows the bones associated with hearing in mammals— the malleus, incus, and ectotympanic— decoupled from the lower jaw, as had been predicted, but were held in place by an ossified cartilage that rested in a groove on the lower jaw.
The new research also suggests that the middle ear evolved at least twice in mammals, for monotremes and for the marsupial-placental group. link