It was thought that all mammals, from egg-laying monotremes such as the duck-billed platypus, to placental mammals such as the dog, developed the tiny auditory bones of the middle ear just once in their common evolutionary history. But a fossil jawbone of a mammal that lived in Australia suggests that the middle-ear bones, which allow animals to hear sounds, evolved independently at least twice.
The delicate bones were thought to have evolved from others that were part of the lower jawbone in mammals' reptilian ancestors. During evolution they became detached to form a separate middle ear.
But palaeontologists from the University of Chicago and the Museum Victoria in Melbourne have discovered a lower jawbone of the oldest known monotreme with middle-ear bones still attached to its jaw.
Finding the jawbone of Teinolophos trusleri, a shrew-like animal, with an attached middle ear suggests that the auditory equipment of this early mammal had evolved separately from the rest of the group, said Tom Rich, curator of vertebrate palaeontology at the museum.
James Hopson, professor of biology at the University of Chicago, said: "The ear bones are still attached to the lower jaw, which implies that this shift had to occur in later monotremes and independently of the shift occurring in the common ancestor of marsupials and placentals."Reuse content