Scientists have found a 115 million old fossil of a jawbone (12 to 15 mm long) of a long extinct mammal (Teinolophos Trusleri; a monotreme like platypus). The fossil was found in a rock on the west coast of Victoria (Australia). The specimen suggests that monotremes split away from other mammals (placental/marsupial mammals) much earlier than previously thought (200 million years, instead of 140m).
It also looks like mammals developed the mechanisms of the inner ear (at least) twice!. Modern mammals have three bones (Malleus, Incus, Stapes) in their middle ear. These bones work together to transmit sound towards the skull. Birds and reptiles have only one bone (Image Courtesy: Nature). Up till now, scientists thought that the inner ear mechanism in mammals must have evolved in an ancestor common to both monotremes and other mammals. This ancient fossil is of a time later than when monotremes split from other mammals (115m years old, split at 200m years). However, its jawbone (and inner ear) has no signs of three inner-ear bones. Since modern monotremes have three bones, this structure must evolved independently of the structure in other mammals.
Such development is known as conversion evolution, when different animals evolve the same features to meet similar challenges. Scientists now have to compare the structures in monotremes and other mammals to see how close the two evolutionary processes were. Should be exciting!