PALEONTOLOGY: Homoplasy in the Mammalian Ear

The separation of the middle ear bones from the mandible is considered a defining feature of all modern mammals. But did this event happen once in a primitive mammalian ancestor or independently in the monotreme lineage and therian (marsupial and placental) lineage? As Martin and Luo discuss in their Perspective, a new fossil-the dentary bone of an ancient toothed monotreme-suggests that the middle ear bones formed independently in these two mammalian lineages, providing a remarkable example of homoplastic evolution.