Making a mammalian ear Mammals have keen hearing owing to their complex inner ear. In our vertebrate ancestors, as in extant reptiles, the three bones that make up the inner ear were instead part of the jaw. Understanding the functional transition of these bones is challenging given their small and delicate nature. Mao et al. describe a new genus and species of stem therian mammal represented by six well-preserved specimens, seemingly caught as they slept huddled together (see the Perspective by Schultz). The unprecedented preservation reveals a clear transitional stage between the two very different functions of the bones. Science , this issue p. 305 ; see also p. 244