Fossil specimen of the Thrinaxodon skull and jaw used for the study. Credit: Matt Wood One of the defining breakthroughs in mammal evolution was the rise of highly sensitive hearing. Modern mammals rely on a middle ear that includes an eardrum and several tiny bones, a system that makes it possible to detect a wide range of sounds at different volumes. This ability likely gave early mammals, many of which were active at night, a crucial edge as they navigated environments dominated by dinosaurs.