Megazostrodon
These early mammals developed many traits which made them well-suited for an active lifestyle. They developed four types of teeth. There is evidence that the inward-closing movement of the mandible suggests a shearing action to chew food. Their skeletons changed so that their limbs were more flexible (they became less laterally splayed, allowing for faster forward motion) and they developed a shorter ribcage and larger lungs to allow for faster respiration. The structure of their jaw bones changed, the lower jaw becoming a single bone — the dentary. The other bones which once made up the jaw moved to the middle ear to create a hearing system.