Chapter nine looks at the mammal-like forms developing in the Jurassic period. Many were small, which initially caused many people to overlook them, concentrating on the much larger dinosaurs in that period. But closer examination shows that the mammal-like forms were developing new ways of living, like burrowing or gliding. Their small size may also have helped them to develop more sensitive hearing by helping to free up jaw bones to be used as inner ear bones.