premise: The Cretaceous is a Paleogene extinction activity.
hypothesis: The Cretaceous ( [ pronkrᵻˈteɪʃəs ] , ) is a geologic period and system that spans 79 million years from the end of the Jurassic Period million years ago ( Mya ) to the beginning of the Paleogene Period Mya . The Cretaceous ended with a large mass extinction , the Cretaceous -- Paleogene extinction event , in which many groups , including non-avian dinosaurs , pterosaurs and large marine reptiles died out . The Cretaceous -- Paleogene ( K -- Pg ) extinction event , also known as the Cretaceous -- Tertiary ( K -- T ) extinction , was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time approximately 65 million years ago . The Paleogene ( [ pronˈpæliːədʒiːn ] or [ ˈpeɪliːədʒiːn ] also spelled Palaeogene or Palæogene ; informally Lower Tertiary ) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya ) to the beginning of the Neogene Period Mya . The Paleogene is most notable for being the time during which mammals diversified from relatively small , simple forms into a large group of diverse animals in the wake of the Cretaceous -- Paleogene extinction event that ended the preceding Cretaceous Period .