premise: The Cretaceous ended.
hypothesis: Cretaceous . 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 end of the Cretaceous is defined by the abrupt Cretaceous -- Paleogene boundary ( K -- Pg boundary ) , a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras . 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 .