premise: The end of the Cretaceous is a mass extinction.
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 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 .