premise: The Cretaceous is an event.
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 . It is the last period of the Mesozoic Era . 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 .