premise: The Cretaceous ended with a mass disruption.
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 .