The Cretaceous-Paleogene ( K-Pg ) extinction event , formerly known as the Cretaceous-Tertiary ( K-T ) extinction , was a mass extinction of some three-quarters of plant and animal species on Earth-including all non-avian dinosaurs-that occurred over a geologically short period of time 66 million years ( Mya ) ago .
The Cretaceous-Tertiary extinction event , or Cretaceous-Palaeogene extinction event , was about 66 million years ago .