Conceptual illustration of the Cretaceous mass extinction event with an Alamosaurus dinosaur (bottom) in the forefront. The K-T extinction, an abbreviation for Cretaceous?揟ertiary extinction, also called the K?揚g extinction or Cretaceous?揚alaeogene extinction was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago.