premise: The Cretaceous is a Paleogene extinction event in 2001.
hypothesis: 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 . As originally proposed in 1980 by a team of scientists led by Luis Alvarez , it is now generally thought that the K -- Pg extinction was caused by a massive comet or asteroid impact , estimated to be 10 km wide , 66 million years ago and its catastrophic effects on the global environment , including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis . Dinosaur . Their dominance continued through the Jurassic and Cretaceous periods and ended when the Cretaceous -- Paleogene extinction event led to the extinction of most dinosaur groups 66 million years ago . The Paleogene ( [ pronˈpæliːədʒiːn ] or [ ˈpeɪliːədʒiːn ] also spelled Palaeogene or Palæogene ; informally Lower Tertiary ) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya ) to the beginning of the Neogene Period Mya .