Cretaceous–Paleogene Extinction Event

The Cretaceous–Paleogene extinction event, often referred to as the Cretaceous–Tertiary extinction event, occurred approximately 65.5 million years ago (Ma) at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant species, most notably dinosaurs, in a geologically short period of time. Also known as the K–T extinction event, it is associated with a geological signature known variously as the K–T boundary, the Cretaceous–Paleogene boundary or the K–Pg boundary, usually a thin band of sedimentation visible wherever rocks of this age are exposed. In the abbreviated versions of the Cretaceous–Paleogene boundary, the K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, the Pg is the abbreviation for the Paleogene Period, and T is the abbreviation for the Tertiary Period (comprising the Paleogene and Neogene). The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. With "Tertiary" being discouraged as a formal time or rock unit by the International Commission on Stratigraphy, the K–T extinction event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers.

Numerous groups of organisms became extinct during the K–Pg extinction event, most notably the non-avian dinosaurs. Non-avian dinosaur fossils are found only below the K–T boundary, indicating that they became extinct during the boundary event. A very small number of dinosaur fossils have been found above the K–T boundary, but they have been explained as reworked fossils, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian clades passed through the boundary with few extinctions, evolving and thriving well past the event. Rates of extinction and radiation varied across different clades of organisms.

Scientists hypothesize that the K–Pg extinctions were caused by one or more catastrophic events, including at least one asteroid impact (especially the one that created the Chicxulub crater) or increased volcanic activity. Several impact craters and massive volcanic activity, such as that in the Deccan Traps, have been dated to the approximate time of the extinction event. These events would have released massive amounts of dust and ash into the atmosphere, reducing surface sunlight, hindering photosynthesis, and severely disrupting Earth's biosphere. Many researchers believe the extinction was more gradual, resulting from the sea level and climate changes already occurring during the late Cretaceous, and aggravated by impact events or increased volcanic activity.