The Cretaceous–Tertiary extinction event, which occurred approximately 65.5 million years ago, was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K–T boundary, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period (a historical term for the period of time now covered by the Paleogene and Neogene periods). 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 event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers.

	Badlands near Drumheller, Alberta, where erosion has exposed the K–T boundary
	An artist's impression of the KT Extinction Asteroid

Scientists theorize that the K–T extinctions were caused by one or more catastrophic events, such as massive asteroid impacts (like the Chicxulub impact), 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 geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate.

There is no evidence of K–T boundary mass extinctions of amphibians, and there is strong evidence that most amphibians survived the event relatively unscathed. Several in-depth studies of salamander genera in fossil beds in Montana show that six of seven genera were unchanged after the event.

The archosaur clade includes two living orders, crocodilians (of which Alligatoridae, Crocodylidae and Gavialidae are the only surviving families) and dinosaurs (of which birds are the sole surviving members), along with the extinct non-avian dinosaurs and pterosaurs.