The Cretaceous–Paleogene (K–Pg) extinction event was a major mass extinction that occurred at the boundary between the Cretaceous and Paleogene periods, marking the disappearance of numerous species of flora and fauna. This geological turnover is theorized to have resulted from a combination of intense environmental stress, including massive volcanism and the impact of a large celestial body. The ensuing global disturbances, such as atmospheric changes, darkness, and ecological collapse, dramatically altered global ecosystems. The immediate fallout led to a rapid restructuring of life on Earth, notably allowing the diversification and dominance of mammalian groups which previously had been constrained by other megafaunal species, fundamentally shifting the course of biological evolution.