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  • The Cretaceous–Paleogene extinction event, formerly known as the Cretaceous–Tertiary extinction, was a mass extinction of some three-quarters of plant and animal species on Earth—including all non-avian dinosaurs—that occurred over a geologically short period of time 66 million years ago. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era which continues today. In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth's crust but abundant in asteroids. As originally proposed by a team of scientists led by Luis Alvarez, it is now generally believed that the K–Pg extinction was triggered by a massive comet/asteroid impact 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.

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