Cretaceous–Paleogene extinction event

The Cretaceous–Paleogene (K–Pg) extinction event,^[a] formerly known as the Cretaceous-Tertiary (K–T) extinction event,^[b] was the mass extinction of three-quarters of the plant and animal species on Earth^[2][3] approximately 66 million years ago. The event caused the extinction of all non-avian dinosaurs. Most other tetrapods weighing more than 25 kg (55 lb) also became extinct, with the exception of some ectothermic species such as sea turtles and crocodilians.^[4] It marked the end of the Cretaceous period, and with it the Mesozoic era, while heralding the beginning of the current geological era, the Cenozoic Era. In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary or K–T boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows unusually high levels of the metal iridium,^[5][6][7] which is more common in asteroids than in the Earth's crust.^[8]

As originally proposed in 1980^[9] by a team of scientists led by Luis Alvarez and his son Walter, it is now generally thought that the K–Pg extinction was caused by the impact of a massive asteroid 10 to 15 km (6 to 9 mi) wide,^[10][11] 66 million years ago causing the Chicxulub impact crater, which devastated the global environment, mainly through a lingering impact winter which halted photosynthesis in plants and plankton.^[12][13] The impact hypothesis, also known as the Alvarez hypothesis, was bolstered by the discovery of the 180 km (112 mi) Chicxulub crater in the Gulf of Mexico's Yucatán Peninsula in the early 1990s,^[14] which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact.^[8] The fact that the extinctions occurred simultaneously provides strong evidence that they were caused by the asteroid.^[8] A 2016 drilling project into the Chicxulub peak ring confirmed that the peak ring comprised granite ejected within minutes from deep in the Earth, but contained hardly any gypsum, the usual sulfate-containing sea floor rock in the region: the gypsum would have vaporized and dispersed as an aerosol into the atmosphere, causing longer-term effects on the climate and food chain. In October 2019, researchers asserted that the event rapidly acidified the oceans and produced long-lasting effects on the climate, detailing the mechanisms of the mass extinction.^[15][16]

Other causal or contributing factors to the extinction may have been the Deccan Traps and other volcanic eruptions,^[17][18] climate change, and sea level change. However, in January 2020, scientists reported that climate-modeling of the mass extinction event favored the asteroid impact and not volcanism.^[19][20][21]

A wide range of terrestrial species perished in the K–Pg mass extinction, the best-known being the non-avian dinosaurs, along with many mammals, birds,^[22] lizards,^[23] insects,^[24][25] plants, and all of the pterosaurs.^[26] In the Earth's oceans, the K–Pg mass extinction killed off plesiosaurs and mosasaurs and devastated teleost fish,^[27] sharks, mollusks (especially ammonites and rudists, which became extinct), and many species of plankton. It is estimated that 75% or more of all animal and marine species on Earth vanished.^[28] However, the extinction also provided evolutionary opportunities: in its wake, many groups underwent remarkable adaptive radiation—sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches. Mammals in particular diversified in the following Paleogene Period,^[29] evolving new forms such as horses, whales, bats, and primates. The surviving group of dinosaurs were avians, a few species of ground and water fowl, which radiated into all modern species of birds.^[30] Among other groups, teleost fish^[31] and perhaps lizards^[23] also radiated into their modern species.

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