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A brief history of mass extinctions

Species go extinct all the time. Scientists estimate that at least 99.9% percent of all species of plants and animals that have ever lived are now extinct. So the demise of dinosaurs like T. rex and Triceratops some 66 million years ago wouldn't be especially noteworthy—except for the fact that around 80% percent of all life alive at the time also died out, in what scientists call a mass extinction.
Mass extinctions—when at least 70% of all species die out in a relatively short time—have happened a handful of times over the course of our planet's history. The largest mass extinction event occurred around 251 million years ago, when perhaps 95% percent of all species went extinct.

Five mass extinctions

Ordovician-Silurian Extinction: Small marine organisms died out. (442 mya)
Devonian Extinction: Many tropical marine species went extinct. (365 mya)
Permian-Triassic Extinction: The largest mass extinction event in Earth's history affected a range of species, including many vertebrates. (251 mya)
Triassic-Jurassic Extinction: The extinction of other vertebrate species on land allowed dinosaurs to flourish. (201 mya)
Cretaceous-Paleogene Extinction: (65.5 mya)

66 million years ago: a mass extinction

Scientists refer to the major extinction that wiped out non-avian dinosaurs as the K-Pg event (formerly called the K-T extinction) because it happened at the end of the Cretaceous Period and the beginning of the Paleogene. Why not C-Pg? Geologists use "K" as a shorthand for Cretaceous based on the German word “kreide”, for which the Cretaceous Period with its chalky sediments is named.
The K-Pg event is so striking that it signals a major turning point in Earth's history, marking the end of the geologic period known as the Cretaceous and the beginning of the Paleogene period. We can see the impact of this event in the fossil record. Fossils that are abundant in earlier rock layers are simply not present in later rock layers, after the event. A wide range of animals and plants suddenly died out, from tiny marine organisms to large dinosaurs.

Changes in the environment

What happened to cause such widespread devastation 66 million years ago? Scientists agree that species go extinct primarily as a result of changes in their environment. The extinction of many species around the world at one time reflects large-scale changes in the global environment.
To explain what caused this mass extinction, scientists have focused on events that would have altered our planet's climate in dramatic, powerful ways. The leading theory is that a huge asteroid slammed into Earth 66 million years ago, blocking sunlight, changing the climate and setting off global wildfires. In recent years, researchers have also been investigating whether other forces, including massive volcanic eruptions and changes in sea level, may have contributed to the environmental changes.

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