Mass extinctions—when at least half 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 250 million years ago, when perhaps 95 percent of all species went extinct.

Top five extinctions

Ordovician-silurian Extinction: Small marine organisms died out. (440 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. (250 mya)
Triassic-jurassic Extinction: The extinction of other vertebrate species on land allowed dinosaurs to flourish. (210 mya)
Cretaceous-tertiary Extinction: (65.5 mya)

65 million years ago: a mass extinction

Scientists refer to the major extinction that wiped out non-avian dinosaurs as the K-T extinction, because it happened at the end of the Cretaceous period and the beginning of the Tertiary period. Why not C-T? Geologists use "K" as a shorthand for Cretaceous. "C" is shorthand for an earlier period, the Cambrian.
The extinction that occurred 65 million years ago wiped out some 50 percent of plants and animals. The 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 Tertiary period.
Around 65 million years ago, something unusual happened on our planet—and we can see it in the fossil record. Fossils that are abundant in earlier rock layers are simply not present in later rock layers. A wide range of animals and plants suddenly died out, from tiny marine organisms to large dinosaurs.
Species go extinct all the time. Scientists estimate that at least 99.9 percent of all species of plants and animals that ever lived are now extinct. So the demise of dinosaurs like T. rex and Triceratops some 65 million years ago wouldn't be especially noteworthy—except for the fact that around 50 percent of all plants and animals alive at the same time also died out in what scientists call a mass extinction.

Changes in the environment

What happened to cause such widespread devastation 65 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 or comet slammed into Earth 65 million years ago, blocking sunlight, changing the climate and setting off global wildfires. In recent years, however, researchers have also been investigating whether other forces, including massive volcanic eruptions and changes in sea level, may have contributed to the general environmental decline.