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Part of the Dinosaurs: Ancient Fossils, New Discoveries exhibition.
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.
Early life forms began to flourish during the Cambrian Explosion, 540 million years ago.
Mass extinctions—when at least half of all species die out in a relatively short time—have occurred only a handful of times over the course of our planet's history. The largest mass extinction event happened around 250 million years ago, when perhaps 95 percent of all species went extinct.
Small marine organisms died out.
Many tropical marine species went extinct.
The largest mass extinction event in Earth's history affected a range of species, including many vertebrates.
The extinction of other vertebrate species on land allowed dinosaurs to flourish.
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.