Around 65 million years ago, something unusual happened on our planet—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.

A Brief History of Earth

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.

Top Five Extinctions

Ordovician-silurian Extinction: 440 million years ago

Small marine organisms died out.

Devonian Extinction: 365 million years ago

Many tropical marine species went extinct.

Permian-triassic Extinction: 250 million years ago

The largest mass extinction event in Earth's history affected a range of species, including many vertebrates.

Triassic-jurassic Extinction: 210 million years ago

The extinction of other vertebrate species on land allowed dinosaurs to flourish.

Cretaceous-tertiary Extinction: 65 Million Years Ago

What to Call It?

Scientists refer to the major extinction that wiped out nonavian 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.

Dawn of a New Age

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.