What Was the Cretaceous–Tertiary Extinction Event?

Mary McMahon
Mary McMahon

The Cretaceous–Tertiary extinction event was a mass extinction which occurred an estimated 65.5 million years ago, wiping out a substantial number of the species on Earth. Estimates of the severity of the event vary, with some biologists suggesting that as many as 85% of plant and animal species might have been lost during this period in Earth's history. Other estimates are more conservative, but still quite remarkable.

The Cretaceous-Tertiary extinction event signalled the end of dinosaurs.
The Cretaceous-Tertiary extinction event signalled the end of dinosaurs.

Among the lay public, the Cretaceous–Tertiary extinction event is probably most notable because it marked the end of the dinosaurs. Many other organisms were lost, however, including a wide range of marine animals. In a sense, it cleared the decks for the rise of birds and mammals, who emerged from the event largely unscathed, as did fungi and ferns, which experienced a notable resurgence after the Cretaceous–Tertiary extinction event.

For geologists, the Cretaceous–Tertiary extinction event is very easy to identify, because it left behind a distinctive band known as the K-T boundary. The K-T boundary occurs all over the Earth and in a variety of rock deposits, suggesting that whatever caused the Cretaceous–Tertiary extinction event, it was widespread, leaving no corner of the Earth untouched. Study of the K-T boundary also reveals a characteristic mineral signature, and often the rock above and below the band is markedly different.

The cause of the Cretaceous–Tertiary extinction event is a subject of some debate amongst scientists, and it may never be fully resolved. Some suggest that it was caused by a series of catastrophes, such as volcanic eruptions, meteor collisions, and supernovae which dramatically changed the Earth's climate, leading to widespread death among organisms which could not survive in Earth's radically altered climate. Some geological evidence does support this theory, with several large meteor craters around the world dating to the right time period.

Others believe that the event was more gradual, although they also link it to climate change. It may have been caused, for example, by radical changes in sea level, which would have changed the Earth's environment. This theory is also supported, as fossil evidence suggests that biodiversity was already on the decline before the Cretaceous–Tertiary extinction event, and the limited array of fossils available for study makes it difficult to precisely determine the amount of time involved in this historic mass extinction.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


Big gaps here. Keep in mind: Floods make fast fossils. How about a worldwide flood, which included earthquakes, storms, and other earth consuming catastrophes -- like the one described in the bible -- not so long ago?

Most dinos were the size of a sheep! At least consider creation, not evolution. Science and creation do fit together and fit better than evolution. Creatures become extinct in our time right now. It's not so unusual.


@hamje32: I think the Loch Ness monster is a dinosaur swimming in the ocean that refuses to die. I guess he never got the memo about extinction.


@miriam98 - I think you’re referring to extinction by nuclear war. I don’t know if that’s really true in either case. The dinosaurs went extinct in the Mesozoic period. Scientists generally think that modern day birds descended from dinosaurs, but I find more interesting the idea that dinosaurs or other creatures long thought to be extinct still live among us.

One example is the Coelacanth fish. This fish was long thought to be extinct but it was found in 1938 near Africa. Other creatures have been found as well that seem somewhat dinosaur-like. The tribal people of Africa tell of an animal called “emela-ntouka.” They describe them as oversized elephants with large horns, but expeditions of scientists say these descriptions are more like dinosaurs such as rhinoceroses.


They say if we have another mass extinction event, no creature would survive except the cockroaches.


Good article. It made me want more information.

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