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THE END OF DINOSAURS

THE CRETACEOUS EXTINCTION

What is an extinction?

Extinction is the sudden or gradual dying out of a species. There are a multitude of reasons for extinction, ranging from disease, human intervention, climate changes, and natural disasters, such as volcanic eruptions or impacting space bodies. Each one can cause the extinction of one or many species of animals, depending on the severity.

When did the idea of extinction become accepted?

During the seventeenth and eighteenth centuries, scientists knew that fossils were the ancient remains of plants and animals. However, most still thought that these fossils represented known, living species that would shortly be discovered living in some remote, unexplored part of the globe.

This changed radically in the 1750s. Explorers in North America found the remains of what they thought were elephants, but in reality the animals were mastodons and mammoths, which died out more than 10,000 years ago toward the end of the Ice Ages. As these and other fossils from the New World were examined, scientists realized the fossils were actually the remains of extinct species. In 1796, Baron Georges Cuvier of the Museum dHistoire Naturelle in Paris (the first comparative anatomist) published a series of papers proving these fossil elephants, and giant mammal bones from other parts of the world, did indeed represent extinct species.

What is the evidence for dinosaur and other life form extinction?

There are several indicators of dinosaur and other life form extinction in the fossil record. But one of the best ways to determine extinction is by the lack of fossils in a rock layer. For example, above the top rock layers from the Cretaceous period, there are no known fossils of dinosaurs; and just above the top rock layers from the Permian period, the number of fossils from animals to plants greatly diminishes. The reason is logical: the animals that die leave behind fossils; when they become extinct, no more fossils are left behind. Such evidence in rock layers makes it seem as if one minute the organisms were there, and the next they disappeared. In reality, most of the extinctions took place over thousands of years.

Along with many animal species, a large number of plants became extinct at the end of the Cretaceous. As with other species, ferns died off, as well, but layers of rock filled with fern spores are indicative of how these plants recover quickly from a mass extinction (iStock).

What is the fern spike?

The fern spike is a layer of rock filled with fern spores; it occurs after a major cataclysmic or localized extinction. Scientists believe that after a major mass extinction, most plants would be wiped out. The first plants to recover are the ferns, which spread their spores into the air: thus, the fern spike. This evidence in rock layers is often used to determine the line between the time before and after a massive global or local extinction.

What were the major extinctions during Earths long history?

Around five major extinctions have occurred over Earths history. Some of the extinctions greatly affected the animals and plants on land, while other extinctions mainly occurred in the oceans. Most of the major extinctions are based on the fossil record, and usually indicate a time when a large percent of the plants and animals living on Earth went extinct, usually for unknown reasons. The following lists some of the known major extinctions on the geologic time scale:

Major Extinctions

Time Period

Date

(millions of years ago)

Percent of Species Extinct (approximate)

Cambrian-Ordovician

438

84%

Devonian-Carboniferous

360

82%

Permian-Triassic

250

97%

Triassic-Jurassic

208

76%

Cretaceous-Tertiary

65

85%

 
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