Dinosaurs and Meteorites

Impact

The clues to mass extinctions are fossils; the remains and traces of ancient animals and plants that have been preserved in layers of rock. Fossils tell of many forms of life that flourished millions of years ago. Then something happened, and they died out. Because scientists are able to date the layers of rock, they can tell when the plants and animals lived and then died out.

The fossil records shows a number of periods in Earth’s history when mass extinctions have taken place. The most famous happen about 65 million years ago, when the dinosaurs died out. At that time, many large groups of plants and animals also became extinct, both on land and in the sea. They disappear from the fossil record along with the dinosaurs.

What happened is one of the big mysteries of science. For years the most likely explanation seemed to be a chang in climate. The fossil remains of plants show that for millions of years Earth’s climate had been warm, and even tropical. But 65 million years ago it had started to cool off. Many forms of life could no longer exist in the regions where they had been living. A number of scientist felt this explanation was not good enough. They didn’t think that change of climate alone could account for the size of the mass extinction. They felt life would have adapted to a gradual change in climate.

The theory of a meteorite grew from a chance discorvery in a bed of limestone in Italy. The limestone had formed as the tiny sea animals and planets died and their remains fell to the bottom of a shallow sea. The remains poled up and hardened into the kind of rock called limestone. From time to time the kinds of sea life changed and with each change, a different layer of rock formed. Millions of years ago, the shallow sea dried up, and the limestone became part of the land.

An American geologist, who was studying the limestone, made an odd discovery. He found a thin layer of clay sandwiched between two layers of limestone. The clay layer meant that at some time in the past a big change had taken place. What ever it was had stopped the formation of limestone. Instead of the remains of sea life raining down on the sea floor, tiny particles of clay had built up. Then the build up of limestone began again.