Many species of animals and plants have disappeared from the earth. They have died out, or become extinct. But sometimes animals or plants can be found buried in rocks. These are called fossils. Imprints in rocks (paw prints, for example) are also called fossils. Not every creature as a fossil. Many simply rot away completely and leave no trace of their existence. Because many creatures and plants have disappeared without leaving any fossils, we will never know anything about them.
The study of fossils, or palaeontology, to give it its scientific , became established at the beginning of the nineteenth century. Before this research began, people did not believe that fossils had once been alive. Large fossil teeth were seen as evidence of a race of giants in the past, while ammonites, a very type of fossil which you might easily find yourself on a beach or among rocks, were called snakestones because of their snake-like shape. People believed that snakes had been to stone by a miracle.
The most famous fossils of all are the dinosaurs. There are, of course, no dinosaurs on display in zoos. They were not to extinction by humans as some animals have been, but became extinct millions of years before our own species developed. The reason why the dinosaurs became extinct is still a mystery. Many theories have connected the disappearance of dinosaurs with major change. One possibility is that a gigantic meteorite crashed into the earth displacing so much dust into the atmosphere that the of sunlight was reduced. The temperature would have fallen and, as a consequence, many types of plants and animals would have become extinct.