Many species of animal and plants have disappeared from the earth. They have died out, or become extinct. But sometimes animals or plant (i) _______ can be found buried in rocks. These are called fossils. Imprints in rocks, paw prints, for example are also called fossils. Not every creature survives as a fossil. Many simply rot away completely and leave no (ii) _______ 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 term, became established at the beginning of the nineteenth century. Before this researeh 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(iii) _______ type of fossil which you might easily find yourself on a beach or among rocks, were called snakcstoncs because of their snake-like shape. People believed that snakes had been turned 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 (iv) _______ to extinction by humans as some animal 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 climatic change. One possibility is that a gigantic meteorite crashed into the earth (v) _______ so much dust into the atmosphere that the heat of sunlight was reduced. The temperature would have fallen and, as a consequence, many types of plants and animals would have become extinct.
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