The extinction of dinosaurs( 來自 yahoo)

1. The extinction of dinosaurs(來自yahoo) • For almost 140 million years, dinosaurs and other large reptiles ruled the land, sky, and sea. Dinosaurs came in sizes and shapes suited to every corner of the world. Then, approximately 65 million years age, these huge reptiles died out and mammals took over the Earth. Few mysteries have ever excited the imaginations of scientists as much as this great ectinction that killed off all the dinosaurs and the other great reptiles. Three of the most common explanations are a gradual change in the Earth's climate, a lack of food, and the Earth's collision with a large asteroid. • Traditionally, scientists believed that the number of dinosaurs declined slowly for millions of years before they eventually disappeared. Many studies of fossils point to a slow change in the environmental conditions on Earth. This led to a generally cooler climate. Toward the end of the Cretaceous Peroid, the climate may have become too cold for the dinosaurs. Many dinosaurs were too big to hibernate in dens. In addition, they lacked fur or feathers for protection against the cold. Smaller animals had the advantage of being able to hibernate during cold periods. Mammals and birds were better suited to withstand the cold because they had fur or feathers for protection. It may have been easier for these animals to migrate to warmer places to avoid the cold weather. In these ways, such animals could have survuved the colder climate that may have slowly killed off the dinosaurs •   Another explanation of dinosaur extinction has to do with food. Some experts think that plant-eating dinosaurs could not digest the new kinds of plans that developed during the Cretaceous Period. Eventually, they starved to death. As the plant-eating dinosaurs died off, so did the meat-eaters who fed on them. Other experts believe that since the dinosaurs could not compete successfully with mammals for food, thay eventually lost the struggle for existence. • Today, some scientists believe that the cause of the extinction was much more sudden and catastrophic. In the late 1970s, scientists discovered evidence for the abrupt end to the Age of Dinosaurs. Dr. Louis Alvarez and his colleagues arrived at a revolutionary hypothesis to explain the extinction of dinosaurs. They suggest that about 65 million years ago, the Earth was struck by a huge asteroid. The asteroid was destroyed in the explosion and billions of tons of dust were thrown up into the air. A thick cloud of dust blocked our sunlight for a long time. Without the sun's energy, plants were not able to make food and they died. The lack of plants killed off many of the plant-eating dinosaurs, which then caused the death of the meat-eating dinosaurs that preyed on them. The darkness caused temperatures to fall below freezing for many months. This sudden change in climate further decreased the dinosaur populations. • It seems that no one theory adequately explains why dinosaurs died out. Perhaps dinosaure simply could not adjust to the changes that were taking place on the Earth toward the end of the Cretaceous Period. Most likely, it was a combination of causes that contributed to the end of the Age of Dinosaurs.