填空题

Science may never answer the most puzzling of all dinosaur questions. What killed these mighty creatures
One of the most popular theories about the death of the dinosaurs is that the world just grew too cold for them. Indeed, for large, coldblooded creatures, even a few nights of cold could spell death. How could the weather have changed Scientists think there might have been a cooling in the earth’s atmosphere during the Late Cretaceous Period.
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Not everyone agrees that a change in weather would have been enough to kill the dinosaurs. Some scientists say that dinosaurs might have been warm-blooded. Warm-blooded animals are able to make their own body heat. They can live in much colder climates than coldblooded animals.
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In the late 1970s, a naturalist named Walter Alvarez was studying the layer of rock that marks the end of the Age of Dinosaurs. He made a puzzling discovery. Alvarez found a layer of clay which contained a great deal of a metal called iridium. This metal is only found in the earth’s core, or in comets and asteroids from outer space. Other scientists in different parts of the world found this iridium-rich clay in the same layer as Alvarez. Could volcanoes have spit up large amounts of iridium from the earth’s core Alvarez and others didn’t think so. There was just too much iridium over too much land.
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Without sunlight, plants would not have been able to grow. With the plants gone, many other forms of life would have starved. A horrible chain of death would have brought down one mighty animal after another, on land and in water.
If a giant crash like this did kill off the dinosaurs, how did other animals survive Many kinds of insects, small mammals, reptiles, birds, and fish continued to live past the end of the Cretaceous Period. How did they manage Some animals can wait out a disaster better than others. Small, slow-moving animals such as turtles and crocodiles can go for longer periods without food than large, active animals. Some sea animals live on types of plants that don’t need much sunlight. And small mammals such as squirrels get through a long winter -they hibernate.
The idea of a great asteroid or comet crash is fascinating. But it would mean the dinosaurs would all have been killed within a very short time -perhaps over a few months or years. What is the dinosaurs did not die out so quickly
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In Montana, located in the north-western United States, dinosaur fossils have been found in a layer of rock that date to around 40,000 years after the end of the Cretaceous Period.
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Could the death of the dinosaurs have been caused by their moving into new areas Illness and disease can be carried by travelling animals. Is it possible that dinosaurs and other creatures died of terrible diseases caught from other animals
Changes long ago on the earth’s surface might have joined pieces of land that were once separate. With new land links, dinosaurs and other animals might have travelled to areas they had never seen before.
Away from home, they would have met up with new kinds of animals and might have caught diseases that they had no defence against!

• A. Many scientists think that the dinosaurs had started to die off millions of years before the end of the Cretaceous Period. And even more amazing, fossils have been found in the United States and southern China that might show dinosaurs lived long after they were supposed to have disappeared.
• B. Other scientists ask why dinosaurs could not just have adapted to the cold weather. In fact, fossils have been found that show dinosaurs might have lived far north as Alaska, and as far south as Antarctica.
• C. The shifts of the earth’s surface might have upset the climate and the colder weather might have attacked the plants first. As they died off, the plant-eating dinosaurs would have starved to death. And without plant-eaters for food, the meat-eating dinosaurs would have been the next to die.
• D. And the fossils in southern China show that dinosaurs might have survived longest in that area.They were found in a layer almost a million years after the last dinosaur was supposed to have died.
• E. If this was what ended the dinosaurs, what do we make of AIvarez’s mystery clay Does this mean there was no asteroid comet crash Some scientists think the dinosaurs might have been affected by such as object from outer space, but only when they were already in trouble.
• F. Alvarez came up with the theory that a large asteroid from outer space must have hit the earth sixty-five million years ago.The crash would have sent great clouds of dust -and iridium -into the air. It would have blocked out the sunlight and may have changed the earth’s temperatures in strange ways.