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  3. The dinosaurs are dead.They ruled the world for millions of years—but once the last one died, that was it.There were no more Mai

The dinosaurs are dead.They ruled the world for millions of years—but once the last one died, that was it.There were no more Mai

admin2021-04-02  10
问题         The dinosaurs are dead.They ruled the world for millions of years—but once the last one died, that was it.There were no more Maiasaura to nurture reptile babies, no fierce Tyrannosaurus, no more spiked Ankylosaurus.

        Dinosaurs are just one of many groups of animals that no longer exist.Scientists don’t always know why animals become extinct.They try to look at different kinds of evidence for answers.The first place scientists look is rocks.Rocks reveal the unusual conditions that might have taken place while the animals lived.
        One popular explanation for the disappearance of dinosaurs is that a large asteroid (小行星)hit Earth 65 million years ago.Dust from the impact could have spread over the whole planet.The dust would have dimmed the sun and killed many plants.Animals that ate plants would have starved, causing the animals that preyed on them to die off also.
        If an asteroid collision really happened, scientists would find iridium (铱，化学元素)in rocks that are about 65 million years old.This rare element is added to Earth’s crust when asteroids hit Earth.It just so happens that around the world, many rocks created at that time do have high amounts of iridium.An asteroid impact would have blown a huge hole in Earth’s surface.In fact, a giant crater (大坑) was found just off the coast of Mexico.Scientists believe it is about 65 million years old.
        Scientists know that dinosaurs vanished around the same time as this asteroid collision.Are the two events related? To find out, scientists need to study the bones of animals that lived then.
        Dinosaurs were not the only animals alive when the asteroid struck Earth.What happened to other groups of creatures? Most frogs, for example, were not affected by the disaster.Most fish were fine too, as were lizards and mammals.Why did these animals go on living, when dinosaurs did not? Is it possible that the dinosaurs disappeared for a different reason?
        Scientists have found fewer and fewer species of dinosaur fossils from the years before dinosaurs vanished altogether.This suggests that dinosaurs disappeared slowly, over thousands and millions of years, not in one crash of an asteroid.It’s possible that dinosaurs did not compete well with the clever, warmblooded mammals that ere increasing in numbers on Earth.
        Right now no one can say for sure what caused the death of the dinosaurs.There’s a lot more work to do—in many areas of science.
Why do scientists choose rocks as the first evidence?
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答案Because rocks can reveal the unusual conditions that might have taken place while the animals lived.
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