茅山道术治疗驼背电影:恐龙的灭亡

Why did all of the remaining dinosaurs die out suddenly at the end of the Cretaceous period? There is a short answer and a long answer to this question.

2 Here is the short answer. No one knows for sure.

3 Here is the long answer. There are a number of theories. Some are scientific, and some are ridiculous. Most scientists believe in one or two main ones, but no one knows for sure.

4 The reason no one knows for sure is because the whole thing happened about 65 million years ago. No one was around to see or hear what was going on back then, and no one was around to keep records.

5 What we do know comes mainly from fossil records. Scientists study the fossils themselves, and they study the rock in which the fossils are found. Fossil records haven't given us definite proof for any one theory. This is because there are some problems with fossil records. For one thing, most fossil evidence of dinosaurs has been found in North America. We don't know as much about dinosaur activity on other parts of the earth. Another problem is that it is very hard to date fossils. Dating fossils means determining how old they are.

6 One really interesting thing about the big dinosaur question is that scientists from many different fields are working to solve the problem. Astronomers, physicists, geologists, paleontologists, ecologists, chemists, geologists and others are all gathering evidence, experimenting, and researching to try to solve the puzzle.

7 The most popular theory is called the Alvarez Hypothesis. This hypothesis says that, 65 million years ago, a big asteroid hit the earth. This asteroid crash scattered dust high up into the atmosphere. It blocked much of the sunlight and caused the earth to become cooler. The crash also started huge fires, high winds, and violent storms that killed some of the dinosaurs. The lack of sunlight and warmth caused many plants to die. This shortage of plants for food caused the rest of the dinosaurs to die out.

8 The second most popular theory says that activity on the earth itself caused the extinction. As the supercontinent of Pangaea was breaking up, movement of the earth's giant land plates caused volcanoes to erupt and the climate to change. These changes affected the earth's atmosphere and temperature, and the dinosaurs died out from the cold or from lack of plants to eat.

9 Other theories suggest that dinosaurs became so big that they couldn't move, or that there were too many dinosaurs and they ate up all of the food.