A、Their blood is in intimate contact with the water they live in. B、Their size is not big enough for them to live on land. C、The

Good afternoon, folks. In today’s lecture, we talk about the dinosaurs. One of the greatest mysteries about the dinosaurs is why they died out suddenly at the end of the Cretaceous Period 65 million years ago. A number of theories have been suggested to account for this mass extinction. The most popular theory is that a comet or meteorite hit the Earth and exploded with a terrific force, throwing up a dust cloud that blocked out the sun. Suddenly, it gets cold everywhere, and everyone dies. Nevertheless, I think this theory doesn’t work. Because it completely ignores most of what is happening on land. If you want to study dinosaurs and understand them, you’ve got to think about frogs and turtles and salamanders and mammals, too. You can’t take dinosaurs out of context. All the big animals were dinosaurs; that’s true. But most animals aren’t big. Go to the Amazon rain forest, or even a forest in New Jersey, and you’ll find that most of the animals there are small. If you exploded a nuclear bomb right now over Brazil blocking the sunlight and chilling the Amazon rain forest, who would die first? The big deer and jaguars or the little frogs? Frogs should die first, because they are the most delicate creatures in any ecosystem; their blood is in intimate contact with the water they live in. Also, a tropical frog exposed to a chill can’t hibernate, so it dies. A big animal, such as a deer or elephant, can stand a major chill and can move to another area. But have you ever heard of frog extinctions at the end of the Cretaceous Period? No. That’s because no frogs died out. No turtles or salamanders died out. None of these delicate animals, with no defense at all against a sudden chill, died out.  But dinosaurs—the biggest, most active animals that should take weeks to kill—did die. This is true of every major extinction that has ever hit the Earth. Not so long ago, there were woolly mammoths and saber-toothed cats in Chicago. There were beavers the size of Buicks all over North America. They disappeared about 10,000 years ago So you may ask: what caused those mass extinctions? Well, it is very hard to come up with a theory that will kill big animals and leave little ones alone. The one thing that would do it is disease. This is a theory developed by American paleontologist Henry Osborn in 1899. He pointed out that when big animals travel, they’ll spread dozens of diseases and disrupt the ecology. Whenever we humans have brought animals from one continent to another, bad things happen. Someone brought starlings from England to North America, and we have a starling problem. Introduce foreign animals and they run amuck. They do not have natural predators and also they spread disease. Whenever there was a mass extinction on land, there were land bridges connecting the continents and big animals moving across them. During most of the history of life, broad oceans have separated the continents. But, periodically, those ocean barriers get drained away and animals can move. Big animals travel very easily. Give an elephant a land bridge and the population will spread at least 1,600 kilometers in two years. Little animals don’t move nearly as fast. It takes a long time for the population of a snake, salamander, or frog to spread. Osborn pointed out that if big animals move across land bridges and start spreading, you’re going to have extinctions caused by disease or disruptions to the ecosystem. There is no way to prevent it. So I like Osborn’s theory. It’s based in ecological reality. We know that foreign animals always cause disaster.

Question No. 16  What is the most popular theory about the dinosaurs’ extinction?
Question No. 17  Why are frogs the most delicate creatures in an ecosystem?
Question No. 18  What is the theory developed by American paleontologist Henry Osborn in 1899?
Question No. 19  Which of the following is NOT regarded as a major extinction of species?
Question No. 20  According to the speaker, what is essential in the spread of diseases by big animals?