题型：阅读理解 题类：模拟题 难易度：普通
吉林省白山市2021届高三英语联考试卷
Crocodiles today look very similar to ones from 200 million years ago. There are also very few species alive today—just 25. Other animals such as lizards (蜥蜴) and birds have achieved a diversity of many thousands of species in the same amount of time or less.
Prehistory also saw types of crocodile we don't see today, including giants as big as dinosaurs, plant-eaters, fast runners and snake-like forms that lived in the sea. The rate of their evolution is generally slow, but occasionally they evolve more quickly because the environment has changed. In particular, their evolution speeds up when the climate is warmer, and their body size increases.
The body size of crocodiles is important because it interacts with how fast animals grow, how much food they need, how big their populations are and how likely they are to become extinct. The limited diversity of crocodiles and their apparent lack of evolution is a result of a slow evolutionary rate. It seems the crocodiles arrived at a body plan that was very efficient and perfect enough that they didn't need to change it in order to survive. This perfection could be one explanation why crocodiles survived Cretaceous period (白垩纪), in which the dinosaurs died out.
Crocodiles generally develop better in warm conditions because they cannot control their body temperature and require warmth from the environment. The climate during the age of dinosaurs was warmer than it is today, and that may explain why there were many more varieties of crocodiles than we see now. Being able to draw energy from the sun means they do not need to eat as much as a warm-blooded animal like a bird or a mammal (哺乳动物).
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