Unit 6 Earth First Vocabulary 词汇课前预先案-2022-2023学年高中英语外研版（2019）必修第二册

Unit 6 Earth First Understanding ideas--Vocabulary 课前预先案 1． Read the whole passage and answer the following questions .课外阅读拓展，世界上最大的鲨鱼知多少？ About 20 million years ago,there lived an ocean giant with teeth the size of a human hand and a jaw wide enough to swallow a person whole.Then,3.6 million years ago,the megalodon disappeared.Exactly why that happened is a scientific mystery,but a new study suggests that the mighty creature may have lost out to the much smaller-but still ferocious-great white shark. The megalodon was the largest shark to ever live.Based on studies of teeth and other fossils found in oceans around the world,scientists believe the megalodon was about three times the size of the largest great white sharks.Because it lived so long ago,it is difficult to know what the megalodon's life was like or exactly what it ate.However,an international team of researchers studied its fossilized teeth to try to find out. In today's ecosystems,animals that are higher up the food chain(meaning they have fewer predators) have lower levels of a mineral called zinc in their bodies.The researchers analyzed teeth from ancient megalodons and great whites and found that members of both species had similar amounts of zinc in their teeth.That meant the two kinds of sharks were in the same spot on the food chain. In their case,they were both apex predators (at the top of the food chain with no natural predators) and ate the same types of prey,such as fish and seals. This finding supported a theory that great whites,though smaller,cou'd have managed to snap up most of the food the megalodons needed to survive.If that was true,the smaller shark played a role in driving the much bigger shark to extinction.Other factors could have included climate and water temperatures.The research also supported the idea that a predator does not have to be the biggest to beat out others of its kind. Scientists will have to do more research to figure out exactly what happened in those ancient