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直到寒武纪末期，也就是大概六亿年前，地球上才出现了宏观物种的大量繁殖，并产生了化石记录，使我们能够追踪生物多样性的兴衰。

It was not until the Cambrian period, beginning about 600 million years ago, that a great proliferation of macroscopic species occurred on Earth and produced a fossil record that allows us to track the rise and fall of biodiversity.

自寒武纪以来，生物多样性总体上有所增加，但也有一些明显的例外。

Since the Cambrian period, biodiversity has generally risen, but there have been some notable exceptions.

由于全球大规模灭绝，生物多样性在至少五个时期内急剧崩溃。

Biodiversity collapsed dramatically during at least five periods because of mass extinctions around the globe.

五个主要的物种灭绝引起了（研究人员）大部分的关注，但它们只是灭绝事件的一方面。

The five major mass extinctions receive most of the attention, but they are only one end of a spectrum of extinction events.

总体而言，在较小的事件中，更多的物种灭绝了，这些事件虽然不那么引人注目，但频率更高。

Collectively, more species went extinct during smaller events that were less dramatic but more frequent.

在五个主要的灭绝事件中，最著名的是白垩纪-第三纪的灭绝，这是恐龙灭绝的事件。

The best known of the five major extinction events, the one that saw the demise of the dinosaurs, is the Cretaceous-Tertiary extinction.

从大约2.8亿年前开始，爬行动物成为陆地环境中主要的大型动物。

Starting about 280 million years ago, reptiles were the dominant large animals in terrestrial environments.

用通俗的语言讲，这就是“恐龙统治地球”的时代，各种各样的爬行动物物种占据了许多生态位。

In popular language this was the era “when dinosaurs ruled Earth,” with a wide variety of reptile species occupying many ecological niches.

但是，任何群体或物种在地球上的统治时间都是有限的。恐龙时代在统治地球超过两亿年之后，在大概六千五百万年前突然结束了。之后哺乳动物开始蓬勃发展，它们是从相对较少种类的小型陆生动物逐渐演变成了今天我们所熟知的无数物种，其中就包括蝙蝠、鲸鱼等物种。

However, no group or species can maintain its dominance indefinitely, and when, after over 200 million years, the age of dinosaurs came to a dramatic end about 65 million years ago, mammals began to flourish, evolving from relatively few types of small terrestrial animals into the myriad of diverse species, including bats and whales, that we know today.

古生物学家将地球历史上的这一点标记为白垩纪时期的结束和第三纪时期的开始，通常缩写为K-T边界。

Paleontologists label this point in Earth’s history as the end of the Cretaceous period and the beginning of the Tertiary period, often abbreviated as the K-T boundary.

这个时间段还以许多其他类型有机物的变化为标志。

This time was also marked by changes in many other types of organisms.

总体而言，约有38％的海洋动物失踪了，在某些群体中这一比例更高。

Overall, about 38 percent of the families of marine animals were lost, with percentages much higher in some groups.

氨类软体动物从非常丰富多样到了灭绝的状态。

Ammonoid mollusks went from being very diverse and abundant to being extinct.

尽管后来数量有反弹，但一种极为丰富的称为有孔虫的浮游海洋动物在当时基本上消失了。

An extremely abundant set of planktonic marine animals called foraminifera largely disappeared, although they rebounded later.

在植物之中，K-T边界原始维管植物的丰富度经历了急剧而短暂地上升，例如蕨类植物，俱乐部苔藓，马尾，针叶树和其他裸子植物。

Among plants, the K-T boundary saw a sharp but brief rise in the abundance of primitive vascular plants such as ferns, club mosses, horsetails, and conifers and other gymnosperms.

此时开花植物（被子植物）的数量减少了，但随后开始急剧增加。

The number of flowering plants (angiosperms) was reduced at this time, but they then began to increase dramatically.

多年以来，科学家一直认为气候变冷是造成这种情况的原因，其中恐龙特别脆弱，因为它们像现代爬行动物一样是冷血动物（他们的体温取决于环境热或冷）。

For many years scientists assumed that a cooling of the climate was responsible, with dinosaurs being particularly vulnerable because, like modern reptiles, they were ectothermic (dependent on environmental heat, or cold-blooded).

现在普遍认为，至少某些恐龙的代谢率足够高，足以使其成为恒温动物（动物通过内部产生热量来维持相对稳定的体温）。

It is now widely believed that at least some species of dinosaurs had a metabolic rate high enough for them to be endotherms (animals that maintain a relatively consistent body temperature by generating heat internally).

然而，恐龙灭绝可能是恒温动物这一观点并未对于K-T灭绝的气候解释提出真正的质疑，因为即使恒温动物也可能受到气候变化的影响。

Nevertheless, climatic explanations for the K-T extinction are not really challenged by the idea that dinosaurs may have been endothermic, because even endotherms can be affected by a significant change in the climate.

1980年，当路易斯·阿尔瓦雷斯（Luis Alvarez）领导的一组物理科学家提出，在6500万年前，一颗10公里宽，以每小时9万公里的速度前进的陨石撞击地球时，K-T灭绝的解释发生了革命性的变化。

Explanations for the K-T extinction were revolutionized in 1980 when a group of physical scientists led by Luis Alvarez proposed that 65 million years ago Earth was struck by a 10-kilometer-wide meteorite traveling at 90,000 kilometers per hour.

他们认为，这次撞击产生了一层厚厚的尘埃云，包围了地球，从而阻止了大部分入射的太阳辐射，并将植物的光合作用降到非常低的水平。

They believed that this impact generated a thick cloud of dust that enveloped Earth, shutting out much of the incoming solar radiation and reducing plant photosynthesis to very low levels.

短期影响可能包括巨大的海浪和大规模火灾。

Short-term effects might have included huge tidal waves and extensive fires.

换句话说，一次灾难性事件引起的一系列事件导致了大灭绝。

In other words, a series of events arising from a single cataclysmic event caused the massive extinctions.

最初，陨石理论是基于单一的证据。

Initially, the meteorite theory was based on a single line of evidence.

在全球各地，地质学家发现大约6500万年前形成的沉积岩层中铱的浓度异常高。

At locations around the globe, geologists had found an unusually high concentration of iridium in the layer of sedimentary rocks that was formed about 65 million years ago.

铱是一种通常在地球表面附近并不常见的元素，但在某些陨石中含量丰富。

Iridium is an element that is usually uncommon near Earth’s surface, but it is abundant in some meteorites.

因此，阿尔瓦雷斯及其同事得出的结论是，沉积在K-T边界的沉积岩中的铱元素可能起源于巨大的陨石或小行星。

Therefore, Alvarez and his colleagues concluded that it was likely that the iridium in sedimentary rocks deposited at the K-T boundary had originated in a giant meteorite or asteroid.

有证据表明，大约六千五百万年前的陨石撞击形成了直径180公里的，以尤卡坦半岛北海岸为中心的圆形地层，因此大多数科学家开始接受陨石撞击理论。

Most scientists came to accept the meteorite theory after evidence came to light that a circular formation, 180 kilometers in diameter and centered on the north coast of the Yucatán Peninsula, was created by a meteorite impact about 65 million years ago.

The word “proliferation” in the passage is closest in meaning to

Paragraph 1 supports which of the following statements about life on Earth before the Cambrian period?

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

According to paragraph 2, why are dinosaurs popularly said to have “ruled Earth” during the Cretaceous period?

According to paragraph 2, which of the following species initially increased in number at the K-T boundary?

Why does the author note that “even endotherms can be affected by a significant change in the climate”?

The word “generated” in the passage is closest in meaning to

The word “extensive” in the passage is closest in meaning to

According to paragraph 4, all of the following contributed to the massive extinctions of the K-T period EXCEPT

According to paragraph 4, which of the following statements explains the importance of the discovery of high levels of iridium in rocks?

According to paragraph 4, which of the following is true about the Yucatán Peninsula?

Which of the following can be inferred from paragraph 4 about the meteorite theory?

Look at the four squares  that indicate where the following sentence could be added to the passage. This focused on the chemical composition of ancient rocks.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Extinction Episodes of the Past It was not until the Cambrian period, beginning about 600 million years ago, that a great proliferation of macroscopic species occurred on Earth and produced a fossil record that allows us to track the rise and fall of biodiversity. Since the Cambrian period, biodiversity has generally risen, but there have been some notable exceptions. Biodiversity collapsed dramatically during at least five periods because of mass extinctions around the globe. The five major mass extinctions receive most of the attention, but they are only one end of a spectrum of extinction events. Collectively, more species went extinct during smaller events that were less dramatic but more frequent. The best known of the five major extinction events, the one that saw the demise of the dinosaurs, is the Cretaceous-Tertiary extinction. Starting about 280 million years ago, reptiles were the dominant large animals in terrestrial environments. In popular language this was the era “when dinosaurs ruled Earth,” with a wide variety of reptile species occupying many ecological niches. However, no group or species can maintain its dominance indefinitely, and when, after over 200 million years, the age of dinosaurs came to a dramatic end about 65 million years ago, mammals began to flourish, evolving from relatively few types of small terrestrial animals into the myriad of diverse species, including bats and whales, that we know today. Paleontologists label this point in Earth’s history as the end of the Cretaceous period and the beginning of the Tertiary period, often abbreviated as the K-T boundary. This time was also marked by changes in many other types of organisms. Overall, about 38 percent of the families of marine animals were lost, with percentages much higher in some groups. Ammonoid mollusks went from being very diverse and abundant to being extinct. An extremely abundant set of planktonic marine animals called foraminifera largely disappeared, although they rebounded later. Among plants, the K-T boundary saw a sharp but brief rise in the abundance of primitive vascular plants such as ferns, club mosses, horsetails, and conifers and other gymnosperms. The number of flowering plants (angiosperms) was reduced at this time, but they then began to increase dramatically. What caused these changes? For many years scientists assumed that a cooling of the climate was responsible, with dinosaurs being particularly vulnerable because, like modern reptiles, they were ectothermic (dependent on environmental heat, or cold-blooded). It is now widely believed that at least some species of dinosaurs had a metabolic rate high enough for them to be endotherms (animals that maintain a relatively consistent body temperature by generating heat internally). Nevertheless, climatic explanations for the K-T extinction are not really challenged by the idea that dinosaurs may have been endothermic, because even endotherms can be affected by a significant change in the climate. Explanations for the K-T extinction were revolutionized in 1980 when a group of physical scientists led by Luis Alvarez proposed that 65 million years ago Earth was struck by a 10-kilometer-wide meteorite traveling at 90,000 kilometers per hour. They believed that this impact generated a thick cloud of dust that enveloped Earth, shutting out much of the incoming solar radiation and reducing plant photosynthesis to very low levels. Short-term effects might have included huge tidal waves and extensive fires. In other words, a series of events arising from a single cataclysmic event caused the massive extinctions.  Initially, the meteorite theory was based on a single line of evidence.  At locations around the globe, geologists had found an unusually high concentration of iridium in the layer of sedimentary rocks that was formed about 65 million years ago.  Iridium is an element that is usually uncommon near Earth’s surface, but it is abundant in some meteorites.  Therefore, Alvarez and his colleagues concluded that it was likely that the iridium in sedimentary rocks deposited at the K-T boundary had originated in a giant meteorite or asteroid. Most scientists came to accept the meteorite theory after evidence came to light that a circular formation, 180 kilometers in diameter and centered on the north coast of the Yucatán Peninsula, was created by a meteorite impact about 65 million years ago.