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 ri and fall of biodiversity. Since the Cambrian period, biodiversity has generally rin, but there have been some notable exceptions. Biodiversity collapd dramatically during at least five periods becau 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 demi of the dinosaurs, is the Cretaceous-T ertiary 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,” when 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 diver 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 T ertiary 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 diver and abundant to being extinct. An extremely abundant t of planktonic marine animals called foraminifera largely disappeared, although they rebounded later. Among plants, the K-T boundary saw a sharp but brief ri in the abundance of primitive vascular plants such as ferns, club moss, hortails, and conifers and other gymnosperms. The number of flowering plants (angiosperms) was reduced at this time, but they then began to increa dramatically.
What caud the changes? For many years scientists assumed that a cooling of the climate was responsible, with dinosaurs being particularly vulnerable becau, 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 ideas that
国防科技大学2020录取分数线dinosaurs may have been endothermic, becau 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 propod that 65 million years ago Earth was stuck 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 ries of events arising from a single cataclysmic event caud the
massive extinctions. ⬛!Initially, the meteorite theory was bad on a single line of evidence. ⬛!At locations around the globe, geologists had found an unusually high concentration of iridium in the layer of dimentary 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
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concluded that it was likely that the iridium in dimentary rocks deposited at the K-T boundary had originated in a giant meteorite or asteroid. Most scientist came to accept the meteorite theory after e
vidence came to light that a circular formation, 180 kilometers in diameter in diameter and centered on the north coast of the Yucatan Peninsula, was created by a meteorite impact about 65 million years ago.
1.The word “proliferation” in the passage is clost in meaning to
A)decline
B)extinction
C)increa
D)migration
2.Paragraph 1 supports which of the following statements about life on Earth
before the Cambrian period?
A)Biodiversity levels were steady, as indicated by the fossil record.
B)Levels of biodiversity could not be tracked.
C)The most dramatic extinction episode occurred.
金山快译在线D)Few microscopic species existed.
3.Which of the ntences below best express the esntial information in the
highlighted ntence in the passage? Incorrect choices change the meaning in important ways or leave out esntial information.
A)The dominance of dinosaurs came to an end 65 million years ago, at which
time mammals began to flourish and diversify.
B)Becau no group of species can remain dominant forever, mammals became
the dominant group when dinosaurs became extinct.
C)After being the dominant group for more than 200 million years, the age of
dinosaurs came to a dramatic end 65 million years ago.
服役是什么意思D)The diver group of mammals that we know today, including bats and
whales, evolved from small terrestrial forms that had been dominated by
dinosaurs.
zowie4.According to paragraph 2, why are dinosaurs popularly said to have “ruled Earth”
during the Cretaceous period?
A)Dinosaurs were the only species of reptile that existed during the whole of the
Cretaceous period.
B)Dinosaurs won the battle for food resources over mammals during the
Cretaceous period.
C)Dinosaurs survived extinction during the Cretaceous period, whereas many
other animal species did not.
D)Dinosaurs were the physically and ecologically dominant animals during the
Cretaceous period.
5.According to paragraph 2, which of the following species initially incread in
number at the K-T boundary?
A)Dinosaurs
B)Foraminifera
C)Ferns
D)Ammonoid mollusks
6.Why does the author note that “even endotherms can be affected by a significant
change in the climate”?
A)T o argue that there was a significant climate at the time that endothermic
dinosaurs became extinct
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B)T o argue that climate change caud some dinosaurs to evolve as endotherms
C)T o support the view that at least some of the dinosaurs that became extinct
were endotherms
D)T o defend climate change as possible explanation for the extinction of
dinosaurs
7.The word “generated” in the passage is clost in meaning to
A)collected
B)produced
C)spread
D)added
8.The word “extensive” in the passage is clost in meaning to
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A)widespread
B)sudden
C)numerous
D)subquent
9.According to paragraph 4, all of the following contributed to the massive翻译员
extinctions of the K-T period EXCEPT:
A)tidal waves
B)fires
C)insufficient solar radiation
D)iridium
巴西世界杯分组10.According to paragraph 4, which of the following statements explains the
importance of the discovery of high levels of iridium rocks?
A)It provided evidence that overexposure to solar radiation led to the K-T
extinction.
B)It showed that more than one cataclysmic event was responsible for the K-T
extinction.
C)It suggested that the cau of the K-T extinction may have been a meteorite
striking Earth.
D)It provided evidence that the K-T extinction occurred 65 million years ago.
11.According to paragraph 4, which of the following is true about the Yucatan
Peninsula?
A)The circular formation there was caud by a meteorite impact 65 million
years ago.
B)Sedimentary rocks from that area have the lowest iridium concentration of any
rocks on Earth.
C)There is evidence that a huge tidal wave occurred there 65 million years ago.
D)Evidence found there challenged the meteorite impact theory.
12.Which of the following can be inferred from paragraph 4 about the meteorite
theory?
A)The data originally prented as evidence for the theory were eventually
rejected.
B)Many scientists did not accept it when it was first propod.
C)It has not been widely accepted as an explanation for the K-T extinction.
D)Alvarez subquently revid it after a circular formation was found in the
Yucatan Peninsula.
13.Look at the four squares [⬛] that indicate where the following ntence could be
added to the passage.
This focud on the chemical composition of ancient rocks.
Where would the ntence best fit?
14.Pro Summary
The K-T extinction 65 million years ago is the best known of the five major extinction episodes since the Cambrian period.
Answer Choices:
A)Collectively, the five major extinction episodes resulted in the elimination of a
larger number of species than did all the minor extinction events.
B)The K-T extinction eliminated the dinosaurs and ammonoid mollusks but was
followed by the diversification of mammals and gymnospermous plants.英语手抄报的内容
C)An extreme cooling of the climate could not have caud the K-T extinction of
dinosaurs, becau, while most dinosaurs depended on environmental heat,
some did not.
D)The K-T extinction of the dinosaurs is the only mass extinction that has been
explained by the impact of a meteorite.
E)In 1980 Luis Alvarez propod that the K-T extinction was caud by ecological
disasters brought about by the impact of a meteorite striking Earth.
F) A high concentration of iridium in dimentary rocks at the K-T boundary and a
large impact crater in the Yucatan Peninsula from 65 million years ago strongly support Alvarez’ hypothesis.
