Cases in which many species become extinct within a geologically short interval of time are called mass extinctions. There was one such event at the end of the Cretaceous period (around 70 million years ago). There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago). The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time.
The fossil record shows at least five mass extinctions in which many families of marine organisms died out. The rates of extinction happening today are as great as the rates during these mass extinctions. Many scientists have therefore concluded that a sixth great mass extinction is currently in progress.
What could cause such high rates of extinction? There are several hypotheses, including warming or cooling of Earth, changes in seasonal fluctuations or ocean currents, and changing positions of the continents. Biological hypotheses include ecological changes brought about by the evolution of cooperation between insects and flowering plants or of bottom-feeding predators in the oceans. Some of the proposed mechanisms required a very brief period during which all extinctions suddenly took place; other mechanisms would be more likely to have taken place more gradually, over an extended period, or at different times on different continents. Some hypotheses fail to account for simultaneous extinctions on land and in the seas. Each mass extinction may have had a different cause. Evidence points to hunting by humans and habitat destruction as the likely causes for the current mass extinction.
American paleontologists David Raup and John Sepkoski, who have studied extinction rates in a number of fossil groups, suggest that episodes of increased extinction have recurred periodically, approximately every 26 million years since the mid-Cretaceous period. The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes. The possibility that mass extinctions may recur periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation upon impact.
Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid-impact hypothesis first suggested by Luis and Walter Alvarez. According to this hypothesis, Earth collided with an asteroid with an estimated diameter of 10 kilometers, or with several asteroids, the combined mass of which was comparable. The force of collision spewed large amounts of debris into the atmosphere, darkening the skies for several years before the finer particles settled. The reduced level of photosynthesis led to a massive decline in plant life of all kinds, and this caused massive starvation first of herbivores and subsequently of carnivores. The mass extinction would have occurred very suddenly under this hypothesis.
One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir). Earth’s crust contains very little of this element, but most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe. A search of sedimentary deposits that span the boundary between theCretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly and precisely at this boundary. This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered.
An asteroid of this size would be expected to leave an immense crater, even if the asteroid itself was disintegrated by the impact. The intense heat of the impact would produce heat-shocked quartz in many types of rock. Also, large blocks thrown aside by the impact would form secondary craters surrounding the main crater. To date, several such secondary craters have been found along Mexico’s Yucatan Peninsula, and heat-shocked quartz has been found both in Mexico and in Haiti. A location called Chicxulub, along the Yucatan coast, has been suggested as the primary impact site.
Paragraph 1: Cases in which many species become extinct within a geologically short interval of time are called mass extinctions. There was one such event at the end of the Cretaceous period (around 70 million years ago). There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago). The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time.
1. Paragraph 1 supports which of the following statements about mass extinctions?
○They take place over a period of 70 million years.
○They began during the Cretaceous period.
○They eliminate many animal species that exist at the time they occur.
○They occur every 250 million years.
Paragraph 2: The fossil record shows at least five mass extinctions in which many families of marine organisms died out. The rates of extinction happening today are as great as the rates during these mass extinctions. Many scientists have therefore concluded that a sixth great mass extinction is currently in progress.
2. According to paragraph 2, scientists base their belief that a mass extinction is going on at present on which of the following?
○The speed with which mass extinctions are happening today is similar to the speed of past extinctions.
○The number of species that have died out since the last extinction event is extremely large.
○Mass extinctions occur with regularity and it is time for another one.
○Fossil records of many marine species have disappeared.
Paragraph 3: What could cause such high rates of extinction? There are several hypotheses, including warming or cooling of Earth, changes in seasonal fluctuations or ocean currents, and changing positions of the continents. Biological hypotheses include ecological changes brought about by the evolution of cooperation between insects and flowering plants or of bottom-feeding predators in the oceans. Some of the proposed mechanisms required a very brief period during which all extinctions suddenly took place; other mechanisms would be more likely to have taken place more gradually, over an extended period, or at different times on different continents. Some hypotheses fail to account for simultaneous extinctions on land and in the seas. Each mass extinction may have had a different cause. Evidence points to hunting by humans and habitat destruction as the likely causes for the current mass extinction.
3. The word extended in the passage is closest in meaning to
○ specific
○ unlimited
○ reasonable
○ long
4. According to paragraph 3, each of the following has been proposed as a possible cause of mass extinctions EXCEPT
○habitat destruction
○continental movement
○fierce interspecies competition
○changes in Earth's temperature
5. Paragraph 3 supports which of the following ideas about mass extinctions?
○Scientists know the exact causes of most mass extinctions.
○Mass extinctions are unlikely to happen again in the future.
○Insects, flowering plants, and bottom-feeding predators in the oceans tend to be the first organisms to disappear during episodes of mass extinctions.
○Some mass extinctions occurred on land and in the seas at the same time.
Paragraph 4: American paleontologists David Raup and John Sepkoski, who have studied extinction rates in a number of fossil groups, suggest that episodes of increased extinction have recurred periodically, approximately every 26 million years since the mid-Cretaceous period. The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes. The possibility that mass extinctions may recur periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation upon impact.
6. 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.
○B(yǎng)ased on their studies of extinction rates of numerous fossil groups, paleontologists David Raup and John Sepkoski have determined that mass extinctions occur about every 26 million years.
○David Raup and John Sepkoski studied extinction rates of numerous fossil groups and suggest that mass extinctions during the Cretaceous period continued for 26 million years.
○Studies that paleontologists David Raup and John Sepkoski conducted of various fossil groups have revealed that extinction rates have increased over the past 26 million years.
○The studies conducted by paleontologists David Raup and John Sepkoski of the fossil remains of species suggest that the extinction rate of species started to increase by the middle of the Cretaceous period.
7. According to paragraph 4, what aspect of extinction episodes does the companion-star hypothesis supposedly clarify?
○Their location
○Their frequency
○Their duration
○Their severity
Paragraph 5: Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid-impact hypothesis first suggested by Luis and Walter Alvarez. According to this hypothesis, Earth collided with an asteroid with an estimated diameter of 10 kilometers, or with several asteroids, the combined mass of which was comparable. The force of collision spewed large amounts of debris into the atmosphere, darkening the skies for several years before the finer particles settled. The reduced level of photosynthesis led to a massive decline in plant life of all kinds, and this caused massive starvation first of herbivores and subsequently of carnivores. The mass extinction would have occurred very suddenly under this hypothesis.
8. The phrase account for in the passage is closest in meaning to
○describe
○challenge
○explain
○test
Paragraph 6: One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir). Earth’s crust contains very little of this element, but most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe. A search of sedimentary deposits that span the boundary between the Cretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly and precisely at this boundary. This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered.
9. According to paragraph 6, what made iridium a useful test of the Alvarez hypothesis?
○Its occurrence in a few locations on Earth against several locations on other planets
○Its occurrence in limited quantities on Earth against its abundance in asteroids
○Its ability to remain solid at extremely high temperatures
○Its ease of detection even in very small amounts
10. In stating that no asteroid itself has ever been recovered, the author emphasizes which of the following?
○The importance of the indirect evidence for a large asteroid
○The fact that no evidence supports the asteroid？impact hypothesis
○The reason many researchers reject the Alvarez hypothesis
○The responsibility of scientists for not making the effort to discover the asteroid itself
Paragraph 7: An asteroid of this size would be expected to leave an immense crater, even if the asteroid itself was disintegrated by the impact. The intense heat of the impact would produce heat-shocked quartz in many types of rock. Also, large blocks thrown aside by the impact would form secondary craters surrounding the main crater. To date, several such secondary craters have been found along Mexico’s Yucatan Peninsula, and heat-shocked quartz has been found both in Mexico and in Haiti. A location called Chicxulub, along the Yucatan coast, has been suggested as the primary impact site.
11. The word intense in the passage is closest in meaning to
○ sudden
○ unusual
○ immediate
○ extreme
12. What is the purpose of paragraph 7 in the passage?
○It proposes a decisive new test of the Alvarez hypothesis.
○It presents additional supporting evidence for the Alvarez hypothesis.
○It explains why evidence relating to the Alvarez hypothesis is hard to find.
○It shows how recent evidence has raised doubts about the Alvarez hypothesis.
Paragraph 1: ■Cases in which many species become extinct within a geologically short interval of time are called mass extinctions. ■There was one such event at the end of the Cretaceous period (around 70 million years ago). ■There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago). ■The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time.
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
In general, it is believed that these two extinctions resulted from drastic environmental changes that followed meteorite impacts or massive volcanic eruptions.
Where would the sentence best fit?
14. 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.
There have been many attempts to explain the causes of mass extinctions.
●
●
●
Answer Choices
○Asteroid impacts, evolutionary developments, and changes in Earth's climate and in the positions of the continents have all been proposed as possible causes of mass extinctions.
○Researchers have observed 26-million-year cycles in extinction rates of a number of fossil groups that could all be attributed to the same cause.
○According to the Alvarez hypothesis, much of the iridium originally present on Earth was thrown into the atmosphere as a result of an asteroid impact that also caused a mass extinction.
○The unusual distribution of iridium on Earth and the presence of craters and heat-shocked quartz are central to the theory that an asteroid impact caused the late Cretaceous event.
○The collision between Earth and a large asteroid resulted in massive damage and generated enough heat to cause irreversible changes in Earth's atmosphere.
○There was a particularly large mass extinction that occurred around 250 million years ago at the end of the Permian period, whose cause could not be determined.
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14. Asteroid impacts, evolutionary…
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地質(zhì)年代中，在一個(gè)短期的時(shí)間間隔有大量物種滅絕的現(xiàn)象就被稱為大規(guī)模物種滅絕。在白堊紀(jì)時(shí)期后期（大約七千萬年前）就曾經(jīng)發(fā)生過一次大規(guī)模物種滅絕。而在二疊紀(jì)時(shí)期后期（大約兩億五千萬年前）還發(fā)生過一次規(guī)模更大的物種滅絕。由于當(dāng)時(shí)滅絕的物種很少為人類所知，所以二疊紀(jì)時(shí)期的大規(guī)模物種滅絕受到的關(guān)注遠(yuǎn)遠(yuǎn)不如其他幾次大規(guī)模物種滅絕。
化石記錄顯示，歷史上至少發(fā)生過五次大規(guī)模物種滅絕，造成大批海洋生物消亡。如今物種滅絕的比率和之前五次大規(guī)模物種滅絕時(shí)期一樣高。因此許多科學(xué)家推斷：當(dāng)前，第六次大規(guī)模物種處于滅絕狀態(tài)。
是什么原因引起如此高的物種滅絕率呢？有幾種假說，包括：地球變暖或變冷；季節(jié)型波動(dòng)的改變或洋流變化；大陸位置移動(dòng)。生物假說包括因昆蟲與開花植物之間的合作式進(jìn)化或海洋底層肉食動(dòng)物進(jìn)化引起的生態(tài)變化。這些生物機(jī)制，有些在極短的時(shí)間內(nèi)就會(huì)滅絕，而有些則很有可能經(jīng)過長時(shí)期在不同時(shí)代或不同大陸緩慢地進(jìn)行。有些假說未能解釋在陸地和海洋同時(shí)發(fā)生的物種滅絕。可能每次大規(guī)模物種滅絕都有不同的原因。但有證據(jù)指出，人類狩獵以及人類破壞棲息地很可能是當(dāng)前大規(guī)模物種滅絕的原因。
美國古生物學(xué)家David Raup 和 John Sepkoski 曾經(jīng)從大量化石群中研究物種滅絕的比率。他們指出，自從白堊紀(jì)時(shí)期中期以來，滅絕的物種不斷增多，大約每隔兩千六百萬年就會(huì)定期發(fā)生一次。白堊紀(jì)時(shí)期后期的恐龍和菊石（的滅絕是一系列此類周期性物種滅絕中更為劇烈的一次。對于周期性出現(xiàn)大規(guī)模物種滅絕的可能性，引發(fā)了這樣的假設(shè)：一顆具有長周期軌道的伴星體使其他天體從正常軌道偏離，導(dǎo)致其中一些天體變成流星掉落到地球，撞擊時(shí)造成大范圍破壞。
各種假說都試圖對白堊紀(jì)時(shí)期后期物種滅絕做出解釋，近年來Luis 和 Walter Alvarez最先提出的小行星撞擊假說備受人們關(guān)注。根據(jù)這一假說，地球與一個(gè)直徑約為10公里的小行星或者總體積與之相當(dāng)?shù)膸讉�(gè)小行星發(fā)生碰撞。碰撞的力量把大量碎片噴射到大氣中，在這些細(xì)小顆粒沉積之前好幾年的時(shí)間里天空都是灰蒙的。光合作用減弱會(huì)造成各種植物的生命大規(guī)模下降。這首先會(huì)造成草食動(dòng)物大規(guī)模餓死，接著就是肉食動(dòng)物大規(guī)模餓死。按照這種假說，大規(guī)模物種滅絕就會(huì)突然間發(fā)生。
Alvarez 假說基于稀土元素銥的出現(xiàn)做了一個(gè)非常有趣的實(shí)驗(yàn)。這種元素在地殼中的含量極少，但在大多數(shù)小行星中的含量卻多得多。因小行星碰撞而被拋進(jìn)到大氣中的碎片可能會(huì)含有大量銥元素，并且大氣流會(huì)把這些物質(zhì)帶到全球各地。白堊紀(jì)時(shí)期與第三紀(jì)時(shí)期交替之間的沉積物的研究顯示：在這兩個(gè)時(shí)期的交替時(shí)期，銥元素的含量急劇增加。盡管還沒有發(fā)現(xiàn)過撞擊的小行星，銥元素異常卻為Alvarez假說提供了有力支持。
按理說，這樣大小的一個(gè)行星，即使受到?jīng)_擊碎裂之后也會(huì)留下一個(gè)巨大的隕石坑。撞擊所釋放的極度高溫使得許多種巖石產(chǎn)生熱沖擊石英。撞擊也會(huì)將一些大石塊拋出去，在主要隕石坑周圍形成次級隕石坑。迄今為止，人們沿著墨西哥尤卡坦半島已經(jīng)找到了一些此類次級隕石坑。并且在海地和墨西哥找到了熱沖擊石英。尤卡坦沿海一個(gè)叫做�？颂K魯伯的地方，曾被認(rèn)為是主要的撞擊點(diǎn)。
菊石：軟體動(dòng)物門頭足綱的一個(gè)亞綱。是已絕滅的海生無脊椎動(dòng)物，生存于中奧陶世至晚白堊世。
