A
Everybody knows that the dinosaurs werekilled by an asteroid. Somethingbig hit the earth 65 million years ago and,when the dust had fallen, so had the great reptiles. There is thus a nice, ifironic, symmetry in the idea that o similar impact brought about thedinosaurs’rise. That is the thesis proposed by Paul Olsen, of ColumbiaUniversity, and his colleagues in this week’s Science.
B
Dinosaurs first appear in the fossil record230m years ago, during the Triassic period. But they were mostly small, andthey shared the earth with lots of other sorts of reptile. It was in thesubsequent Jurassic, which began 202million years ago, that they overran theplanet and turned into the monsters depicted in the book and movie “JurassicPark”(侏罗纪公园) .(Actually, though, the dinosaurs that appeared on screen were from the stillmore recent Cretaceous (白垩纪) period.) Dr Olsen and his colleagues are not the first to suggestthat the dinosaurs inherited the earth as the result of an asteroid strike. Butthey are the first to show that the takeover did, indeed, happen in ageological eyeblink.
C
Dinosaur skeletons are rare. Dinosaurfootprints are, however, surprisingly abundant. And the sizes of the prints areas good an indication of the sizes of the beasts as are the skeletonsthemselves. Dr Olsen and his colleagues therefore concentrated on prints, notbones.
D
The prints in question were made in easternNorth America, a part of the world then full of rift valleys similar to thosein East Africa today. Like the modern African rift valleys, the Triassic (三叠纪) /Jurassic American onescontained lakes, and these lakes grew and shrank at regular intervals becauseof climatic changes caused by periodic shifts in the earth’s orbit. (Asimilarphenomenon is responsible for modern ice ages.) That regularity, combined withreversals
in the earth’s magnetic field, which are detectable in the tiny fields ofcertain magnetic minerals, means that rocks from this place and period can bedated to within a few thousand years. As a bonus, squishy (adj.粘糊糊的) lake-edge sediments are justthe things for recording the tracks of passing animals. By dividing the labourbetween themselves, the ten authors of the paper were able to study such tracksat 80 sites.
E
The researchers looked at 18 so-calledichnotoxo(群落). These arerecognisable types of footprint that cannot be matched precisely with thespecies of animal that left them. But they can be matched with a general sortof animal, and thus act as an indicator of the fate of that group, even whenthere are no
bones to tell the story.
F
Five of the ichnotaxa disappear before theend of the Triassic, and four march confidently across the boundary into theJurassic. Six, however, vanish at the boundary, or only just splutter acrossit; and three appear from nowhere, almost as soon as the Jurassic begins.
G
That boundary itself is suggestive. Thefirst geological indication of the impact that killed the dinosaurs was anunusually high level of iridium in rocks at the end of the Cretaceous, when thebeasts disappear from the fossil record. Iridium is normally rare at theearth’s surface, but it is more abundant in meteorites. When people began tobelieve the impact theory, they started looking for other Cretaceous-end anomalies. One that turned up was a surprising abundance of fern spores in rocks Just abovethe boundary layer-a phenomenon known as a “fern spike”(蕨类）
H
That matched the theory nicely. Many modernferns are opportunists. Theycannot compete against plants with leaves, but if apiece of land is cleared by, say, a volcanic eruption, they are often the firstthings to set up shop there. An asteroid strike would have scoured much of theearth of its vegetable cover, andprovided a paradise for ferns. A fern spike inthe rocks is thus a good indication that something terrible has happened.
I
Both an iridium (铱) anomaly and a fern spike appearin rocks at the end of the Triassic, too. That accounts for the disappearingichnotaxa: the creatures that made them did not survive the holocaust. Thesurprise is how rapidly the new ichnotaxa appear. Eubrontes giganteus, forexample, is there a mere 10,000 years after the iridium anomaly. The Eubrontes(一种大脚印）printswere made by theropods-the dinosaur group that went on to produce suchnightmares as Allosaurus(异龙)and Tyrannosaurus(暴龙) -and Eubrontes is already 20% bigger than any theropod trackrecorded from the Triassic.
J
DrOlsen and His colleagues suggest that the explanation for this rapidincrease insize may be a phenomenon called ecological release. This is seen today whenreptiles (which, in modern times, tend io be small creatures) reach islandswhere they face no competitors. The most spectacular example is ontheIndonesian island of Komodo, where local lizards have grown so large thatthey are often referred to as dragons. The dinosaurs, in other words, couldflourish only when the competition had been knocked out.
K
That leaves the question of where theimpact happened. No large hole in theearth’s crust seems to be 202m years old.It may, of course, have been overlooked. Old craters are eroded and buried, andnot always easy to find. Alternatively, it may have vanished. Althoughcontinental crust is more or less permanent, the ocean floor is constantlyrecycled by the tectonic processes
that bring about continental drift. There is no ocean floor left that is morethan 200m years old, soa crater that formed in the ocean would have beenswallowed up by now.
L
There is a third possibility, however. Thisis that the crater is known, but has been misdated. The Manicouagan“structure”, a center in Quebec, is thought to be 214m years old. It ishuge-some 100km across-and seems to be the largest of between three and fivecraters that formed within a few hours of each other as the lumps of adisintegrated comet hit the earth one by one. Such an impact would surely havehad a perceptible effect on the world, but the rocks from 214m years ago do notrecord one. It is possible, therefore, that Manicouagan ( 根陨石坑) has been misdated. That willbe the next thing to check.
Question 1-6...................................................................................
Do the following statements agree with the information given in Reading Passage1?
In boxes1-6 on your answer sheet, write
TRUE if the sataement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if there is no information on this
1 Dr Paul Olsen and his colleagues believe that asteroid knock may also lead to
dinosaurs’boom
2 Books and movie like Jurassic Park often exaggerate the size of the
dinosaurs.
3 Dinosaur footprints are more adequate than dinosaur skeletons.
4 The prints were chosen by Dr Olsen to study because they are convenient totracked down into a date of geological precise within thousands years.
5 Ichnotaxa showed that footprints of dinosaurs offer exact information of thetrace left by an individual species.
6 We can find more Iridium in the earth’s surface than in meteorites.
Questions 7-13...............................................................................
Complete the following summary of the paragraphs of Reading Passage, using no
more than two words from the Reading Passage for each answer. Write youranswers in
boxes 7-13 0n your answer sheet.
Dr Olsen and his colleagues applied a phenomenon named 7 to explain the largesize of the Eubrontes, which is a similar case to that nowadays reptiles invadea place where there are no 8 ; for example, on an island called Komodo,indigenous huge lizards grow so big that people even referring them as9However, there were no old impact trace being found? The answer may be that wehave 10 the evidence. Old craters are difficult to spot or it probably 11 dueto the effect of the earth moving. Even a crater formed in Ocean had been 12under the impact of crust movement. Beside, the third hypothesis is that thepotential evidences- some craters may be 13 .
参考答案：
1. YES 2. NOT GIVEN 3. YES 4. YES 5. NO 6. NO
7. ecological release 8. competitor 9. dragons
10. overlooked 11. vanished 12. swallowed up 13. Misdated
第三篇：职场性格
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发表于 2018-11-22 09:53:56
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