Title：Extinctions at the End of the Cretaceous

　　It has long been recognized that the dinosaurs disappeared from the fossil record at the end of the Cretaceous period (65 million years ago), and as more knowledge has been gained, we have learned that many other organisms disappeared at about the same time. Themicroscopic plankton (free-floating plants and animals) with calcareous shells suffered massively.The foundation of the major marine food chain that led from the minute plankton to shelled animals to large marine reptiles had collapsed.

　　On land it was not only the large animals that became extinct. The mammals, most ofwhich were small, lost some 35 percent of their species worldwide. Plants were also affected. For example, in North America 79 percent did not survive, and it has been noted that the survivors were often deciduousthey could lose their leaves and shut downwhile others could survive as seeds. As in the sea, it seems that on the land one key food chain collapsed: the one with leaves as its basic raw material. These leaves were the food of some of the mammals and of the herbivorous dinosaurs, which in turn were fed on by the carnivorous dinosaurs. Furthermore, it is most likely that these large dinosaurs had slow rates of reproduction, which always increases the risk of extinction. Crocodiles, tortoises, birds, and insects seem to have been little affected. The two first named are known to be able to survive for long periods without food, and both can be scavengers (feed on dead material). Indeed, with the deaths of so many other animals and with much dead plant material, the food chain based on detritus would have been well-supplied. Many insects feed on dead material; furthermore, most have at least one resting stage in which they are very resistant to damage. In unfavorable conditions some may take a long time to develop: there is a record of a beetle larva living in dead wood for over 40 years before becoming an adult. Some birds were scavengers, but the survival of many lineages is a puzzle.

　　What happened in the biological story just after these extinctionswhat is found in and just above the boundary layer between the deposits of the Cretaceous and those of the Tertiary (65¨C2.6 million years ago), termed the K/T boundary For a very short period the dominant microorganisms in marine deposits were usually diatoms and dinoflagellates(both single-celled types of plankton).The important feature for the survival of both these groups was the ability to form protective cysts (sacs around organisms) that rested on the sea floor. Above these, in the later deposits, are the remains of other minute plankton, but the types are quite different from those of the Late Cretaceous. In terrestrial deposits a sudden and dramatic increase in fern plant spores marks the boundary in many parts of the world; ferns are early colonizers of barren landscapes. The fern spike (sudden increase), as it istermed, has been found also in some marine deposits (such was the abundance of fern spores blown around the world), and it occurs in exactly the same layer of deposit where the plankton disappear. We can conclude that the major marine and terrestrial events occurred simultaneously.

　　Many theories have been put forward for the extinction of the dinosaurs, but most of them can be dismissed. Since 1980 there have been more focused, but still controversy-ridden, investigations. In that year Louis and Walter Alvarez and colleagues from the University of California published their research on the amounts of various metals in the boundary between Cretaceous and Tertiary rocks (K/T boundary) in Italy, Denmark, and New Zealand. Theyhad found, accidentally, that a rare metal, iridium, suddenly became very abundant exactly at the boundary and then slowly fell away. This phenomenon, known as the iridium spike, has now been identified in K/T boundary deposits in over a hundred other sites in the world. Iridium occurs in meteorites and volcanic material, but in the latter case it is accompanied by elevated levels of nickel and chromium. These other metals are not especially abundant at the K/T boundary. The Alvarezes concluded that the iridium spike was due to a large asteroid that struck Earth 65 million years ago.

　　1. The word recognized in the passage is closest in meaning to

　　A. suspected

　　B. argued

　　C. assumed

　　D. accepted

　　2. According to paragraph 1, which of the following was true of small planktonic organisms at the end of the Cretaceous A. They lost their main sources of food.

　　B. They developed calcareous shells.

　　C. They decreased greatly in number as did many other types of organisms.

　　D. They replaced other minute organisms as a food source in the major marine food chain

　　3. Which of the following statements is NOT supported by the information provided in paragraph 2 about extinctions at the end of the Cretaceous

　　A. About 35 percent of mammal species were lost.

　　B. 79 percent of North American plants disappeared.

　　C. Most birds, tortoises, and crocodiles escaped extinction.

　　D. Deciduous trees were especially likely to go extinct.

　　4. According to paragraph 2, which of the following factors probably contributed to the extinction of the dinosaurs

　　A. The length of time it took dinosaurs to reproduce

　　B. Large quantities of dead material disturbing their habitats

　　C. Increased competition for food from scavengers

　　D. An increase in carnivore populations

　　5. In paragraph 2, why does the author provide the information that there is a record of a beetle larva living in dead wood for over 40 years before becoming an adult

　　A. To help explain why insects were less likely to go extinct than other species

　　B. To show that not all species that relied on trees disappeared during the late Cretaceous

　　C. To suggest that insects that lived long ago had much longer life spans than those living today

　　D. To support the claim that conditions at the end of the Cretaceous were highly unfavorable

　　6. The word simultaneously in the passage is closest in meaning to

　　A. rapidly

　　B. repeatedly

　　C. at the same time

　　D. for different reasons

　　7. According to paragraph 3, which of the following is true of plankton after the extinctions at the end of the Cretaceous A. Diatoms and dinoflagellates suddenly became very rare.

　　B. Single-celled types of plankton were replaced by more complicated microorganisms.

　　C. The plankton found in later deposits are closely related to single-celled Late Cretaceous microorganisms.

　　D. The only types of Late Cretaceous plankton to survive extinction were those protected by cysts.

　　8. 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.

　　A. The fern spike is the term given to this sudden increase in the abundance of fern spores blown into the oceans around the world.

　　B. The sudden increase in fern spores occurred in exactly the same parts of the world where the plankton was disappearing.

　　C. Fern spores have even been found in some marine deposits from exactly the same layer as that showing the disappearance of the plankton.

　　D. Most marine deposits from around the world contain either plankton or a lot of fern spores but not both together in the same layer.

　　9. The word dismissed in the passage is closest in meaning to

　　A. further explored

　　B. rejected

　　C. completely revised

　　D. replaced

　　10. The word elevated in the passage is closest in meaning to

　　A. high

　　B. varying

　　C. unusual

　　D. adequate

　　11. According to paragraph 4, what evidence is there that an asteroid hit Earth

　　A. The fact that the metals nickel and chromium were found at sites around the world

　　B. The presence in K/T boundary deposits of large amounts of a metal that is found in meteorites

　　C. The fact that iridium amounts decreased at the same time that the Cretaceous ended and the Tertiary began

　　D. The differences in the types of metals found in deposits in Italy, Denmark, and New Zealand

　　12. Paragraph 4 suggests which of the following about the Alvarezes' theory

　　A. Their theory was accepted until 1980, when more focused investigations showed it to be incorrect.

　　B. It rules out the possibility that the iridium spike was the result of volcanic activity.

　　C. Before it can be accepted, more evidence must be gathered from locations outside Europe and New Zealand.

　　D. Experts believe the research done by the Alvarezes was too broad.

　　13. Look at the four squares that indicate where the following sentence could be added to the passage. This change in plankton found in marine deposits is what marks the boundary between the Cretaceous and the Tertiary. Where would the sentence best fit? Click on a square to add the sentence to the passage.

　　(题目不完整)

　　14. Drag your choices to the spaces where they belong. To review the passage, click on View Text.

　　Answer Choices A. Aside from the dinosaurs, most of the organisms affected were very small, such as single-celled plankton and insects.

　　B. Herbivores and carnivores were both affected, but the hardest-hit group was the scavengers, including many birds and some mammals.

　　C. Two major food chains were eliminated, one in the oceans based on plankton, and one on land based on leaves.

　　D. Examinations of marine and terrestrial deposits show clearly that ocean extinctions occurred well before those on land. E. In geological samples from around the world, the K/T boundary is marked by a sudden change in plankton and by a spike in fern spores.

　　F. Sudden, high levels of iridium found at the K/T boundary suggest that a meteorite might have caused the extinctions.