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  3. It has long been recognized that the dinosaurs disappeared from the fossil r

It has long been recognized that the dinosaurs disappeared from the fossil r

游客2025-01-19  23
问题     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. The microscopic 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 of which 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 deciduous—they could lose their leaves and shut down—while 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. 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 extinctions? What is found in and just above the boundary layer between the deposits of the Cretaceous and those of the Tertiary 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 is termed, 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. In 1980, Louis Alvarez 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. They had 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. [br] According to the first paragraph, which of the following was true of small planktonic organisms at the end of the Cretaceous?
选项 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.
答案 C
解析 第1段第1句提到在白垩纪晚期，恐龙以及许多其他的生物几乎同时消失了。紧接着第2、3句便是对此的进一步描述，从suffered massively和The foundation of…had collapsed可知，小型浮游生物和其他生物体一样也处在消失的状态中，数量急剧减少，因此C项表述与此相符。A项“它们失去了主要的食物来源”、B项“它们长出了钙质的外壳”和D项“它们取代了其他微型生物作为主要海洋食物链的食物来源”均与文章内容不符，可排除。
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