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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  25
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 third paragraph, which of the following is true of plankton after the extinctions at the end of the Cretaceous?
选项 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.
答案 D
解析 第3段开头是用自问自答的方式来说明白垩纪与第三纪的地层之间，K-T界线的沉积物层内部和上面有什么沉积物。其中第3、4句提到，在很短的一段时间里，硅藻和腰鞭毛虫是主要的海洋沉积物，因为它们能在海床上形成保护性囊泡，由此可判断，白垩纪末期能幸存下来的浮游生物是那些受囊泡保护的硅藻和腰鞭毛虫，因此D项正确。第3段第3句提到，在一段时间内，海洋沉积物中的微生物主要是硅藻和腰鞭毛虫，因此A项 “硅藻和腰鞭毛虫突然变得非常稀少”错误。第3段提到，在一段时间内海洋沉积物中主要是单细胞浮游生物 (硅藻和腰鞭毛虫)，之后是其他微型浮游生物(other minute plankton)，并未提到more complicated microorganisms “更复杂的微生物”，可排除B项。第3段第6句提到，之后在沉积物中发现的浮游生物残骸与白垩纪末期有很大的不同(the types are quite different from those of the Late Cretaceous)，因此C项 “在之后的沉积物中发现的浮游生物与白垩纪末期的单细胞微生物密切相关”错误，可排除。
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