2. 职称英语
  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2023-12-25  38
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] In the second paragraph, 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?
选项 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.
答案 A
解析 第2段倒数第3句提到，在休眠期，昆虫对外来损害具有很强的抵抗力。第2段倒数第2句提到，在不利的条件下，昆虫发育的时间会很长，由此可推断，作者说甲虫幼虫在枯木中生活了40多年才发育成成虫是为了作为例子来解释说明昆虫在不利的条件下可以存活下来的原因，故A项正确。B项“以证明并非所有依赖树木的物种都在白垩纪晚期消失了”、C项“表明生活在很久以前的昆虫比现在的昆虫寿命长得多”均不符合题意。D项“支持白垩纪末期环境非常不利的观点”，文中没有说明这个记录来源于哪个时候，只说到有这样一个记录(there is a record of)。因此也不选D项。
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