2. 外语
  3. Disaster struck 250 million years ago, when the worst devastation in the earth’s history occurred. Called the end-Permian mass e

Disaster struck 250 million years ago, when the worst devastation in the earth’s history occurred. Called the end-Permian mass e

admin2013-06-12  70
问题    Disaster struck 250 million years ago, when the worst devastation in the earth’s history occurred. Called the end-Permian mass extinction, it marks a fundamental change in the development of life.
   The history of life on the earth is replete with catastrophes of varying magnitudes. The one that has captured the most attention is the extinction of the dinosaurs and other organisms 65 million years ago between the Cretaceous and Tertiary periods--which claimed up to half of all species. As severe as that devastation was, it pales in comparison to the greatest disaster of them all: the mass extinction some 250 million years ago, at the end of the Permian period.  Affectionately called "the mother of mass extinctions" among paleontologists (with apologies to Saddam Hussein), it yielded a death toll that is truly staggering. About 90 percent of all species in the oceans disappeared during the last several million years of the Permian. On land, more than two thirds of reptile and amphibian families vanished. Insects, too, did not escape the carnage: 30 percent of insect orders ceased to exist, marking the only mass extinction insects have ever undergone.
   But from catastrophes, opportunities arise. For several hundred million years before the end-Permian event,  the shallow seas had been dominated by life-forms that were primarily immobile. Most marine animals lay on the seafloor or were attached to it by stalks, filtering the water for food or waiting for prey. In the aftermath of the extinction, many once minor groups-active, predatory relatives of modern-day fish, squids, snails and crabs —were able to expand. Some completely new lineages appeared. This ecological reorganization was so dramatic that it forms a fundamental boundary in the history of life.  Not only does it demarcate the Permian and Triassic periods, it also establishes the close of the Paleozoic era and the start of the Mesozoic era. The modern tidal pool reflects what lived and what died 250 million years ago.
   Over the past few years, exciting new insights into the causes and consequences of the end-Permian mass extinction have poured in from virtually every branch of the earth sciences Some of these findings include  detailed studies of rapid changes in ocean chemistry, more thorough documentation of extinction patterns and new analyses showing that large volcanic eruptions occurred at the Permo-Triassic boundary.
   How much do mass extinctions contribute to the evolution of a group, as compared with long-term adaptive trends? For example, sea urchins are ubiquitous in modern oceans but were relatively uncommon during the Permian. Only a single genus, Miocidaris, is known for certain to have survived the extinction. Did Mioeidaris survive by pure chance, or was it better adapted? Would sea urchins today look any different had it not been for the end-Permian extinction?
   To resolve such questions, we need to learn more about the causes of the catastrophe and how those species that survived differed from those that disappeared.  The key sources for this information are rock layers and fossils.  Unfortunately, samples from the late Permian and early Triassic are notoriously difficult to come by. The fossil record across the boundary is plagued by poor preservation, a lack of rock to sample and other problems, including access.  An extensive drop in sea level during the late Permian limited the number of marine rocks deposited on land, and many areas where the best rocks were preserved (most notably, in southern China) have been relatively hard for some geologists to reach.
   As such, it has proved difficult to ascertain just how quickly life was snuffed out or if the deaths were subject to any regional variations. Some creatures, especially those sensitive to changes in the environment, died off rapidly, as shown by Erik Flugel and his colleagues at the University of Erlangen, who arrived at this conclusion after examining reefs in southern China and Greece. Other evidence indicates more gradual loss of life.
   Intensive studies of newly found and critical boundary layers in Italy, Austria and southern China have helped our understanding. They indicate that the duration of the extinction is shorter than previously thought, implying that abruptly calamitous environmental conditions must have set in.
In the mass extinction 65 million years ago, ______ of all species on earth vanished.
选项 A、two thirds
B、half
C、90%
D、30%
答案B
解析
转载请注明原文地址:https://kaotiyun.com/show/t54O777K
0

专业英语八级

相关试题推荐
随机试题
最新回复(0)