首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
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
admin
2013-06-12
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
不消说,相识的人数是随了年龄增加的,一个人年龄越大,走过的地方、当过的职务越多,相识的人理该越增力了。可是,相识的人并不就是朋友。我们和许多人相识,或是因为了事务关系,或是因了偶然的机缘棗如在别人请客的时候同席吃过饭之类。见面时点头或握手,有事时走访或通信
ThelargestcityinNewZealandis
Ifadultslikedtoreadbooksthatwereexceedinglydifficult,they’dallbereadingProust.Mostdon’t.Sowhy,readingex
Ifadultslikedtoreadbooksthatwereexceedinglydifficult,they’dallbereadingProust.Mostdon’t.Sowhy,readingex
Althoughitmighthavehappenedanywhere,myencounterwiththegreenbananastartedonasteepmountainroadintheinteriorof
Morethananyothercountry,Americadefinesitselfbyacollectivedream:thedreamofeconomicopportunityandupwardmobility
WhichisthecorrectinformationabouttheBest361CollegessurveycarriedoutbythePrincetonReview?
Inmanyclassroomsaroundthecountry,teachersareemphasizing,andperiodicallytesting,students’readingfluency,thecurren
ConsideringhowjazzistranscribedinChinese(jueshi),youmaybemisledintoassumingthatitisanaristocraticculturalfor
SteveandYaserfirstmetintheirchemistryclassofanAmericanuniversity.YaserwasaninternationalstudentfromJordan.He
随机试题
上消化道出血可单纯表现为呕血或黑便,也可两者兼有,这取决于
以下关于氧气供气系统叙述正确的为()。
下列基础中不属于深基础的是:
债券筹资的优点包括()。
当个人贷款业务银行问竞争十分激烈时,当地的收入房价比又远低于全国平均水平,有位个人VIP客户提出可在银行购买适量基金和保险产品,但二套房贷款利率优惠至基准利率,银行客户经理可同意受理。()
质量委员会的基本职责是()质量改进工作并使其制度化。
学生发展的可能性和可塑性转变为现实的条件是()
试述教师备课的基本内容。
信息安全包括物理安全,安全控制和【 】。
进了海滨浴场,大海一下子呈(1)在我们面前。大海真美呀!远远望去,浩荡的海水无边无际,几(2)分不清哪里是水哪里是天,白色的海(3)齐刷刷地扑向海滩,一批(4)失了,又过来一批……辽阔的沙滩被冲得像溜冰场一样平整、光滑;沙滩上,穿着各色
最新回复
(
0
)