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
52
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
HowtoConquerPublicSpeakingFearⅠ.IntroductionA.Publicspeaking—acommonsourceofstressforeveryoneB.Thetr
Bigcrimesdeservetoughresponses.Inanycountrythetheftandpublicationof250,000secretgovernmentdocumentswoulddeserv
Bigcrimesdeservetoughresponses.Inanycountrythetheftandpublicationof250,000secretgovernmentdocumentswoulddeserv
MadeleinehadmetLeonardinanupper-levelsemioticsseminartaughtbyarenegadefromtheEnglishdepartment.MichaelZipperst
Whathandshakemean?Inone’sowncountry,1.Insomecultures,people—shakehands—bowstoeachother—【1】
KatharineJeffertsSchoricalledonthepeopletoconcentrateon______.
TheoriesofHistoryⅠ.Howmuchweknowabouthistory?A.Writtenrecordsexistforonlyafractionofman’stimeB.Theacc
ItwasadaythatMichaelEisnerwouldundoubtedlyliketoforget.SittinginaLosAngeleswitnessboxforfourhourslastweek
ConsideringhowjazzistranscribedinChinese(jueshi),youmaybemisledintoassumingthatitisanaristocraticculturalfor
ConsideringhowjazzistranscribedinChinese(jueshi),youmaybemisledintoassumingthatitisanaristocraticculturalfor
随机试题
成本领先战略
蛋白质的空间构象主要取决于
苍耳子和辛夷功效的共同点
下列属于残疾的第三级预防的是
城市规划作为国家的宏观调控的手段,其作用更多地体现在:(1)提供城市社会发展的保障措施(2)作为平衡城市各集团利益的措施(3)为各城市的协调发展提供基础(4)保证土地的使用更符合国家利益(5)在修正市场失败的基础上支持土地和房地产市
某饭店有380间客房,全部客房年度固定成本总额为3600万元,单位变动成本为120元/间,预测出租率85%,年度利润目标为1200万元,则该饭店客房的保本点价格约为()元。
2016年7月2日,甲公司向乙公司订购一套总价值150万元的精密仪器设备,双方签订了买卖合同,约定由乙公司代办托运。7月8日,甲公司为筹集购买精密仪器设备的货款,通过X借贷平台向丙公司借款100万元,双方当日签订了借款合同,约定借款期限为3个月,自资金到达
下列几种搭配不能正确体现聚落的建筑风格与自然环境关系的是()。
在城市中,高层建筑越来越多,一旦发生火灾,下列做法不正确的是()
WhichofthefollowingisNOTtrueaccordingtothepassage?
最新回复
(
0
)