托福阅读真题Official 44 Passage 1（三）

托福阅读真题Official 44 Passage 1（三）

From Fish to Terrestrial Vertebrates

One of the most significant evolutionary events that occurred on Earth was the transition of water-dwelling fish to terrestrial tetrapods(four-limbed organisms with backbones).Fish probably originated in the oceans,and our first records of them are in marine rocks.However,by the Devonian Period(408 million to 362 million years ago),they had radiated into almost all available aquatic habitats,including freshwater settings.One of the groups whose fossils are especially common in rocks deposited in fresh water is the lobe-finned fish.

The freshwater Devonian lobe-finned fish rhipidistian crossopterygian is of particular interest to biologists studying tetrapod evolution.These fish lived in river channels and lakes on large deltas.The delta rocks in which these fossils are found are commonly red due to oxidized iron minerals,indicating that the deltas formed in a climate that had alternate wet and dry periods.If there were periods of drought,any adaptations allowing the fish to survive the dry conditions would have been advantageous.In these rhipidistians,several such adaptations existed.It is known that they had lungs as well as gills for breathing.Cross sections cut through some of the fossils reveal that the mud filling the interior of the carcass differed in consistency and texture depending on its location inside the fish.These differences suggest a saclike cavity below the front end of the gut that can only be interpreted as a lung.Gills were undoubtedly the main source of oxygen for these fish,but the lungs served as an auxiliary breathing device for gulping air when the water became oxygen depleted,such as during extended periods of drought.So,these fish had already evolved one of the prime requisites for living on land:the ability to use air as a source of oxygen.

A second adaptation of these fish was in the structure of the lobe fins.The fins were thick,fleshy,and quite sturdy,with a median axis of bone down the center.They could have been used as feeble locomotor devices on land,perhaps good enough to allow a fish to flop its way from one pool of water that was almost dry to an adjacent pond that had enough water and oxygen for survival.These fins eventually changed into short,stubby legs.The bones of the fins of a Devonian rhipidistian exactly match in number and position the limb bones of the earliest known tetrapods,the amphibians.It should be emphasized that the evolution of lungs and limbs was in no sense an anticipation of future life on land.These adaptations developed because they helped fish to survive in their existing aquatic environment.

What ecological pressures might have caused fishes to gradually abandon their watery habitat and become increasingly land-dwelling creatures.Changes in climate during the Devonian may have had something to do with this if freshwater areas became progressively more restricted.Another impetus may have been new sources of food.The edges of ponds and streams surely had scattered dead fish and other water-dwelling creatures.In addition,plants had emerged into terrestrial habitats in areas near streams and ponds,and crabs and other arthropods were also members of this earliest terrestrial community.Thus,by the Devonian the land habitat marginal to freshwater was probably a rich source of protein that could be exploited by an animal that could easily climb out of water.Evidence from teeth suggests that these earliest tetrapods did not utilize land plants as food;they were presumably carnivorous and had not developed the ability to feed on plants.

How did the first tetrapods make the transition to a terrestrial habitat.Like early land plants such as rhyniophytes,they made only a partial transition;they were still quite tied to water.However,many problems that faced early land plants were not applicable to the first tetrapods.The ancestors of these animals already had a circulation system,and they were mobile,so that they could move to water to drink.Furthermore,they already had lungs,which rhipidistians presumably used for auxiliary breathing.The principal changes for the earliest tetrapods were in the skeletal system—changes in the bones of the fins,the vertebral column,pelvic girdle,and pectoral girdle.

Question 5 of 14

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage?Incorrect choices change the meaning in important ways or leave out essential information.

A.Because the lungs of these fish were able to provide only a small amount of oxygen,these fish obtained most of their oxygen through their gills during periods of drought.

B.During periods of extended drought,these fish used their lungs to increase their intake of oxygen beyond the levels absorbed by the gills in normal times.

C.Although these fish primarily used their gills to obtain oxygen,they used their lungs to obtain oxygen from the air when there was not enough in the water.

D.During periods of extended drought,the gills became an auxiliary breathing device and the lungs became the main source of oxygen for these fish.

正确答案：C

题目详解

题型分类：简化句子题

原文分析：本题型要从长难句的内容和逻辑上进行分析。前半句主干信息是鳃肯定是鱼获取氧气的主要来源，这一点毫无置疑。后半句用but来起强调作用，说明肺的主要任务是当鱼处于缺氧的水中时，它会起到辅助呼吸的作用。

选项分析：

C选项符合原句的逻辑和主干内容。Although对应原句中but表示的转折关系，primarily used their gills对应原句中the main source of oxygen，因此前半句关于gills的功能描述符合原句。后半句说明lungs在when there was not enough in the water时的功能是obtain oxygen from the air，对应原句中gulping air when the water became oxygen depleted，符合原句的后半部分内容。

A选项是逻辑错误，原文用but表示转折强调；选项中变为because表达因果关系。

B选项是内容错误，说在干旱时期，鱼会使用肺来获取氧气，并且氧气量要超过正常时期鳃获取的量。原文中无此意。

D选项是内容错误，说干旱时期，鳃变成了辅助，肺成了主要的，而原文中说的是鳃始终是主要呼吸工具，肺是辅助的。因此该选项与原句表述不符。

Question 6 of 14

According to paragraph 3,the structure of the fins of rhipidistian crossopterygian may have allowed these fish to

A.reduce the amount of oxygen needed for survival

B.develop thick,sturdy bones

C.move more efficiently in water

D.move short distances over areas that were mostly dry

正确答案：D

题目详解

题型分类：事实信息题

原文定位：定位词：the structure of the fins。利用定位词定位到第三段前三句话。第一句说明这是鱼类进化的第二个适应性。第二句介绍了fins的生理特征。第三句说明fins可以被作用一种陆上运动装备，可以使鱼从一个将近干涸的池塘跳到邻近的有充足水源和氧气的池塘中去存活。

选项分析：

D选项说可以使鱼短距离地移动离开几乎干涸的区域，move short distances对应原句allow a fish to flop its way from one pool...to an adjacent pond...，that were mostly dry对应原句from one pool of water that was almost dry。因此该选项符合定位句的表达。

A选项说可以使鱼减少生存所需的氧气量。无中生有。

B选项说可以使鱼长出厚实的，强壮的骨骼。而原文说The fins were thick,fleshy,and quite sturdy,with a median axis of bone down the center并没有说bones是sturdy。与原文描述不符。

C选项说可以使鱼在水中移动得更有效率。无中生有。

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