托福閱讀真題Official 44 Passage 1（三）

托福閱讀真題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.

正確答案：C

題目詳解

題型分類：簡化句子題

原文分析：本題型要從長難句的內容和邏輯上進行分析。前半句主干信息是鰓肯定是魚獲取氧氣的主要來源，這一點毫無置疑。后半句用but來起強調作用，說明肺的主要任務是當魚處于缺氧的水中時，它會起到輔助呼吸的作用。

選項分析：

C選項符合原句的邏輯和主干內容。Although對應原句中but表示的轉折關系，primarily used their gills對應原句中the main source of oxygen，因此前半句關于gills的功能描述符合原句。后半句說明lungs在when there was not enough in the water時的功能是obtain oxygen from the air，對應原句中gulping air when the water became oxygen depleted，符合原句的后半部分內容。

A選項是邏輯錯誤，原文用but表示轉折強調；選項中變?yōu)閎ecause表達因果關系。

B選項是內容錯誤，說在干旱時期，魚會使用肺來獲取氧氣，并且氧氣量要超過正常時期鰓獲取的量。原文中無此意。

D選項是內容錯誤，說干旱時期，鰓變成了輔助，肺成了主要的，而原文中說的是鰓始終是主要呼吸工具，肺是輔助的。因此該選項與原句表述不符。

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

正確答案：D

題目詳解

題型分類：事實信息題

原文定位：定位詞：the structure of the fins。利用定位詞定位到第三段前三句話。第一句說明這是魚類進化的第二個適應性。第二句介紹了fins的生理特征。第三句說明fins可以被作用一種陸上運動裝備，可以使魚從一個將近干涸的池塘跳到鄰近的有充足水源和氧氣的池塘中去存活。

選項分析：

D選項說可以使魚短距離地移動離開幾乎干涸的區(qū)域，move short distances對應原句allow a fish to flop its way from one pool...to an adjacent pond...，that were mostly dry對應原句from one pool of water that was almost dry。因此該選項符合定位句的表達。

A選項說可以使魚減少生存所需的氧氣量。無中生有。

B選項說可以使魚長出厚實的，強壯的骨骼。而原文說The fins were thick,fleshy,and quite sturdy,with a median axis of bone down the center并沒有說bones是sturdy。與原文描述不符。

C選項說可以使魚在水中移動得更有效率。無中生有。

>> 雅思 托福 免費測試、量身規(guī)劃、讓英語學習不再困難<<