托福閱讀真題Official 44 Passage 1（一）

托福閱讀真題Official 44 Passage 1（一）

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 1 of 14

Paragraph 1 supports which of the following statements about fish evolution?

A.Lobe-finned fish were among the earliest types of fish to appear.

B.Fish began living in freshwater habitats only after originating elsewhere.

C.Lobe-finned fish radiated into almost all available aquatic habitats.

D.During the Devonian,lobe-finned fish were more common in marine than in freshwater habitats.

正確答案：B

題目詳解

題型分類：事實(shí)信息題

原文定位：定位詞fish evolution。用定位詞定位到文章第一句，可以發(fā)現(xiàn)本段整體上都是介紹fish evolution的，因此本題可以分類為大定位事實(shí)信息題，需要查找的信息較多較瑣碎，需要閱讀全段，之后結(jié)合四個(gè)選項(xiàng)一一確認(rèn)。根據(jù)第一段第一句，了解fish evolution是一個(gè)意義重大的進(jìn)化事件，第二句說(shuō)明魚起源于海洋。第三句說(shuō)明到了Devonian Period，魚就擴(kuò)展到了包括淡水在內(nèi)的幾乎所有可利用的水生環(huán)境中。最后指出lobe-finned fish就是其中一種常見(jiàn)的例子。

選項(xiàng)分析：

B選項(xiàng)符合第二句和第三句，這里的elsewhere指的就是ocean。

A選項(xiàng)說(shuō)lobe-finned fish是出現(xiàn)的最早的魚類之一。無(wú)中生有。

C選項(xiàng)說(shuō)lobe-finned fish擴(kuò)展到幾乎所有的可利用的水生環(huán)境中，原文說(shuō)的是fish。與原文表述不符。

D選項(xiàng)做比較說(shuō)，比起淡水環(huán)境，lobe-finned在海洋中更常見(jiàn)，無(wú)中生有。

Question 2 of 14

According to paragraph 2,what do the minerals in the delta rocks containing rhipidistian crossopterygian fossils reveal?

A.These deltas formed in dry periods but gradually became wetter.

B.These deltas contain different types of iron minerals than do the surrounding areas.

C.Most rhipidistian crossopterygian fish died when the climate became dry.

D.Rhipidistian crossopterygian fish lived in areas that experienced alternate dry and wet periods.

正確答案：D

題目詳解

題型分類：事實(shí)信息題

原文定位：定位詞：the minerals in the delta rocks。用定位詞定位到文章第二段第三句，本句說(shuō)“三角洲巖石的化石通常是紅色的，這是因?yàn)楹醒趸F礦物，這表明三角洲形成于干濕交替的氣候中”。

選項(xiàng)分析：

D選項(xiàng)符合定位句子表達(dá)的意思。lived in areas that experienced alternate dry and wet periods對(duì)應(yīng)原文中的formed in a climate that had alternate wet and dry periods。

A選項(xiàng)說(shuō)三角洲形成在干燥時(shí)期，然后逐漸變潮濕。而原文說(shuō)的是formed in a climate that had alternate wet and dry periods，干濕交替。因此該選項(xiàng)與原文描述不符。

B選項(xiàng)做比較說(shuō)，比起周圍地區(qū)，這類三角洲包含不同類型的鐵礦物。無(wú)中生有。

C選項(xiàng)說(shuō)大部分的rhipidistian crossopterygian fish在氣候變干燥的時(shí)候死亡。無(wú)中生有。

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