托福閱讀真題Official 44 Passage 1（五）

托福閱讀真題Official 44 Passage 1（五）

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 9 of 14

In paragraph 4,why does the author point out that crabs and other arthropods were already living on land when the ancestors of the first tetrapods began living there?

A.To account for the presence of dead fish along the edges of ponds and streams during the Devonian

B.To support the claim that climate change caused freshwater habitats to become more restricted during the Devonian

C.To identify a consequence of the emergence of plants into terrestrial habitats near ponds and streams

D.To identify a possible reason for why certain fish gradually became terrestrial organisms

正確答案：D

題目詳解

題型分類：修辭目的題

題干分析：題干問作者為何在文中寫某個(gè)信息，先利用該信息到原文定位，定位到第三段第五句。題干中問的信息處于第五句的后半部分，我們看完整句子是In addition開頭的，說明和上一句是并列關(guān)系，因此第四句和第五句都是說明具體的食物來源，都是為第三句服務(wù)的。第三句和第二句又都是為了說明導(dǎo)致魚類進(jìn)化的兩個(gè)主要原因：氣候變化（第二句）和新的食物來源（第三句），因此追溯到第三段首句，也是本段的主旨句：什么生態(tài)壓力導(dǎo)致魚逐漸放棄水生環(huán)境，而成為陸生生物？本題為段落主旨的修辭目的題。

選項(xiàng)分析：

D選項(xiàng)說a possible reason for why...對(duì)應(yīng)首句What ecological pressures might have caused...這一問題給出答案，certain fish gradually became terrestrial organisms對(duì)應(yīng)首句中fishes to gradually abandon their watery habitat and become increasingly land-dwelling creatures。符合主旨。

A選項(xiàng)說解釋死魚的存在。死魚為具體的例子，而不是主旨。

B選項(xiàng)提到氣候變化。而題干問的信息，是食物來源的例子，或是說明本段主旨的例子，不是氣候變化的例子。

C選項(xiàng)說明植物出現(xiàn)的結(jié)果。植物部分也是具體例子，不是主旨。

Question 10 of 14

According to paragraph 4,teeth of the earliest tetrapods suggest that these tetrapods

A.competed with other animals for protein

B.were probably carnivores

C.could easily climb out of water

D.were able to eat plants

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：teeth of the earliest tetrapods。利用定位詞定位到第四段的最后一句。本句說牙齒證明了最早的四足動(dòng)物并不以陸上植物為食，它們很可能是食肉動(dòng)物，并且并沒有具備以植物為食的能力。

選項(xiàng)分析：

B選項(xiàng)符合定位句表達(dá)。were probably carnivores對(duì)應(yīng)原句they were presumably carnivorous。

A選項(xiàng)說與其它動(dòng)物競(jìng)爭(zhēng)蛋白質(zhì)。無中生有。

C選項(xiàng)說能夠輕易爬出水域。無中生有。

D選項(xiàng)說能夠食用植物。而原文說did not utilize land plants as food，and had not developed the ability to feed on plants與原文矛盾。

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