托福阅读真题Official 44 Passage 1（五）

托福阅读真题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

正确答案：D

题目详解

题型分类：修辞目的题

题干分析：题干问作者为何在文中写某个信息，先利用该信息到原文定位，定位到第三段第五句。题干中问的信息处于第五句的后半部分，我们看完整句子是In addition开头的，说明和上一句是并列关系，因此第四句和第五句都是说明具体的食物来源，都是为第三句服务的。第三句和第二句又都是为了说明导致鱼类进化的两个主要原因：气候变化（第二句）和新的食物来源（第三句），因此追溯到第三段首句，也是本段的主旨句：什么生态压力导致鱼逐渐放弃水生环境，而成为陆生生物？本题为段落主旨的修辞目的题。

选项分析：

D选项说a possible reason for why...对应首句What ecological pressures might have caused...这一问题给出答案，certain fish gradually became terrestrial organisms对应首句中fishes to gradually abandon their watery habitat and become increasingly land-dwelling creatures。符合主旨。

A选项说解释死鱼的存在。死鱼为具体的例子，而不是主旨。

B选项提到气候变化。而题干问的信息，是食物来源的例子，或是说明本段主旨的例子，不是气候变化的例子。

C选项说明植物出现的结果。植物部分也是具体例子，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

正确答案：B

题目详解

题型分类：事实信息题

原文定位：定位词：teeth of the earliest tetrapods。利用定位词定位到第四段的最后一句。本句说牙齿证明了最早的四足动物并不以陆上植物为食，它们很可能是食肉动物，并且并没有具备以植物为食的能力。

选项分析：

B选项符合定位句表达。were probably carnivores对应原句they were presumably carnivorous。

A选项说与其它动物竞争蛋白质。无中生有。

C选项说能够轻易爬出水域。无中生有。

D选项说能够食用植物。而原文说did not utilize land plants as food，and had not developed the ability to feed on plants与原文矛盾。