托福阅读真题Official 44 Passage 1（一）

托福阅读真题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.

正确答案：B

题目详解

题型分类：事实信息题

原文定位：定位词fish evolution。用定位词定位到文章第一句，可以发现本段整体上都是介绍fish evolution的，因此本题可以分类为大定位事实信息题，需要查找的信息较多较琐碎，需要阅读全段，之后结合四个选项一一确认。根据第一段第一句，了解fish evolution是一个意义重大的进化事件，第二句说明鱼起源于海洋。第三句说明到了Devonian Period，鱼就扩展到了包括淡水在内的几乎所有可利用的水生环境中。最后指出lobe-finned fish就是其中一种常见的例子。

选项分析：

B选项符合第二句和第三句，这里的elsewhere指的就是ocean。

A选项说lobe-finned fish是出现的最早的鱼类之一。无中生有。

C选项说lobe-finned fish扩展到几乎所有的可利用的水生环境中，原文说的是fish。与原文表述不符。

D选项做比较说，比起淡水环境，lobe-finned在海洋中更常见，无中生有。

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 minerals in the delta rocks。用定位词定位到文章第二段第三句，本句说“三角洲岩石的化石通常是红色的，这是因为含有氧化铁矿物，这表明三角洲形成于干湿交替的气候中”。

选项分析：

D选项符合定位句子表达的意思。lived in areas that experienced alternate dry and wet periods对应原文中的formed in a climate that had alternate wet and dry periods。

A选项说三角洲形成在干燥时期，然后逐渐变潮湿。而原文说的是formed in a climate that had alternate wet and dry periods，干湿交替。因此该选项与原文描述不符。

B选项做比较说，比起周围地区，这类三角洲包含不同类型的铁矿物。无中生有。

C选项说大部分的rhipidistian crossopterygian fish在气候变干燥的时候死亡。无中生有。