托福阅读真题Official 44 Passage 1（二）

托福阅读真题Official 44 Passage 1（二）

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 3 of 14

The word“advantageous”in the passage is closest in meaning to

A.beneficial

B.necessary

C.remarkable

D.common

正确答案：A

题目详解

题型分类：词汇题

题干分析：词汇所在句子If there were periods of drought,any adaptations allowing the fish to survive the dry conditions would have been advantageous译为“如果处在干旱期，任何能使鱼类在干旱环境中存活下来的适应性都是有利的”。advantageous意思为“有利的”。名词形式：advantage意思为“好处，优势，益处”。

选项分析：

A选项beneficial有利的，正确。

B选项necessary必需的。

C选项remarkable显著的。

D选项common普通的。

Question 4 of 14

In paragraph 2,why does the author include the information that mud inside rhipidistian crossopterygian fossils differed in consistency and texture depending on where the mud was located?

A.To provide evidence that rhipidistian crossopterygian lived in river channels and lakes on large deltas

B.To identify an effect of the oxidation of iron minerals on the evolution of rhipidistian crossopterygian

C.To help explain why scientists have concluded that rhipidistian crossopterygian probably had lungs

D.To explain why scientists decided to cut cross sections through some fossils of rhipidistian crossopterygian

正确答案：C

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，定位到第二段第七句。紧接着第八句说These differences suggest a saclike cavity below the front end of the gut that can only be interpreted as a lung说明了作者要引出这些差别来说明肺在这类鱼体内的存在。本题考察句子之间的关系，these这类的指代词起到关键作用，引出下一句是作者意图所在。

选项分析：

C选项To help explain...probably had lungs符合作者要表达的意图，对应第八句的These differences suggest...can only be interpreted as a lung为了证明肺的存在。

A选项说为了证明rhipidistian crossopterygian生活的地方。无中生有。

B选项说为了说明铁矿物的氧化在鱼进化中的作用。不是文中对应的信息。

D选项说为了解释为什么科学家要做横切面。不是文中对应的信息。