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OFFICIAL44 Look at the four squares [■] that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.

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From Fish to Terrestrial Vertebrates
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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. [■] 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.

13.Look at the four squares [■] that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.

These would have been deposited by the receding waters of droughts, during which many aquatic animals must have died..

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【题目翻译】请看四个方块[■],它指示了下一句话可以添加到段落中的位置。这个句子最适合哪儿? A:它们可能是由于干旱退去的水域而沉积下来的,在此期间许多水生动物肯定已经死亡。 【判定题型】:根据题目问法,题目要求将句子插入到文中最恰当的空格处,故判断本题为句子插入题。 【待插入句分析】These would have been deposited by the receding waters of droughts, during which many aquatic animals must have died. 待插入句子中these 指示代词是有效线索,前面话需要明确these指的是什么。 【原文分析】这一段介绍了某些生态压力导致鱼类逐渐放弃在水中的栖息地,逐渐成为陆生生物。 【选项分析】 A.后面句子中in addition 与前面句子形成平行结构。 B.后面句子和之前用thus 构成因果关系。 C.后面句子说动物不食用植物。 选择A选项是因为these 指代的是scattered dead fish and other water-dwelling creatures,这部分内容也和待插入句子中的during which many aquatic animals must have died相呼应。

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