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从水生鱼类过渡到陆生四足动物（长有四肢的脊椎生物）是地球上最为重要的生物进化事件之一。

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).

鱼类很可能起源于海洋，有关鱼类最早的记录来自海洋岩石。

Fish probably originated in the oceans, and our first records of them are in marine rocks.

但在泥盆纪（4.08亿年到3.62亿年以前）鱼类已遍布包括淡水环境在内的几乎所有的水生环境。

However, by the Devonian Period (408 million to 362 million years ago), they had radiated into almost all available aquatic habitats, including freshwater settings.

肉鳍鱼类是其中的代表之一，它们的化石常见于淡水湖的岩石之中。

One of the groups whose fossils are especially common in rocks deposited in fresh water is the lobe-finned fish.

研究四足动物进化的生物学家们对生活在泥盆纪淡水中的总鳍鱼纲扇鳍鱼目的肉鳍鱼类尤其感兴趣。

The freshwater Devonian lobe-finned fish rhipidistian crossopterygian is of particular interest to biologists studying tetrapod evolution.

这些鱼类曾经生活在大型三角洲地带的河道和湖泊之中。

These fish lived in river channels and lakes on large deltas.

由于含有氧化铁矿物，发现这些化石的三角洲岩石通常是红色的，这表明形成三角洲时期的气候已经出现了干湿季节的交替。

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.

如果有干旱季节出现的话，那么任何有助于鱼类在干燥条件下生存的适应都是有益的。

If there were periods of drought, any adaptations allowing the fish to survive the dry conditions would have been advantageous.

在这些扇鳍鱼类中，的确存在一些这样的适应。

In these rhipidistians, several such adaptations existed.

众所周知，它们用肺以及鱼鳃来呼吸。

It is known that they had lungs as well as gills for breathing.

一些化石的横截面表明，骨架内泥沙的密实度和纹理会根据其在体内位置的不同而不同。

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.

这些差异表明在鱼肠前端之下的囊状空腔只能被认为是肺。

These differences suggest a saclike cavity below the front end of the gut that can only be interpreted as a lung.

对于这些鱼类来说，鱼鳃毫无疑问是主要的获得氧气的来源，但当水中含氧量下降时，例如干旱期变长时，肺则用来作为辅助的呼吸器官来吸入空气。

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.

因此这些鱼类早已进化出在陆地上生存的主要必备条件之一：将空气作为氧气来源的能力。

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.

第二种适应则是肉鳍的身体结构。

A second adaptation of these fish was in the structure of the lobe fins.

这些厚厚的肉质鳍片非常结实，有一个骨轴贯穿中心。

The fins were thick, fleshy, and quite sturdy, with a median axis of bone down the center.

这些鳍片可以在陆地移动时起到辅助作用，或许足够可以帮助一条鱼从一个几乎干涸的池塘笨拙地移动到另外一个拥有足够水源和氧气的邻近池塘来生存。

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.

这些鳍片最终演变成短而粗硬的腿。

These fins eventually changed into short, stubby legs.

泥盆纪扇鳍鱼的鳍片的骨头在数量和位置上恰恰与已知最早的四足动物——两栖动物的肢骨相符合。

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.

值得强调的是，肺和四肢的进化绝不是因为它们预料到未来会在陆地上存活。

It should be emphasized that the evolution of lungs and limbs was in no sense an anticipation of future life on land.

这些适应是为了帮助鱼类在当时的水生环境中继续生存。

These adaptations developed because they helped fish to survive in their existing aquatic environment.

那么是怎样的生态压力导致鱼类逐步脱离水生栖息地并逐渐成为陆生生物的呢？

What ecological pressures might have caused fishes to gradually abandon their watery habitat and become increasingly land-dwelling creatures?

如果淡水区域逐渐受限，泥盆纪的气候变化或许与此有关系。

Changes in climate during the Devonian may have had something to do with this if freshwater areas became progressively more restricted.

另一个刺激因素可能是新的食物来源。

Another impetus may have been new sources of food.

池塘和溪流的周边肯定会散落着死掉的鱼或其他水栖生物。

The edges of ponds and streams surely had scattered dead fish and other water-dwelling creatures.

此外，植物会出现在靠近池塘和溪流的陆生环境，螃蟹和其他节肢动物同样也是这一最早的陆生群落的成员。

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.

因此，在泥盆纪，淡水周边的陆地栖息地很可能为那些能够轻松登上陆地的动物提供了丰富的蛋白质来源。

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.

来自牙齿的证据显示，这些最早的四足动物并不以陆地植物为食，它们很可能是肉食动物，还没有进化出靠植物为食的能力。

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.

最初的四足动物是如何向陆生环境过渡的呢？

How did the first tetrapods make the transition to a terrestrial habitat?

就像莱尼蕨类这样的早期陆生植物一样，四足动物仅仅需要部分过渡，它们依旧与水紧密相连。

Like early land plants such as rhyniophytes, they made only a partial transition; they were still quite tied to water.

然而，早期陆生植物所面临的问题并不会困扰最早的四足动物。

However, many problems that faced early land plants were not applicable to the first tetrapods.

这些动物的祖先早已具备了循环系统，而且由于它们可以移动，所以可以来到水边饮水。

The ancestors of these animals already had a circulation system, and they were mobile, so that they could move to water to drink.

此外，它们已经有了肺，扇鳍鱼可能用肺来辅助呼吸。

Furthermore, they already had lungs, which rhipidistians presumably used for auxiliary breathing.

最早四足动物的主要变化在于骨骼系统——鳍片、脊柱、骨盆带以及肩胛带的骨骼变化。

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.

Paragraph 1 supports which of the following statements about fish evolution?

According to paragraph 2, what do the minerals in the delta rocks containing rhipidistian crossopterygian fossils reveal?

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?

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

According to paragraph 3, the structure of the fins of rhipidistian crossopterygian may have allowed these fish to

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

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?

According to paragraph 5, which of the following was true of the first tetrapods?

Look at the four squares  that indicate where the following sentence could be added to the passage. These would have been deposited by the receding waters of droughts, during which many aquatic animals must have died.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

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.