When one considers the many ways by which organisms are completely destroyed after death, it is remarkable that fossils are as common as they are. Attack by scavengers and bacteria, chemical decay, and destruction by erosion and other geologic agencies make the odds against preservation very high. However, the chances of escaping complete destruction are vastly improved if the organism happens to have a mineralized skeleton and dies in a place where it can be quickly buried by sediment. Both of these conditions are often found on the ocean floors, where shelled invertebrates (organisms without spines) flourish and are covered by the continuous rain of sedimentary particles. Although most fossils are found in marine sedimentary rocks, they also are found in terrestrial deposits left by streams and lakes. On occasion, animals and plants have been preserved after becoming immersed in tar or quicksand, trapped in ice or lava flows, or engulfed by rapid falls of volcanic ash.
The term "fossil" often implies petrifaction, literally a transformation into stone. After the death of an organism, the soft tissue is ordinarily consumed by scavengers and bacteria. The empty shell of a snail or clam may be left behind, and if it is sufficiently durable and resistant to dissolution, it may remain basically unchanged for a long period of time. Indeed, unaltered shells of marine invertebrates are known from deposits over 100 million years old. In many marine creatures, however, the skeleton is composed of a mineral variety of calcium carbonate called aragonite. Although aragonite has the same composition as the more familiar mineral known as calcite, it has a different crystal form, is relatively unstable, and in time changes to the more stable calcite.
Many other processes may alter the shell of a clam or snail and enhance its chances for preservation. Water containing dissolved silica, calcium carbonate, or iron may circulate through the enclosing sediment and be deposited in cavities such as marrow cavities and canals in bone once occupied by blood vessels and nerves. In such cases, the original composition of the bone or shell remains, but the fossil is made harder and more durable. This addition of a chemically precipitated substance into pore spaces is termed "permineralization."
Petrifaction may also involve a simultaneous exchange of the original substance of a dead plant or animal with mineral matter of a different composition. This process is termed " replacement" because solutions have dissolved the original material and replaced it with an equal volume of the new substance. Replacement can be a marvelously precise process, so that details of shell ornamentation, tree rings in wood, and delicate structures in bone are accurately preserved.
Another type of fossilization, known as carbonization, occurs when soft tissues are preserved as thin films of carbon. Leaves and tissue of soft-bodied organisms such as jellyfish or worms may accumulate, become buried and compressed, and lose their volatile constituents. The carbon often remains behind as a blackened silhouette.
Although it is certainly true that the possession of hard parts enhances the prospect of preservation, organisms having soft tissues and organs are also occasionally preserved. Insects and even small invertebrates have been found preserved in the hardened resins of conifers and certain other trees. X-ray examination of thin slabs of rock sometimes reveals the ghostly outlines of tentacles, digestive tracts, and visual organs of a variety of marine creatures. Soft parts, including skin, hair, and viscera of ice age mammoths, have been preserved in frozen soil or in the oozing tar of oil seeps.
The probability that actual remains of soft tissue will be preserved is improved if the organism dies in an environment of rapid deposition and oxygen deprivation. Under such conditions, the destructive effects of bacteria are diminished. The Middle Eocene Messel Shale (from about 48 million years ago) of Germany accumulated in such an environment. The shale was deposited in an oxygen-deficient lake where lethal gases sometimes bubbled up and killed animals. Their remains accumulated on the floor of the lake and were then covered by clay and silt. Among the superbly preserved Messel fossils are insects with iridescent exoskeletons (hard outer coverings), frogs with skin and blood vessels intact, and even entire small mammals with preserved fur and soft tissue.
Paragraph 1: When one considers the many ways by which organisms are completely destroyed after death, it is remarkable that fossils are as common as they are. Attack by scavengers and bacteria, chemical decay, and destruction by erosion and other geologic agencies make the odds against preservation very high. However, the chances of escaping complete destruction are vastly improved if the organism happens to have a mineralized skeleton and dies in a place where it can be quickly buried by sediment. Both of these conditions are often found on the ocean floors, where shelled invertebrates (organisms without spines) flourish and are covered by the continuous rain of sedimentary particles. Although most fossils are found in marine sedimentary rocks, they also are found in terrestrial deposits left by streams and lakes. On occasion, animals and plants have been preserved after becoming immersed in tar or quicksand, trapped in ice or lava flows, or engulfed by rapid falls of volcanic ash.
1.The word "agencies" in the passage is closest in meaning to
O combinations
O problems
O forces
O changes
2.In paragraph 1, what is the author's purpose in providing examples of how organisms are destroyed?
O To emphasize how surprising it is that so many fossils exist
O To introduce a new geologic theory of fossil preservation
O To explain why the fossil record until now has remained incomplete
O To compare how fossils form on land and in water
3.The word "terrestrial" in the passage is closest in meaning to
O land
O protected
O alternative
O similar
Paragraph 2: The term "fossil" often implies petrifaction, literally a transformation into stone. After the death of an organism, the soft tissue is ordinarily consumed by scavengers and bacteria. The empty shell of a snail or clam may be left behind, and if it is sufficiently durable and resistant to dissolution, it may remain basically unchanged for a long period of time. Indeed, unaltered shells of marine invertebrates are known from deposits over 100 million years old. In many marine creatures, however, the skeleton is composed of a mineral variety of calcium carbonate called aragonite. Although aragonite has the same composition as the more familiar mineral known as calcite, it has a different crystal form, is relatively unstable, and in time changes to the more stable calcite.
4.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
O When snail or clam shells are left behind, they must be empty in order to remain durable and resist dissolution.
O Although snail and clam shells are durable and resist dissolving, over time they slowly begin to change.
O Although the soft parts of snails or clams dissolve quickly, their hard shells resist dissolution for a long time.
O Empty snail or clam shells that are strong enough not to dissolve may stay in their original state for a long time.
5.Why does the author mention "aragonite" in the passage?
O To emphasize that some fossils remain unaltered for millions of years
O To contrast fossil formation in organisms with soft tissue and in organisms with hard shells
O To explain that some marine organisms must undergo chemical changes in order to fossilize
O To explain why fossil shells are more likely to survive than are fossil skeletons
Paragraph 3: Many other processes may after the shell of a clam or snail and enhance its chances for preservation. Water containing dissolved silica, calcium carbonate, or iron may circulate through the enclosing sediment and be deposited in cavities such as marrow cavities and canals in bone once occupied by blood vessels and nerves. In such cases, the original composition of the bone or shell remains, but the fossil is made harder and more durable. This addition of a chemically precipitated substance into pore spaces is termed "permineralization."
6.The word "enhance" in the passage is closest in meaning to
O control
O limit
O combine
O increase
7.Which of the following best explains the process of permineralization mentioned in paragraph 3?
O Water containing calcium carbonate circulates through a shell and deposits sediment.
O Liquid containing chemicals hardens an already existing fossil structure.
O Water passes through sediment surrounding a fossil and removes its chemical content.
O A chemical substance enters a fossil and changes its shape.
Paragraph 4: Petrifaction may also involve a simultaneous exchange of the original substance of a dead plant or animal with mineral matter of a different composition. This process is termed " replacement" because solutions have dissolved the original material and replaced it with an equal volume of the new substance. Replacement can be a marvelously precise process, so that details of shell ornamentation, tree rings in wood, and delicate structures in bone are accurately preserved.
8.The word "precise" in the passage is closest in meaning to
O complex
O quick
O exact
O reliable
Paragraph 5: Another type of fossilization, known as carbonization, occurs when soft tissues are preserved as thin films of carbon. Leaves and tissue of soft-bodied organisms such as jellyfish or worms may accumulate, become buried and compressed, and lose their volatile constituents. The carbon often remains behind as a blackened silhouette.
9.Paragraph 5 suggests which of the following about the carbonization process?
O It is completed soon after an organism dies.
O It does not occur in hard-shell organisms.
O It sometimes allows soft-tissued organisms to be preserved with all their parts.
O It is a more precise process of preservation than is replacement.
Paragraph 6: Although it is certainly true that the possession of hard parts enhances the prospect of preservation, organisms having soft tissues and organs are also occasionally preserved. Insects and even small invertebrates have been found preserved in the hardened resins of conifers and certain other trees. X-ray examination of thin slabs of rock sometimes reveals the ghostly outlines of tentacles, digestive tracts, and visual organs of a variety of marine creatures. Soft parts, including skin, hair, and viscera of ice age mammoths, have been preserved in frozen soil or in the oozing tar of oil seeps.
10.The word "prospect" in the passage is closest in meaning to
O completion
O variety
O possibility
O speed
Paragraph 7: The probability that actual remains of soft tissue will be preserved is improved if the organism dies in an environment of rapid deposition and oxygen deprivation. Under such conditions, the destructive effects of bacteria are diminished. The Middle Eocene Messel Shale (from about 48 million years ago) of Germany accumulated in such an environment. The shale was deposited in an oxygen-deficient lake where lethal gases sometimes bubbled up and killed animals. Their remains accumulated on the floor of the lake and were then covered by clay and silt. Among the superbly preserved Messel fossils are insects with iridescent exoskeletons (hard outer coverings), frogs with skin and blood vessels intact, and even entire small mammals with preserved fur and soft tissue.
11.According to paragraph 7, how do environments containing oxygen affect fossil preservation?
O They increase the probability that soft-tissued organisms will become fossils.
O They lead to more bacteria production.
O They slow the rate at which clay and silt are deposited.
O They reduce the chance that animal remains will be preserved.
12.According to the passage, all of the following assist in fossil preservation EXCEPT
O the presence of calcite in an organism's skeleton
O the presence of large open areas along an ocean floor
O the deposition of a fossil in sticky substances such as sap or tar
O the rapid burial of an organism under layers of silt
■Another type of fossilization, known as carbonization, occurs when soft tissues are preserved as thin films of carbon. ■Leaves and tissue of soft-bodied organisms such as jellyfish or worms may accumulate, become buried and compressed, and lose their volatile constituents. ■The carbon often remains behind as a blackened silhouette.■
13.Look at the four squares [■] that indicate where the following sentence can be added to the passage.
But the evidence of past organic life is not limited to petrifaction.
Where would the sentence best fit?
Click on a square [■] to insert the sentence in the passage.
14.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 answer choices 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.
The remains of ancient life are amazingly well preserved in the form of fossils.
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Answer Choices
O Environmental characteristics like those present on ocean floors increase the likelihood that plant and animal fossils will occur.
O Fossils are more likely to be preserved in shale deposits than in deposits of clay and silt.
O The shells of organisms can be preserved by processes of chemical precipitation or mineral exchange.
O Freezing enables the soft parts of organisms to survive longer than the hard parts.
O Comparatively few fossils are found in the terrestrial deposits of streams and lakes.
O Thin films of carbon may remain as an indication of soft tissue or actual tissue may be preserved if exposure to bacteria is limited.
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參考答案：
1. 3
2. 1
3. 1
4. 4
5. 3
6. 4
7. 2
8. 3
9. 2
10. 3
11. 4
12. 2
13. 1
14. Environmental characteristics ...
The shells of organisms ...
Thin films of carbon ...
如果想想生物在死之后被完全摧毀的種種方式，能夠這樣頻繁出現(xiàn)化石是一件很令人驚訝的事。食腐動(dòng)物和細(xì)菌的破壞、化學(xué)性腐爛、腐蝕以及其它地質(zhì)因素都會(huì)非常不利于保存。不過(guò)，如果生物體碰巧具有礦化的骨骼并且死于可以迅速被沉積物掩埋的地方，擺脫被完全摧毀的幾率便會(huì)大大增加。海底通常就具有上述的兩方面條件，這里生活著很多帶殼的無(wú)脊椎動(dòng)物（沒(méi)有脊椎的動(dòng)物），不斷累積的似雨的沉積顆粒會(huì)把它們掩埋起來(lái)。雖然多數(shù)的化石是在海洋沉積巖中發(fā)現(xiàn)的，但是在溪流和湖泊留下的陸相沉積物中也發(fā)現(xiàn)過(guò)。有時(shí)，浸入焦油和流沙、陷入冰或熔巖流或被急速降落的火山灰吞噬的動(dòng)植物得以保存下來(lái)。
術(shù)語(yǔ)“化石”常常意味著石化，字面意思就是變成了石頭。生物體死后，軟組織一般會(huì)被食腐動(dòng)物和細(xì)菌吃掉�？赡軙�(huì)留下蝸�；蚋蝌劭諝�，如果空殼足夠堅(jiān)固并且能抵御分解，就有可能在很長(zhǎng)一段時(shí)間內(nèi)基本上保持原樣。事實(shí)上，我們現(xiàn)在所知的在沉積物中發(fā)現(xiàn)的海洋無(wú)脊椎動(dòng)物保存良好的殼已超過(guò)了1億年之久。不過(guò)，很多海洋生物的骨骼是由稱為霰石的各式碳酸鈣礦物質(zhì)組成的。雖然霰石與我們更為熟悉的礦物方解石具有同樣的組成，但是它的晶型不同，相對(duì)不穩(wěn)定，最終會(huì)變成更穩(wěn)定的方解石。
很多其他過(guò)程也許會(huì)改變哈喇殼或蝸牛殼并且增加它被保存下來(lái)的幾率。含有溶解的二氧化硅、碳酸鈣或鐵的水可能會(huì)在封閉的沉積物中流動(dòng)，并沉積到諸如骨髓腔和骨頭管道內(nèi)，這些骨頭管道曾經(jīng)由血管和神經(jīng)占據(jù)。這種情況下，骨和殼的原始組成沒(méi)有改變，但是形成的化石更堅(jiān)硬并且更持久。這種在孔隙中填充化學(xué)沉積物的過(guò)程就叫做“完全礦化”。
石化還可能同時(shí)涉及死亡的動(dòng)植物的原有物質(zhì)與不同組成的礦物質(zhì)的交換作用。該過(guò)程叫做“置換作用”，因?yàn)槿芤喝芙饬嗽嘉镔|(zhì)并將其置換成為等體積的新物質(zhì)。置換是一個(gè)讓人難以置信的精確過(guò)程，貝殼裝飾的細(xì)節(jié)、樹(shù)木的年輪以及骨骼的精細(xì)結(jié)構(gòu)都被精準(zhǔn)地保存下來(lái)。
另一種類型的石化，稱為“碳化”，當(dāng)軟組織以碳薄膜的形式保存時(shí)會(huì)發(fā)生碳化。樹(shù)葉和軟體動(dòng)物例如水母或蠕蟲(chóng)的組織可能會(huì)堆積起來(lái)，被掩埋并被壓實(shí)，然后其中的揮發(fā)性成分會(huì)消失。碳通常以一種黑色輪廓的形式被保留下來(lái)。
雖然擁有堅(jiān)硬的部分的確會(huì)增加保存的可能性，但是具有軟組織和器官的生物偶爾也會(huì)被保存下來(lái)。在針葉樹(shù)以及某些其它的樹(shù)種的凝固樹(shù)脂中就發(fā)現(xiàn)了昆蟲(chóng)甚至是很小的無(wú)脊椎動(dòng)物。對(duì)巖石薄片的X射線檢查有時(shí)會(huì)發(fā)現(xiàn)可怕的觸角輪廓、消化道和很多種海洋生物的視覺(jué)器官。凍土或石油滲漏時(shí)滲出的焦油中保存了包括皮膚、毛發(fā)和冰河時(shí)代猛犸象的內(nèi)臟在內(nèi)的軟組織。
如果生物體死于一個(gè)快速沉積和缺氧的環(huán)境，會(huì)有助于軟組織殘骸的保存。在這種條件下，細(xì)菌的破壞性影響會(huì)降低。德國(guó)始新世中期的麥塞爾頁(yè)巖（來(lái)自4800萬(wàn)年前）就是在這種環(huán)境下積累起來(lái)的。該頁(yè)巖在一個(gè)缺氧的湖泊里沉積，那里時(shí)有致命的氣體冒出并殺死動(dòng)物。動(dòng)物的殘骸在湖底聚集，然后被粘土和淤泥所覆蓋。在保存完好的麥塞爾化石中有帶閃亮外骨骼（硬質(zhì)外部覆蓋物）的昆蟲(chóng)，皮膚和血管完好無(wú)損的青蛙，甚至是毛皮和軟組織都完整保存的小型哺乳動(dòng)物。
