Day 29

Passage 29

Methods of Fossil Formation

① Fossils are the remains or traces of prehistoric plants and animals, which are buried and preserved in sedimentary rock, or trapped, in organic matter. Many factors can influence how fossils are preserved. The fossilization of an organism depends on the chemistry of the environment and on the biochemical makeup of the organism. As a result, not all organisms in a community will be preserved. The hard, indigestible skeletons and shells of animals and the woody material of plants are usually the best preserved. Fossils of organisms made of softtissues that decay readily are rarer.

② Plants are most commonly fossilized through carbonization. In this process, the mobile oils in the plant's organic matter are leached out and the remaining matter is reduced to a carbon film. Plants have an inner structure of rigid organic walls that may be preserved in this manner, revealing the framework of the original cells, whereas animals have softtissues with a less rigid cellular structure and are rarely preserved through carbonization. Another common mode of preservation of plants is petrifaction, which is the crystallization of minerals inside cells. One of the best-known forms of petrifaction is silicification, a process in which silica-rich fluids enter the plant's cells and crystallize, making the cells appear to have turned to stone (petrified). Petrifaction may also occur in animals when minerals such as calcite, silica, or iron fill the pores and cavities of shells or bones.

③ Replacement fossilization occurs when an organism is buried in mud and its remains are replaced by N-sulfide (pyrite) or phosphate (apatite) minerals. This process may replace softtissues, preserving rarely seen details of the organism's anatomy. Paleontologists have used mild acids to etch the phosphatized fossil remains of ancient fish found in Brazil to reveal structures such as gills and muscles.

④ There is another method of preservation called recrystallization, for many animal shells. It destroys the microscopic details of the shell but does not change the overall shape. Shells preserved in this way are composed of the mineral aragonite, a form of calcium carbonate that breaks down over millions of years to form the more stable mineral calcite. Snail shells and bivalve shells from the Jurassic Period and later are still composed principally of aragonite, while most older shells that have been preserved have recrystallized to calcite.

⑤The softtissues of animals, on the other hand, are preserved only under extremely unusual conditions, and the preserved tissue usually lasts for only a short geological period. In the Siberian permafrost (earth that remains frozen year-round), for example, entire mammoths have been preserved in ice for thousands of years. The remains of the mammoths' last meals have sometimes been preserved in the stomachs, allowing paleontologists to study the animals' diet. Another process called mummification may occur in hot, arid climates, which can dehydrate organisms before their soft tissues have decayed fully. The skin itself is preserved for only a short time, but the impressions of the skin in the surrounding sediment can be preserved much longer if the sediment turns to rock. Paleontologists have found skin impressions of dinosaurs preserved by this method. Besides, when whole organisms become trapped and preserved in organic matter such as amber, natural asphalt, or pea, they can be preserved for millions of years, with details still intact of their softtissues, such as muscles and hair-like bristles.

⑥In addition, acidic conditions may slowly dissolve away the skeletons of fossil animals preserved in rock, leaving a space where the organism used to be. The impression that is left in the rock becomes a mold. This process commonly occurs in fossil shells where the calcite shell dissolves easily with acid. The impression of the outside of the shell is the external mold. Sometimes the inside of the shell is filled with sediment before the shell is dissolved, leaving an internal impression of the shell called an internal mold. If the space where the shell used to be is then filled with a new mineral, the replica of the shell forms a cast. At other times, when animals walk through soft sediment such as mud, their feet, tails, and other body parts leave impressions that may harden and become preserved to form a mold. When such an impression is filled with a different sediment, the sediment that fills the mold forms a cast. Molds and casts of dinosaur tracks are relatively common and help paleontologists understand how these creatures moved.

——2012年7月28日北美机经

Why does the author mention the preservation of mammoths' last meals in Paragraph 5?

A. To illustrate some animal remains can last only a short time.

B. To indicate the paleontological value of preservation.

C. To give an example of fossil preserved in extreme conditions.

D. To contrast this kind of preservation with mummification.

核心词汇：

词汇练习：

阅读下列句子，用所给单词（或词组）的正确形式填空：

sedimentary rigid reveal remains preserve pore paleontologist

microscopic intact indigestible cavity geological carbonization

1. Although it is certainly true that the possession of hard parts enhances the prospect of preservation, organisms having softtissues and organs are also occasionally____ . (TPO20:Fossil Preservation)

2. The leaves of an oak tree taste foul because they are rich in tannins, a chemical that renders them distasteful or____ to many organisms.(Online Test:Opportunists and Competitors)

3. Certainly a great art exists, and by its existence____ that ancient human beings were not without intelligence,skill,and sensitivity.(Online Test:Lascaux Cave Paintings)

4. Another type of fossilization, known as____ , occurs when softtissues are preserved as thin films of carbon. (TPO-20: Fossil Preservation)

5. Even if the new population is of a diff erent species, it can approximately fill the niche vacated by the extinct population and keep the food web____ .(TPO-3:The Long-term Stability of Ecosystems)

6. A slightly younger fossil formation containing animal____ is the Tommotain formation, named after a locale in Russia.(TPO-5:The Cambrian Explosion)

7. When such statues are viewed in isolation, out of their original context and without knowledge of their function, it is easy to criticize them for their____ attitudes that remained unchanged for three thousand years.(TPO-11:Ancient Egyptian Sculpture)

8. 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____ particles. (TPO-20:Fossil Preservation)

9. They possess drought-resisting adaptations: loss of water through the leaves is reduced by means of dense hairs covering waxy leaf surfaces, by the closure of____ during the hottest times to reduce water loss, and by the rolling up or shedding of leaves at the beginning of the dry season.(TPO-26:Survival of Plants and Animals in Desert Conditions)

10. Even today, ____meteorites continually bombard Earth, falling on both the land and the sea.(TPO-8:Extinction of the Dinosaurs)

11. Geothermal energy is in a sense not renewable, because in most cases the heat would be drawn out of a reservoir much more rapidly than it would be replaced by the very slow____ processes by which heat flows through solid rock into a heat reservoir. (TPO-21:Geothermal Energy)

12. Large quantities of water may also be stored in limestones when joints and cracks have been enlarged to form____ .(TPO-12:Water in the Desert)

13. This 700-million-year-old formation gives few clues to the origins of modern animals, however, because____ believe it represents an evolutionary experiment that failed. It contains no ancestors of modern animal groups.(TPO-5:The Cambrian Explosion)

参考答案：

1. preserved 2. indigestible 3. reveals 4. carbonization 5. intact 6. remains 7. rigid 8. sedimentary 9. pores 10. microscopic 11. geological 12. cavities 13. paleontologists