2. 外语
  3. 　　　 Generally, in order to be preserved in the fossil record, organisms must possess hard body parts 　　　 such as shells or b

　　　 Generally, in order to be preserved in the fossil record, organisms must possess hard body parts 　　　 such as shells or b

admin2004-12-30  98
问题 　　　     Generally, in order to be preserved in the fossil record, organisms must possess hard body parts
　　　 such as shells or bones. Soft, fleshy structures are quickly destroyed by predators pr decayed by
　　　 bacteria. Even hard parts left on the surface for and length of time will be destroyed Therefore,
　　　 organisms must be buried rapidly to escape destruction by the elements and to be protected
(5) 　 agents of weathering and erosion Marine organisms thus are better candidates for
　　　 fossilization than those living on the land because the ocean is typically the site of sedimentation,
　　　 whereas the land is largely the site of erosion.
　　　     The beds of ancient lakes were also excellent sites for rapid burial of skeletal remains of
　　　 freshwater organisms and skeletons of other animals, including those of early humans Ancient
(10)　 swamps were particularly plentiful with prolific growths of vegetation, which fossilized in
　　　 abundance. Many animals became trapped in bogs overgrown by vegetation. The environment of
　　　 the swamps kept bacterial decay to a minimum, which greatly (aide(D)in the preservation of plants
　　　 and animals. The rapidly accumulating sediments in flood plains, deltas, and stream channels
　　　 buried freshwater organisms, along with other plants and animals that happened to fall into the
(15)　 water.
　　　     Only a small fraction of all the organisms that have ever lived are preserved as fossils. Normally,
　　　 the remains of a plant or animal are completely destroyed through predation and decay. Although
　　　 it seems that fossilization is common for some organisms, for others it is almost impossible. For
　　　 the most part, the remains of organisms are recycled in the earth, which is fortunate because
(20)　 otherwise soil and water would soon become depieted of essential nutrients. Also, most of the
　　　 fossils exposed on Earth’s surface are destroyed by weathering processes. This makes for an
　　　 incomplete fossil record with poor or no representation of certain species.
　　　     The best fossils are those composed of unaltered remains. Generally, it is the inorganic hard parts,
　　　 composed mostly of calcium carbonate, that form the vast majority of unaltered fossils. Calcite and
(25)　 aragonite also contributed to a substantial number of fossils of certain organisms.
选项 A、is not likely to appear in the fossil record
B、is not heavy enough to sink below the surface
C、is not attractive to predators
D、takes a long time to decay
答案A
解析
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