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【考研英语】14.Rare Fossil Record  

2016-05-29 09:58:54|  分类: 【英语】写作 |  标签: |举报 |字号 订阅

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 The preservation1 of embryos2 and juveniles3 is a rate occurrence in the fossil record. The tiny, delicate skeletons are usually scattered4 by scavengers or destroyed by weathering before they can be fossilized. Ichthyosaurs had a higher chance of being preserved than did terrestrial creatures because, as marine5 animals, they tended to live in environments less subject to erosion. Still, their fossilization required a suite6 of factors: a slow rate of decay of soft tissues, little scavenging by other animals, a lack of swift currents and waves to jumble7 and carry away small bones, and fairly rapid burial. Given these factors, some areas have become a treasury8 of well-preserved ichthyosaur fossils.

  The deposits at Holzmaden, Germany, present an interesting case for analysis. The ichthyosaur remains9 are found in black, bituminous marine shales11 deposited about 190 million years ago. Over the years, thousands of specimens13 of marine reptiles14, fish and invertebrates15 have been recovered from these rocks. The quality of preservation is outstanding, but what is even more impressive is the number of ichthyosaur fossils containing preserved embryos. Ichthyosaurs with embryos have been reported from 6 different levels of the shale10 in a small area around Holzmaden, suggesting that a specific site was used by large numbers of ichthyosaurs repeatedly over time. The embryos are quite advanced in their physical development; their paddles, for example, are already well formed. One specimen12 is even preserved in the birth canal. In addition, the shale contains the remains of many newborns that are between 20 and 30 inches long.

  Why are there so many pregnant females and young at Holzmaden when they are so rare elsewhere? The quality of preservation is almost unmatched and quarry16 operations have been carried out carefully with an awareness17 of the value of the fossils. But these factors do not account for the interesting question of how there came to be such a concentration of pregnant ichthyosaurs in a particular place very close to their time of giving birth.


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