The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world.
The earliest animals whose remains have been found were all very simple kinds and lived in the sea. Later forms are more complex, and among these are the sea-lilies, relations of the starfishes, which had long arms and were attached by a long stalk to the sea bed, or to rocks.
When an animal dies, the body, its bones, or shell, may often be carried away by streams into lakes or the sea and there get covered up by mud. If the animal lived in the sea its body would probably sink and be covered with mud. More and more mud would fall upon it until the bones or shell become embedded and preserved.
Many factors can influence how fossils are preserved in rocks. Remains of an organism may be replaced by minerals, dissolved by an acidic solution to leave only their impressions, or simply reduced to a more stable form.
答案:
本篇介绍的是史前类动物。文章结构的脉络清晰:首先介绍什么是史前动物和我们研究史前动物的依据化石,以及化石形成的过程。随后,作者即按照动物进化的顺序我们可以见到其化石的最早的动物,水生壳类动物,脊椎类动物逐一加以介绍。
41.文章一开始,作者告诉我们,早在人类出现以前地球上就许多物种,现在有些物种的后代依然生存,而另外一些则没有留下后裔。在本题空白处后面文章又说岩石上偶尔会留下数百万年前就死掉了的动物精确的印记。显然,空白处应该是关于岩石与灭绝了的动物之间的关系(7个选项中有A﹑B﹑E﹑G四项提到了rock,但另外大三项意思上不符)。此外,空白处前面的extinct和no descendant均为否定意义和表达,而空白处的后面accurate和much则为肯定意义的表达;这意味着空白处的内容应该有一个结构上的转折只有B项符合这一条件。所以正确答案只能是B。
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