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 impression, or simply reduced to a more stable form.
启发思维:
这篇文章讲述的中心是化石,也可说是灭绝动物的尸体。前三段介绍化石如何的形成以及保存化石的条件,在其后的段落中按时间次序介绍了早期海洋动物,爬行动物以及哺乳动物之前的一些物种情况。
再读选项,大致确定7个选项的启示是:
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