［ D］The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world.
［ E］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 star-fishes, which had long arms and were attached by a long stalk to the sea bed, or to rocks.
［ F］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 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.
［ G］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.

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