The correct answer is - an index fossil.
The index fossils are fossils that are found in lots of places around the world, but they are only found in particular types of rocks that allowed for those fossils to be preserved. Because of this, this types of fossils are used as indicators of certain geological periods, and because of them scientists are able to very easily arrange the things in order.
a substance in a physical state in which it does not resist change of shape and will expand indefinitely to fill any container.
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