Geologists and other scientists use fossils to correlate the ages of different rock strata (thin layers or beds of rock that differ in some way from adjacent layers) in different places on Earth. If two different rock strata contain the same set of fossil species, then the two different rock strata were probably deposited about the same time. Fossils also can give clues about the environment of Earth in the past. For example, certain fossils are only found in the ocean. When these fossils are found in a rock strata, it is a sure sign that the rock strata was deposited in an ocean even if it is now on top of a high mountain.

Fossils can also help to establish the relative ages of rocks (which rocks are older and which are younger). If a fossil species can be assigned an absolute date by radioactive...