Here's why:
* Index fossils are guide fossils: They are fossils of organisms that lived for a relatively short period of time and were geographically widespread.
* They act as time markers: Because they existed for a limited time, their presence in rock layers indicates the age of those layers.
* Similar fossils, similar ages: Finding the same index fossil in two different locations strongly suggests that the rocks formed during the same time period, even if the locations are far apart.
Example: If you found a trilobite fossil in a rock layer in North America and another trilobite fossil in a rock layer in Europe, you could reasonably conclude that those rock layers formed during the same geological period, even though the continents are separated by an ocean.
