Final answer:
Index fossils are used to determine the age of rock layers by identifying the presence of specific organisms that existed during a certain time period.
Step-by-step explanation:
Index fossils are organisms that are used to determine the age of rock layers. They are typically organisms that were widespread and existed during a specific time period. They are useful because they are easy to identify and only existed for a relatively short period of time.
For example, the brachiopod species Pentamerus existed during the Silurian period and is commonly used as an index fossil for that time period. The presence of Pentamerus in a rock layer can help geologists determine the age of the rock.
Learn more about Index fossils and their use in determining the age of rock layers