Answer:
A fossil found in rocks from one time period
Explanation:
An index fossil is an fossil of an organism which lived at only one time during the evolutionary history. The index fossils are useful since they are only obtained from rock of one particular evolutionary time zone. Â Restricted presence of index fossils represents the fact that they lived during a relatively brief period of geologic history. These fossils are used for dating and correlating the various time zones of evolutionary history
