Answer:
Special fossils called "index fossils" indicate to geologists the relative age or the specific time period in geological time. Index fossils are used as valuable markers or reference points for correlating and dating rock layers or geological formations. These fossils are typically abundant, widespread, and have a short-lived existence in the geological record. By identifying and comparing index fossils found in different rock layers or locations, geologists can determine the relative age of those rocks and establish a chronological sequence of events in geological time.