Final answer:
An index fossil must have existed for a short geologic time period in order to provide a precise dating reference. These fossils help scientists date rock layers based on the fossil's unique and recognizable characteristics.
Step-by-step explanation:
In order for a fossil to be used as an index fossil, the species must have existed for a short geologic time period. Index fossils are incredibly useful for both dating and correlating the strata in which they are found because they have distinct characteristics: they are easily recognizable, were geographically widespread, were abundant, and lived for a relatively brief period geologically. These organisms provide a narrow, identifiable span of geologic time, enabling scientists to determine the age of the rock layers in which they are found and, consequently, to understand Earth's history better. For example, the presence of Belemnites, which only existed during the Mesozoic era, in a rock layer instantly informs us that the rock is from that period, between 252 and 66 million years ago.