What is an important characteristic of an index fossil?

Index fossils are defined as those fossils that are found geographically over a large extent and those that existed for a shorter period of time in geological history.

These fossils are very important because the study of these fossils enabled the paleontologists and geologists to demarcate the different geological periods or the boundaries, that further helped in the construction of the geological time scale. In addition to this, it also helped in the method of rock correlation.

For example, Trilobites, Ammonites and Brachiopods.