Here's why:

* Wide Distribution: Index fossils represent organisms that lived over a wide geographic area.

* Short Time Span: They existed for a relatively short period of geological time.

* Easily Recognizable: Index fossils are unique and easily identifiable.

These characteristics make them useful for dating rock layers because:

* If an index fossil is found in a rock layer, it indicates that the rock layer formed during the time the organism lived.

* By comparing the presence of index fossils in different rock layers, geologists can determine the relative ages of the layers.

Example: Trilobites are a good example of an index fossil. They lived for millions of years, but their species changed rapidly. Finding a specific trilobite species in a rock layer helps pinpoint the age of the layer with a relatively high degree of accuracy.