As you can see, index fossils are an extremely useful element for geologists and paleontologists. But not every fossil qualifies as an index fossil. In order to be useful as an index fossil it needs to be widespread and it needs to live only within a certain period of time. When these two criteria are met, it makes for a great index fossil. An example of a great index fossil is trilobites. Trilobites, when they lived, were pretty much everywhere. They also lasted all the way to the end of the Palezoic period, making it easy to date. Here is a chart of the most useful index fossils:
