An index fossil is a fossil used to determine the relative age of a given rock. It is typically a plant, invertebrate, or trace fossil.
An index fossil must meet certain criteria to be useful. First, it must be the fossil of an organism that lived for a limited period of geologic time. It also must be abundant and easily recognizable. Finally, it must have a broad geographic range. When fossils meet these criteria, they help geologists and paleontologists date the rocks in which they are found.
