Index fossils are fossils of organisms that lived during a specific time period and were widely distributed across different geographic areas. These fossils are useful for dating and correlating the age of rock layers, as they help geologists identify the relative age of the rocks in which they are found. To be considered an index fossil, an organism must have existed for a relatively short time but have been abundant and widespread. Examples of index fossils include trilobites and ammonites, which help scientists understand the geological history of Earth and the evolution of life.