More than any other type of fossil, index fossils define geological time periods for geologists by providing the most useful information about the age of the rock they are found in. Index fossils are also called key fossils or type fossils.
The best index fossils have four characteristics: They are unique, prevalent, plentiful and restricted in geologic time. Most fossils are from ocean rocks, so most major index fossils are from marine organisms. However, young rocks found in certain regions are useful for revealing the fossils of land organisms.
Many kinds of organisms are unique but few are widespread. Index fossils that began life as floating eggs and in infant stages are important because ocean currents helped them populate the world. The most successful ones became abundant, but this also exposed them to environmental change and extinction. This thrive-or-die characteristic makes the best index fossils.
Trilobites are excellent index fossils for Paleozoic rocks. These mobile animals lived across oceans and constantly evolved into new species from the Middle Cambrian time to the end of the Permian Period, which is almost the entire Paleozoic period. These organisms inhabited large, even global areas, and their large fossils can be studied without a microscope. Similar index fossils include rugose corals, crinoids, mollusks, ammonites, bryozoans and brachiopods.
Other index fossils such as floating plankton are very useful because of their small to microscopic size. Their tiny bodies rained down all over the ocean and are found in many kinds of rocks, even wellbore cuttings. Index microfossils are a great asset to the petroleum industry, and have enabled geologists to break down geologic time into fine detail.
Index fossils of terrestrial rocks formed on land reveal clues about quickly-evolving small rodents as well as larger animals that ranged over wide geographic ranges.
Index fossils define the ages, epochs, periods and eras in the formal architecture of geologic time. Related fossil types are the characteristic fossil, which belongs to a time period without defining it, and guide fossils, which narrow down but don’t specify a time period.