A good index fossil is one with four characteristics: it is distinctive, widespread, abundant and limited in geologic time. Because most fossil-bearing rocks formed in the ocean, the major index fossils are marine organisms, but certain land organisms are useful in young rocks and in specific regions.
Any type of organism can be distinctive, but not so many are widespread. Many important index fossils are of organisms that start out life as floating eggs and infant stages, which allowed them to populate the world using ocean currents. The most successful of these became abundantand at the same time, they became the most vulnerable to environmental change and extinction. That boom-and-bust character is what makes the best index fossils.
Consider trilobites, a very good index fossil for Paleozoic rocks. Trilobites lived in all parts of the ocean and were constantly evolving new species during their existence, which lasted from Middle Cambrian time to the end of the Permian Period, almost the entire length of the Paleozoic. Because they were mobile animals they tended to inhabit large, even global areas. In addition, their fossils are large enough to study without a microscope. Other index fossils of this type include ammonites, crinoids, rugose corals, brachiopods, bryozoans and mollusks.
Other major index fossils are small or microscopic, part of the floating plankton in the world ocean. These are handy because of their small size. They can be found even in small bits of rock, such as wellbore cuttings. Because their tiny bodies rained down all over the ocean, they can be found in all kinds of rocks. Therefore the petroleum industry has made great use of index microfossils, and geologic time is broken down in quite fine detail by various schemes based on graptolites, fusulinids, diatoms and radiolarians.
For terrestrial rocks, which form on land, regional or continental index fossils may include small rodents that evolve quickly as well as larger animals that have wide geographic ranges.
Index fossils are used in the formal architecture of geologic time for defining the ages, epochs, periods and eras of the geologic time scale. Related fossil types include the characteristic fossila fossil that belongs to a time period but doesn't define itand the guide fossil, one that helps narrow down a time range rather than nail it down.