A good index fossil is from an animal that lived over a limited geologic time. This allows geologists to determine the age of the rock by the presence of the fossil. Other characteristics include the fossil is easy to distinguish, the animal lived over a large range of Earth and that the animal was abundant.
Index fossils help geologists to age rocks in which they form. When layers of rock have index fossils of two different eras, the index fossils are also useful in dating layers between which they form.
Many index fossils are types of marine life. Small organisms, including certain forms of plankton, grow in all areas of the ocean. Their size means they are located in small bits of rock that might not contain other forms of life. Because these fossils are easy to find and easily distinguished, their presence provides confirmation of other processes of aging rocks, including radiocarbon dating.
On the continents, rodents are often useful as index fossils. While widespread, rodents evolve quickly, limiting their geologic time frame. Larger animals with widespread distribution also provide indexing information. The presence of these larger animals often defines provincial time divisions. Some fossils are characteristic of certain periods without defining the period. Guide fossils help to narrow the range of a rock formation but may not provide exact information.