Index fossils are used to determine the age of rock layers
An index fossil is a fossil that can be used to assign a relative age to a layer or layers of rock, thus placing the layer and any other fossils in it at the proper place on the geologic time scale.
Good Index Fossils are Common
To be useful as an index fossil, the fossil must be common. The more common a fossil, the more likely a paleontologist will find it in a layer of rock. Rare forms are not useful, because their absence does not necessarily mean that the rocks are of a different age--it may simply mean that the rare animal wasn't nearby.
Good Index Fossils are Widespread
The best index fossils are animals that are present in rocks all over the world, and present in a wide variety of rock types. Swimming or floating animals tend to make better index fossils because their remains can be found almost anywhere, while plants and most bottom-dwellers live only within a narrow range of conditions.
Good Index Fossils Represent a Limited Time Span
The presence of a fossil animal that flourished for several geologic time periods does little to narrow down the age of a rock layer. Fossil animals that were around for a relatively short time allow more precise definition of a layer's relative age.
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