Faunal stages are subdivisions of rock layers used primarily by paleontologists who study fossils rather than by geologists who study rock formations. Typically, a faunal stage will consist of a series of rocks that contain similar fossils. There will be one or more index fossils that are usually common, easily recognized, and limited to a single, or at most a few, stages. Thus, for example, a North American paleontologist finding fragments of the trilobite Olenellus would identify the beds as being from the Waucoban Stage whereas fragments of a later trilobite such as Elrathia would identify the stage as Albertan. Stage is a term defining a package of rocks; it is equivalent to the term age defining a period of time, although the two words are often used interchangeably in informal literature.
Faunal stages were very important in the 19th century and early 20th Century as they were the major tool available for dating rock beds until the development of seismology and radioactive dating in the second half of the 20th Century.
Faunal stages are regional. They often include many formations of differing rock types that were being laid down in different environments at the same time. In recent years, regional and global correlations of rock sequences have become relatively certain and there is less need for faunal labels to refine the age of formations. There has been a tendency to use European and, to a lesser extent, Asian, age names for the same time period world wide even though the faunas in other regions may have little in common with the stage as originally defined.