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 litera