A fossil with widespread geographical range but which is restricted in time to a brief existence. In archaeology, it is a theory that proposes that strata containing similar fossil assemblages will tend to be of similar age. This concept enables archaeologists to characterize and date strata within archaeological sites using diagnostic artifact forms, making an animal species the basis for dating by faunal association. Artifacts that share the attributes of index fossils are useful in the cross-dating and correlation of deposits that contain them and in the construction of chronologies.