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Certain fossils were readily identified with specific geological formations in the stratigraphic sequence of England, and later similar formations in western Europe.

These fossils were found to be useful even where the sedimentological characteristics of the formations differed over distances.

Lyell noted that of the various assemblages of marine mollusks found, those from... As was explained earlier, at specific stratigraphic boundaries certain types of fossils either appear or disappear or both in some cases.

Such biostratigraphic boundaries separate larger or smaller units of time that are defined as eons, eras, periods, epochs, and ages.

A biostratigraphic zone is a body of rock defined or characterized by its fossil content.

The clustering of fossil extinctions often represents missing or condensed sections.

Informal biostratigraphy is concerned with using fossil taxa to help define ancient environments, a type of study called paleoecology (the study of ancient ecology preserved in sedimentary rocks).

Trilobites appeared before ammonites, for example, and dinosaurs became abundant before mammals.The inception (“*” in the illustration below), or lowest occurrence of a species or lineage, is a reliable datum only in core or outcrop samples because caving is virtually unavoidable in cutting samples; however, it can help refine the stratigraphy.The overlap of species extinctions and inceptions allows the development of range zones (see figure below), which can be correlated from site to site.Biostratigraphers study the distribution of fossils in sedimentary strata.They have two motives – reconstructing the history of life and developing a relative time scale for other geologic studies.