For instance, rock units in the Grand Canyon can be found over parts of four states. Principle of Faunal Succession Fossils have appeared throughout geologic time in patterns that are never repeated. Once a fossil disappears from the record, it never reappears.
Fossils define a specific time horizon.
If a species existed for only a short time, its fossils will define a short period of time. We can use fossils to correlate rock units from one place to another, even though those rocks may not be identical. Fossils used in this way are called index fossils . Limited time in existence as a species. Marine Transgressions and Regressions Shorelines move throughout time.