The principle of fossil succession states that groups of fossils appear in a chronological order through their vertical placement in sedimentary rocks. In the same way that the oldest rocks are found in the bottom layer of the earth, the oldest fossils also followed the same chronology as they appear in the same set of rock layers, or strata.
Fossils of species that did not coexist in the same geological period were found to be in different strata. The principle provided a way for geologists to determine the age of a particular rock layer based on the fossils embedded in it. Fossil succession is used not only as a basis for rock age determination but also as a way to improve fossil fuel discoveries for coal extraction.
The concept was formulated by William Smith, an English civil engineer, in the 1800s. He discovered the vertical layers of rocks and fossils where he excavated. Together with fossil collectors Benjamin Richardson and Joseph Townsend, Smith analyzed the rock strata and segregated them by color and hardness. The team noticed that the rock layers contained varying kinds of fossils, with each layer dominated by a different fossil group from the others. Smith deduced that any rock strata may be segregated according to the fossils that they contain. This was the idea behind the principle of fossil succession, also called faunal succession.