What Is Faunal Succession

Fossils are key to understanding the Law of Faunal Succession

Figuring out when things happened during the long expanse of the Earth’s history became a lot easier when William Smith (1769-1839) discovered the law of faunal succession. The concept grew of out his work on the science of stratigraphy (which he pretty much invented). Stratigraphy is a branch of geology concerned with rock layers and layering (stratification). It relates primarily to sedimentary and layered volcanic rocks.


Smith saw that certain rock layers were likely to contain certain types of fossils. Fossils are the remains or traces of animals (fauna or faunal means “animal”), plants and other living things from long ago. Fossils occur in sedimentary rock, which is formed by compacting particles of sand, mud, decaying and other materials under pressure over long periods of time. (Igneous and metamorphic rock are formed under very hot conditions that usually destroy the distinguishable remains of organic material, so they generally do not contain any fossils.)

Sequence of Rock Layers

Based on the Law of Superposition discovered in the 17th century, which states that in a sample of sedimentary rock layers, each layer is older than the layer above it and younger than the layer below it, Smith figured out that the older the rock, based on how far down in the pile of layers it was, the more primitive were the fossils it contained.

Odd Fossil Out

Study of the fossils and the rock layers led Smith to be able to distinguish between strata of similar rocks by the assemblages (groups) of fossils that were present. Very similar looking rock strata might contain exactly the same fossils, except for one species. Depending on whether the odd species was simpler or more complex in structure than the fossils around it, Smith could assign the strata a place on a time line. Smith described his finding: “For it was the nice distinction which those similar rocks required, which led me to the discovery of organic remains peculiar to each Stratum.”

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A Definite Order

Smith’s discovery became known as the “Law of Faunal Succession.” It states that fossils occur in a definite, unchanging sequence in the geologic record. It is a concept used by all geologists today in their study of rocks, time lines and evolution. The U.S. Geological Survey summarizes the Law of Faunal Succession this way: “Fossils appear and disappear in a definite order.”

History of the World

Using the Law of Faunal Succession and rock samples from around the world, geologists have been able to form a detailed history of the Earth and build a geologic time scale going at least as far back as the Cambrian Period (543 to 490 million years ago), when most of the major groups of fossil animals appear in the geological record.