Faunal succession describes a principle in geology and paleontology, stating that fossilized organisms succeed each other in a definite, recognizable order through geological time. This means specific groups of plants and animals appear and disappear from the fossil record at distinct points in Earth’s history. This ordered progression helps understand Earth’s timeline and provides a framework for interpreting the record of life preserved within sedimentary rocks.
The Core Principle
Different species have inhabited Earth at various times. Once a species becomes extinct, it does not reappear in the fossil record. This means rock layers from different geological periods contain distinct fossil assemblages. For instance, rock layers with dinosaur fossils are older than those with large mammal fossils, reflecting the sequence of life forms over millions of years. This ordered appearance and disappearance is a direct consequence of biological evolution and extinction events.
This consistent pattern allows scientists to identify specific time intervals based on the unique combination of fossils in a rock layer. Certain fossils, known as “index fossils,” are useful for this. Index fossils are widespread geographically, abundant within specific rock layers, and represent species that existed for a relatively short geological period before becoming extinct. Their distinct characteristics and limited temporal range make them markers for dating and correlating rock units.
The Discovery of Faunal Succession
The principle of faunal succession was first recognized by William Smith in the late 18th and early 19th centuries. Smith, an English canal surveyor, observed that specific rock layers consistently contained the same types of fossils, even when the rock type changed across locations. For example, he noticed a particular ammonite type always in a certain shale layer, and a distinct brachiopod always in an overlying limestone.
These observations, made while mapping geological strata for canal construction, allowed him to predict which fossils would be found in unexposed rock layers. Smith’s work compiling geological maps based on fossil content advanced the understanding of stratigraphy. His insights demonstrated that fossils followed a predictable sequence, providing a tool for geological mapping and correlation.
Applying Faunal Succession in Science
Faunal succession serves various applications in modern geology and paleontology. Scientists use this principle to determine the relative age of sedimentary rock layers by identifying their characteristic fossils. If a rock layer contains a specific index fossil, its age can be inferred relative to other layers with different index fossils. This allows for constructing a chronological sequence of geological events without needing absolute numerical dates.
Geologists also use faunal succession to correlate rock layers across vast geographical distances, even between continents. By matching rock units with identical or similar fossil assemblages, scientists can establish that these layers formed at approximately the same time. This correlation helps develop a global geological timescale, mapping Earth’s history in distinct time intervals. The principle also aids in reconstructing ancient environments; organisms in the fossil record provide clues about past conditions, such as whether an area was a shallow sea, deep ocean, or terrestrial forest.