In 1981, Jack Sepkoski introduced the idea of three major groups of marine animals that evolved over time, from the Cambrian period to the present day (known as the Phanerozoic era). He used a method called factor analysis of the fossil record to study this. Each group usually shows a pattern where biodiversity increases rapidly at first, then slows down, followed by extinctions. However, the Modern Fauna has not yet shown the part of the pattern where biodiversity decreases.

Cambrian fauna

Fauna I, also called "Cambrian," is a group of fossils known for having many trilobites. This group includes most of the fossils that first appeared during the Cambrian explosion, a time when many new life forms quickly evolved. Most of these fossils later went extinct during the Ordovician-Silurian extinction event. This fauna includes trilobites, small shelly fossils (which Sepkoski grouped into "Polychaeta," including cribricyathids, coleolids, and volborthellids), Monoplacophora, inarticulate brachiopods, and hyoliths.

Paleozoic fauna

Fauna II is also called the Paleozoic. It is known for having many brachiopod fossils. This group includes most of the fossils found during the Great Ordovician Biodiversification Event. It was largely extinct during the Capitanian mass extinction and the Permian-Triassic extinction. This fauna includes fossils from the following classes: Articulata, Crinoidea, Ostracoda, Cephalopoda, Anthozoa, Stenolaemata, and Stelleroidea.

Modern fauna

Fauna III, also called "Modern," is described as a group rich in mollusks. This group mainly developed during the Mesozoic-Cenozoic Radiation, a period of diversification that is still ongoing. The following classes are included: Gastropoda, Bivalvia, Osteichthyes, Malacostraca, Echinoidea, Gymnolaemata, Demospongiae, and Chondrichthyes.

Kindred concepts

In the mid-1800s, John Phillips proposed three major time periods: Palaeozoic, Mesozoic, and Cenozoic. Later, Brenchley and Harper, writing after Sepkoski, suggested that two earlier evolutionary animal groups existed before Sepkoski’s three: Ediacaran and Tomottian. They also noted similarities with four types of plant groups: Early Vascular, Pteridophytes, Gymnospores, and Angiospores. Additionally, they identified three major groups of land-dwelling tetrapods: "Megadynasty I (Carboniferous-early Permian)" included primitive amphibians and reptiles, such as Dimetrodon; "Megadynasty II (early Permian -mid-Triassic)" included mammal-like therapsids; and "Megadynasty III (late Triassic-Cretaceous)" included the age of the dinosaurs.