In 1981, Jack Sepkoski introduced the idea of three major groups of marine animals that have existed from the Cambrian period to the present day, known as the entire Phanerozoic era. He used a method called factor analysis of the fossil record to study these groups. Each evolutionary fauna usually shows an increase in the number of different species over time, following a pattern that starts slowly, grows quickly, and then slows down again. This pattern is often called a logistic curve. After reaching a peak, these groups often experience mass extinctions. However, the Modern Fauna has not yet shown the part of the curve where the number of species begins to decrease.

Cambrian fauna

Fauna I, known as the Cambrian period, is a group of fossils rich in trilobites. This group includes most of the fossils that first appeared during the Cambrian explosion and mostly went extinct during the Ordovician-Silurian extinction event. The fossils found in this group include trilobites, small shelly fossils (grouped by Sepkoski into "Polychaeta," but including cribricyathids, coleolids, and volborthellids), Monoplacophora, inarticulate brachiopods, and hyoliths.

Paleozoic fauna

Fauna II, also called the "Paleozoic," is a group of ancient life forms known for having many brachiopods. This group includes most of the fossils found during the Great Ordovician Biodiversification Event. Most of these life forms disappeared during the Capitanian mass extinction and the Permian-Triassic extinction. This fauna includes fossils from these classes: Articulata, Crinoidea, Ostracoda, Cephalopoda, Anthozoa, Stenolaemata, and Stelleroidea.

Modern fauna

Fauna III, also called "Modern," is a group that has many types of mollusks. It began mainly during the Mesozoic and Cenozoic eras, which are still continuing today. The following classes are included: Gastropoda, Bivalvia, Osteichthyes, Malacostraca, Echinoidea, Gymnolaemata, Demospongiae, Chondrichthyes.

Kindred concepts

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