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 this. An evolutionary fauna usually shows an increase in the number of different species over time, following a logistic curve, which is then followed by extinctions. However, the Modern Fauna has not yet shown the part of the curve where the number of species starts to decrease.

Cambrian fauna

Fauna I, also called "Cambrian," is a group of fossils known for having many trilobites. It includes most of the fossils that first appeared during the Cambrian explosion and mostly disappeared during the Ordovician-Silurian extinction event. This group contains trilobites, small shelly fossils (classified by Sepkoski into "Polychaeta," which includes cribricyathids, coleolids, and volborthellids), Monoplacophora, inarticulate brachiopods, and hyoliths.

Paleozoic fauna

Fauna II, also called the "Paleozoic," is described as a group rich in brachiopods. This group contains many of the fossils found during the Great Ordovician Biodiversification Event. Most of these organisms died out during the Capitanian mass extinction and the Permian-Triassic extinction. This fauna is identified by fossils from the following groups: Articulata, Crinoidea, Ostracoda, Cephalopoda, Anthozoa, Stenolaemata, and Stelleroidea.

Modern fauna

Fauna III, also called "Modern," is a group with many types of mollusks. This group mainly developed 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-19th century, John Phillips proposed three major systems: Palaeozoic, Mesozoic, and Cenozoic. Later, after Sepkoski, Brenchley and Harper suggested that two earlier evolutionary faunas existed before Sepkoski’s three: Ediacaran and Tomottian. They also noted similarities with four groups of plants that evolved on land: Early Vascular, Pteridophytes, Gymnospores, and Angiospores. Additionally, they identified three groups of land-dwelling tetrapods: "Megadynasty I (Carboniferous-early Permian)" which included primitive amphibians and reptiles, especially Dimetrodon; "Megadynasty II (early Permian -mid-Triassic)" which included mammal-like therapsids; and "Megadynasty III (late Triassic-Cretaceous)" which included the age of the dinosaurs.