In 1981, Jack Sepkoski introduced the idea of three major evolutionary faunas of marine animals from the Cambrian period to the present day, which covers the entire Phanerozoic era. This idea was developed by analyzing the fossil record using a method called factor analysis. An evolutionary fauna usually shows an increase in the number of different species over time, following a pattern shaped like an S-curve, which is then followed by mass extinctions. However, the Modern Fauna has not yet shown the part of the curve where biodiversity begins to decrease.

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 began to develop. Many of these species later disappeared during the Ordovician-Silurian extinction event. Fauna I includes trilobites, small shelly fossils (classified by Sepkoski as "Polychaeta," which also includes cribricyathids, coleolids, and volborthellids), Monoplacophora, inarticulate brachiopods, and hyoliths.

Paleozoic fauna

Fauna II, also called "Paleozoic," is described as a group with many brachiopod fossils. It has the most fossils found during the Great Ordovician Biodiversification Event. This fauna largely disappeared during the Capitanian mass extinction event and the Permian-Triassic extinction event. Fossils from this group include the following classes: Articulata, Crinoidea, Ostracoda, Cephalopoda, Anthozoa, Stenolaemata, and Stelleroidea.

Modern fauna

Fauna III, known as "Modern," has many types of mollusks. It began during the Mesozoic and Cenozoic eras and is still happening today. The following groups are included: Gastropoda, Bivalvia, Osteichthyes, Malacostraca, Echinoidea, Gymnolaemata, Demospongiae, and Chondrichthyes.

Kindred concepts

In the mid-1800s, John Phillips proposed three major time periods: Paleozoic, Mesozoic, and Cenozoic. Later, scientists Brenchley and Harper wrote about two earlier groups of animals that existed before Sepkoski’s three: Ediacaran and Tomottian. They also noted similarities among four groups of plant life: Early Vascular, Pteridophytes, Gymnospores, and Angiospores. Additionally, they identified three groups of land-dwelling four-legged animals: "Megadynasty I (Carboniferous to early Permian)" which included early amphibians and reptiles, such as Dimetrodon; "Megadynasty II (early Permian to mid-Triassic)" which included mammal-like reptiles; and "Megadynasty III (late Triassic to Cretaceous)" which included the age of the dinosaurs.