Evolutionary Fauna

The concept of the three great Evolutionary Faunas of marine animals from the Cambrian to the Present (that is, the entire Phanerozoic) was introduced by Jack Sepkoski in 1981 using factor analysis of the fossil record. An evolutionary fauna typically displays an increase in biodiversity following a logistic curve followed by extinctions (although the Modern Fauna has not yet exhibited the diminishing part of the curve).

Fauna I, known as "Cambrian", described as a "Trilobite-rich assemblage", encompasses the bulk of the fossils which first appeared in the Cambrian explosion, and largely became extinct in the Ordovician-Silurian extinction event. This fauna comprises trilobites, small shelly fossils (grouped by Sepkoski into "Polychaeta", but including cribricyathids, coleolids, and volborthellids), Monoplacophora, inarticulate brachiopods and hyoliths.

Fauna II, known as "Paleozoic", described as a "Brachiopod-rich assemblage", accounts for most of the fossils appearing in the Great Ordovician Biodiversification Event, and largely became extinct in the Permian-Triassic extinction event. This fauna is marked by fossils of the following classes: Articulata, Crinoidea, Ostracoda, Cephalopoda, Anthozoa, Stenolaemata, Stelleroidea.

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