Fauna includes all of the animals that have existed historically in a region, an environment, or an era. In palaeontology, it is often categorised or named according to the animal types, distribution or geologic periods. Its boundaries are not definite. Examples of spatial classification are the North China fauna and the Southeast fauna. Examples of geologic period classification are the Cambrian Period fauna and the Early Cambrian Epoch fauna. Examples of biological classification are the Actinoceras fauna and the Orthocera fauna.