All the species of animals found in a particular region, period, or special environment. Five faunal realms, based on terrestrial animal species, are generally recognized: Holarctic, including Nearactic (North America) and Paleartic (Eurasia and northern Africa); Paleotropical (tropical Africa and Southeast Asia); Neotropical (Central and South America); Australian; and Antarctic.
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Paleontologists sometimes refer to a sequence of faunal stages, which is a series of rocks all containing similar fossils.
The name comes from Fauna, a Roman fertility and earth goddess, the Roman god Faunus, and the related forest spirits called Fauns. All three words are cognates of the name of the Greek god Pan, and panis is the Greek equivalent of fauna. Fauna is also the word for a book that catalogues the animals in such a manner. The term was first used by Linnaeus in the title of his 1747 work Fauna Suecica.
Infauna are aquatic animals that live within the bottom substratum rather than on its surface. Bacteria and microalgae may also live in the interstices of bottom sediments. On average, infaunal animals become progressively rarer with increasing water depth and distance from shore, whereas bacteria show more constancy in abundance, tending toward one billion cells per milliliter of interstitial seawater. (Infauna are benthos that live buried in underwater mud.)
Macrofauna are benthic or soil organisms which are at least one millimeter in length.
Megafauna are large animals of any particular region or time. For example, Australian megafauna.
Meiofauna are small benthic invertebrates that live in both marine and fresh water environments. The term Meiofauna loosely defines a group of organisms by their size, larger than microfauna but smaller than macrofauna, rather than a taxonomic grouping. In practice these are organisms that can pass through a 1 mm mesh but will be retained by a 45 μm mesh, but the exact dimensions will vary from researcher to researcher. Whether an organism will pass through a 1 mm mesh will also depend upon whether it is alive or dead at the time of sorting.