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Fauna is a generic term for the list of animal species occurring in a particular, large region. Fauna can refer to a prehistoric collection of animals, as might be inferred from the fossil record, or to a modern assemblage of species living in a region. The botanical analogue is known as flora. More locally, a faunation refers to the communities of individuals of the various animal species and occurring in a particular place. Because many zoologists are specialized in the animals they study, faunas are often considered on the basis of systematic groups, as is the case of bird species (avifauna) or reptiles and amphibians (herpetofauna).

A faunal region is a zoogeographic designation of large zones containing distinct assemblages of species that are more-or-less spatially isolated from other provinces by physical barriers to migration, such as a large body of water, a mountain range, or extensive desert. Faunal provinces are less distinct sub-units of faunal regions. These various designations are typically separated by zones of rapid transition in species types.

In the Americas, for example, there are two major faunal regions, with a zone of rapid transition occurring in Central America. The South American zoofauna includes many species and even families that do not occur naturally in North America, and vice versa. The South and North American faunal regions are divided by the narrow Isthmus of Panama, which has been submerged by oceanic waters at various times in the geological past, or has otherwise presented a significant barrier to the migration of many species of animals. However, during periods in the past when animals were able to pass through this barrier, significant mixtures of the two faunas occurred. Lingering evidence of relatively recent episodes of prehistoric faunal blending include the presence of the opossum (Didelphis virginiana) and California condor (Gymnogyps californianus) in North America, and white-tailed deer (Odocoileus virginianus) and cougar (Felis concolor) in South America.

Another famous faunal transition is known as Wallace's Line, after the nineteenth century naturalist who first identified it, A. R. Wallace (he was also with Charles Darwin, the co-publisher of the theory of evolution by natural selection). Wallace's Line runs through the deepwater oceanic straits that separate Java, Borneo, and the Philippines and Southeast Asia more generally to the north, from Sulawesi, New Guinea, and Australia to the south. The most extraordinary faunistic difference across Wallace's Line is the prominence of marsupial animals in the south, but there are also other important dissimilarities.

One of the most famous faunal assemblages in the fossil record is that of the Burgess Shale of southeastern British Columbia. This remarkable fauna includes 15-20 extinct phyla of metazoan animals that existed during an evolutionary radiation in the early Cambrian era, about 570 million years ago. Most of the phyla of the Cambrian marine fauna are now extinct, but all of these lost animals represented innovative and fantastic experiments in the form and function of the invertebrate body plan (and also undoubtedly, in invertebrate physiology, behavior, and ecology, although these cannot be inferred from the fossil record).

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