The Neogaean, or Neotropical, realm extends south from the tropical lowlands of Mexico through Central America into South America as far as the temperate and subantarctic zones and includes the West Indies (Figure 2). Among endemic mammal groups, the Didelphimorphia (an order of marsupials) and several distinctive placental orders, such as the Edentata (and several extinct orders), have been present since the Paleocene (65.5 million to 55.8 million years ago). By the Oligocene (33.9 million to 23 million years ago) the platyrrhines (New World monkeys) and a group of rodents (the Caviomorpha) had entered South America by means that are still not understood. Among birds, two entire orders—the flightless Rheiformes (rheas) and Tinamiformes (tinamous)—and 30 families are endemic. Some fish and invertebrate taxa also are endemic. Many of these endemic taxa are believed to date from Gondwanan times (the Early Cretaceous), when the southern continents formed a single landmass. Evidence for this view is provided by the presence in Africa and Australia of their nearest relatives—e.g., the flightless birds, lungfish, bony fish families such as Cichlidae, and many invertebrates (notably the primitive Onychophora, known as velvet worms).

In the West Indies, which are an impoverished region within Neogaea, distinctive mammals include two endemic insectivore families, Solenodontidae (solenodon, almiqui) and the recently extinct Nesophontidae. The Galapagos Islands have an impoverished fauna ultimately derived from South America.

Antarctic realm

The Antarctic, or Archinotic, realm encompasses the Antarctic continent, subantarctic islands, and elements of southwestern New Zealand. The existence of the realm—or rather of its ghost, because nowhere today does it exist in an umixed state—is justified by the common occurrence in New Zealand and South America of such groups as the Eustheniidae (a family of stoneflies), the crustacean order Stygocaridacea, and certain freshwater snails. It is plausible that the marsupial family Microbiotheriidae, which is confined to Chile and is more closely related to the Australian marsupials than to other South American ones, is a relic of an Antarctic connection.