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The dynamic model of the world postulated by plate tectonic theory provides a new basis for speculation about the biogeography of living organisms. Previous interpretations of the history of the marsupial mammals have constituted important arguments for both the stabilist and mobilist (or land bridge) concepts of zoogeography. However, the geological record of the marsupials has never permitted an adequate test of the views of these divergent schools of thought. Now, however, plate tectonics places geographic constraints on biogeographic hypotheses and helps to establish levels of probability among the various hypotheses proposed on biological grounds. In this paper consideration of relevant geological and biological evidence has led to ideas regarding the origin, evolution, and dispersal of the marsupials that are consistert with the new paleogeography. They are offered as logical alternatives to those currently held. Further fossil evidence, especially from the Cretaceous, will be necessary to test the various hypotheses developed from the information presently available.

Plate tectonic models of the Late Jurassic and Early Cretaceous world show the Gondwana continents in an intermediate dispersal stage. Lingering contacts between South America and Africa and archipelagic links of South America with Laurasia and to the united Australoantarctic continent are corroborated by the wide distribution of similar terrestrial biota. Pantothere stocks (possibly peramurids) ancestral to the higher theria apparent were widely distributed, although among the southern continents they are so far known only in Africa. Therians with fully tribosphenic molars of metatherian-eutherian grade are known from the mid-Cretaceous of North America and Asia; their absence elsewhere in the world at that time is regarded as an artifact. Metatherians of didelphid-grade, ancestral to all later marsupials, apparently arose in the environmently varied South American continent in Late Cretaceous (post-Albian) time. Adaptive radiation through a wide range of environments extending from latitude 0° -70°S produced species which dispersed northward through the tropical middle American archipelago and southward following an archipelagic path to temperate Australoantarctica. Increasing geographic and climatic isolation in Late Cretaceous time led to endemic continental faunas, although the close proximity of North and South America still allowed limited exchange of marsupicarnivorans. Marsupial evolution was drastically reduced in Tertiary time in North America, probably due to competition with the adaptively similar placental insectivores of Holarctica. Only in the isolated southern continents did marsupials form important elements in the terrestrial fauna. Their importance is directly correlated with their degree of isolation from Holarctica. Australia remained a part of East Antarctica and at high latitudes until Eocene time. Dispersal of mammals to the region involved island-hopping along the volcanic archipelago bordering the Pacific side of Antarctica. Although this temperate region seems to have acted as a strong filter to the dispersal of mammals (no placentals were able to reach Australia), other terrestrial organisms found their way across it readily, giving the flora and other elements of the fauna of these temperate southern continents a remarkable similarity. Little is known of the early adaptive radiation that presumably resulted after the arrival of marsupials in Australia. A second major period of diversification may have followed the rifting and drift of Australia north into warmer latitudes. Marsupials shared the South American continent with placental edentates and protungulates, and their interaction probably shaped to some degree the adaptive radiation of each.

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