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THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 174 10 The Impact of Climatic Changes on the Development of the Australian Flora David C. Christophel University of Adelaide ABSTRACT Australia in the Tertiary provides an excellent opportunity to examine evolution in an isolated system because for almost 30 million years (m.y.) following the Eocene break with Antarctica, the Australian plate had no major contact with any other. On at least two occasions during that interval, global climatic events were reflected in the fossil plant record. The mid-Eocene cooling is demonstrated by two neighboring floras from either side of the event being dominated by totally different plants. The terminal Eocene cooling is clearly marked by both pollen and megafossil shifts. Thus, through the Tertiary we see a basically greenhouse Eocene Gondwanic flora respond to climatic deterioration and evolve into the sclerophyll and arid communities that dominate the continent today. UNIQUENESS OF THE AUSTRALIAN SYSTEM Although the geological and biological uniqueness of Australia is well known and documented, there are certain aspects of that uniqueness that are particularly relevant to the consideration of climate change and the effect on vegetation though the Tertiary Period. The first of these is related to Australia's geographical isolation during much of the Tertiary. It is generally agreed (e.g., Frakes et al., 1987) that from the Late Eocene (from roughly 38 m.y. ago (Ma) to 8 Maâabout 30 m.y.) the Australian Plate was isolated from all other continental plates. This has two important consequences. The first is that climatic changes, and their reflected vegetation patterns, are not significantly masked or diluted by events on other continents. Second, the consequences of change can be followed through time. As an example, a megafossil flora observed for the Early Miocene must have been based on a gene pool present in the Australian Oligocene since no credible external source is available. A corollary is that if a sudden floristic change is observed over a specific time period, the causal factors must be local (e.g., climatic change) because no outside influence can be seriously considered. Therefore, if one thinks of this time interval as an evolutionary experiment, the variables are far more limited than for any other such "experiments" on different continents.