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THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 182 TABLE 10.1 Main Plant Groups Represented in Australian Megafossil Deposits PALAEOGENE NEOGENE Proteaceae Banksia/Hakea Myrtaceae (non-Eucalyptus) Eucalyptus Gymnostoma Casuarina Podocarpaceae Cupressaceae Araucariaeae Epacridaceae Nothofagus Chenopodiaceae Elaeocarpaceae Asteraceae Restionaceae Poaceae Lauraceae Acacia/Cassia NOTE: Taxa underlined in the right-hand column may be considered as likely direct replacements (either taxonomically or vegetationally) for those opposite them in the left-hand column. DISCUSSION If the four major types currently found in Australia are examined once more, they can be viewed in the light of major Tertiary climatic changes to the continent. The closed forests of the north and east of the continent clearly have their affinities with the greenhouse and near-greenhouse phases of Eocene Australia. Gondwanic families dominate, and in many cases the relationships are reflected at the generic level. Although there is no Eocene evidence for Eucalyptus, Acacia, or other taxa listed as Neogene in Table 10.1, Beadles's hypothesis suggests that they nonetheless could have evolved under depauperate soil conditions, but in quantities too small to be observed in the fragmentary fossil record. Then too, they may well have been initiated or survived in the mid-Eocene cooling represented by McGowran's (in Frakes et al., 1987) 8-m.y. ''hole" in Australia's fossil record. Whatever their Eocene status, the sclerophyllous plant elements that dominate the open forest and the heath scrub today either evolved or spread during the Oligocene-Miocene refrigeration. This of course was aided by the inability of the Gondwanic closed forest components to survive over large areas during this climatic deterioration. The mid-Miocene warming suggested by Frakes et al. (1987) may well have guaranteed the survival of some of those Gondwanic elements that struggled through the refrigeration, and may also be reflected in the mixture of floral provinces in some vegetation types such as the forests at Wilson's Promontory near the southern tip of Victoria. As might be expected, the macrofossil record for arid floras is poor, although Chenopodiaceae and Mimosaceae pollen is well documented from the late Miocene and Pliocene of several localities (Martin, 1981). Although for Charles Darwin the flowering plants represented the "abominable mystery," for Australian researchers it is perhaps Acacia. Although the genus is one of the few to occur in all major Australian habitats, and contains more than 650 species in Australia (Morley and Tolkein, 1983), there is only one confirmed report of fossil leaves from the late Miocene (Christophel, 1989), and pollen is not common. Thus, the explanation for the origin and spread of a genus whose distribution suggests it to be Gondwanic, and hence ancient, remains shrouded but is almost certain to be related, when once unraveled, to the changing Tertiary climates. ACKNOWLEDGMENTS Much of the research for this project was supported by grants from the Australian Research Council, Alcoa of Australia, and the Adelaide University/CSIRO Granting Scheme. The figures for this chapter were prepared by Linda Allen and Leonie Jane Scriven. REFERENCES Bailey, I. W., and E. W. Sinnott (1916). The climatic distribution of certain kinds of angiosperm leaves, American Journal of Botany 3, 24-39. Beadle, N. C. W. (1966). Soil phosphate and its role in molding segments of the Australian flora and vegetation, with special reference to xeromorphy and sclerophylly, Ecology 47, 992-1007. Christophel, D. C. (1981). Tertiary megafossil floras as indicators of floristic associations and paleoclimate, in Ecological Biogeography of Australia, A. Keast, ed., W. Junk Publishers, The Hague, pp. 379-390. Christophel, D. C. (1989). Evolution of the Australian flora through the Tertiary, P. Syst. Evol. 162, 63-78. Christophel, D. C., and D. T. Blackburn (1978). Tertiary megafossil flora of Maslin Bay, South Australia: A preliminary report , Alcheringa 2, 311-319. Christophel, D. C., and D. R. Greenwood (1987). A megafossil flora from the Eocene of Golden Grove, South Australia, Transactions of the Royal Society of South Australia 111, 155-162. Christophel, D. C., and D. R. Greenwood (1988). A comparison of Australian tropical rainforest and Tertiary fossil leafbeds, in The Ecology of Australia's Wet Tropics, R. Kitching, ed., Proceedings of the Ecological Society of Australia 15, Surrey Beatty & Sons Pty. Ltd., Chipping Norton, New South Wales, pp. 139-148. Christophel, D. C., and D. R. Greenwood (1989). 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