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Effects of Past Global Change on Life
A second important, unique feature of the Australian system is the nature of its plant fossil record. Australia has perhaps the only tropical to subtropical rain forest system in the world that has a well-documented macrofossil record; hence, its evolution can be traced through time. There are three factors contributing to this situation: first, a large number of Australia's macrofossil deposits are preserved as mummified leaves, allowing maximum taxonomic and physiognomic information to be gleaned from them (Christophel, 1981). Second, the isolation of the continent alluded to earlier means that there is a far better chance of actually identifying taxa and communities, and tracing them through time, without having to search for floras of other continents for matches. A consequence is that far greater confidence can be placed on labeling an unidentifiable fossil as extinct because the likelihood of an external taxonomic affinity is much reduced. Finally, a large number of deposits are known from the portion of the Eocene Epoch at or near the time of the early Tertiary plate separation, providing a better than average understanding of the gene pool from which later floristic elements must have been derived. A similar, though somewhat weaker case, can be made for the documentation of some of Australia's less mesic vegetation types, the qualifying feature here being the more recent evolution of these vegetation forms and their components, and hence the greater chance of external influence following Miocene collision with the Sundra Plate (Kemp, 1981).
GEOLOGICAL AND PLATE TECTONIC SETTING
The major events in the physical movements of the Australian Plate during the Tertiary are not contentious. There is general agreement that during the early Paleogene, Australia was attached to Antarctica via its southeastern corner and Tasmania (Figure 10.1) and that, although by mid-Miocene the shelves between components were likely submerged, they were still joined and oceanic circulation over that shelf was minimal. Near the end of the Eocene the rate of northward movement of the Australian Plate increased two- to threefold, and it continued at that rate until the leading, northern edge collided with the island arcs of the Sundra Plate in the Middle- to Late Miocene (Galloway and Kemp, 1981). Tectonic activity was minimal across most of the plate during this isolated rafting period, with the uplift of the eastern highlands likely occurring at an early stage of the Miocene (Ollier, 1986).
MODERN VEGETATION OF AUSTRALIA
In order to assess the impact of Tertiary climatic change on the makeup of the modern Australian flora, it is first necessary to categorize the floristic or vegetational elements in the modern-day flora. A somewhat simplified vegetation map of Australia is shown in Figure 10.2. By distilling that further, it is possible to identify four categories of vegetation: (1) the closed forest systems, (2) the open forest or woodland systems, (3) the heath scrub or mallee systems, and (4) the great arid and semiarid systems that occupy a high percentage of the continental mass. A more thorough treatment of specific vegetation types in Australia may be found in Specht (1981a,b).
In examining the vegetation types one at a time, the first type to be considered is the closed forest system. As may be seen from Figure 10.3A, this system can also
FIGURE 10.1 Reconstruction of Australia in the Eocene showing location and paleolatitude of several Eocene megafossil localities referred to in the text. The estimated altitudes and inferred forest type of each Eocene flora are shown graphically to the right of the map. MMF is microphyll mossy forest, SNVF is simple notophyll vine forest, CNVF is complex notophyll vine forest, and CMVF is complex mesophyll vine forest. (Modified from Christophel and Greenwood, 1989.)