Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Plant Communities – Early Oligocene-late Early Miocene

Macphail has suggested that in Australia the history of the vegetation prior to the Quaternary the best known period is probably the Oligocene to the Miocene. The main problem with this history is the poor biostratigraphic control, especially within the Late Oligocene to Early Miocene, that makes it difficult to summarise. Most spores and pollen originated locally, adding to to problem. What has been indicated by these phenomena is that the vegetation is increasingly provincial and heterogeneous, plant community composition and presumably structure. Macphail has listed a number of conclusions that can be arrived at with at least a moderate degree of confidence.

• Within the Oligo-Miocene flora the number of first appearances and extinctions of spore and pollen species is probably not a reflection of a stable period within the Australian flora. He suggests that ongoing diversification is probably 'concealed' by the morphologies of spores and pollen that are conservative, especially tricolpate and tricolporate types.
• Rainfall that was increasingly seasonal, associated with global cooling, had led to the evolution of floras that were of a cool-cold type, after crossing biological thresholds beyond which it was necessary for the flora to adapt to survive the cooling and more seasonal conditions. The new floras were of the araucarian dry rainforest type, as well as low, open communities in the form of sedgeland and swamps. High elevation, high latitudes and continental interiors are the regions where these occur first. Among the taxa to have their distribution most affected are the Nothofagus (Brassospora spp.) and angiosperms that were thermophilous.

Based on present evidence it appears Nothofagus mesothermal rainforest was increasingly being confined to Tasmania and southeastern Victoria, though a much wider distribution was maintained by some Brassospora spp. such as Nothofagidites falcatus. The presence in lowland rainforest communities of Nothofagus (Brassospora spp.) has been confirmed by macrofossils, though they challenge the importance of the presence in this vegetation of forms such as Lyphozonia spp., that are believed to have been the dominant taxon around the edges of Tasmanian lakes and rivers. It is not certain if the same applied to situations such as upland areas. Though at middle to high elevations Nothofagus microthermal rainforest appears to have evolved. Some evidence suggests that the Brassospora spp. had been increasingly confined to upland areas of mainland Australia. Macphail suggests that at best they were minor elements of ?gallery wet forest along the inland rivers as well as in the southwestern region of Australia that was dominated by one or more of the families Casuarinaceae, Lauraceae and Myrtaceae.

In southeastern Australia the associated decline of thermophilous angiosperms, and in Tasmania, of Casuarinaceae, was offset by herb and cryptogam floras that were becoming increasingly diverse. In the Murray Basin and along the South Australian coast Araucariaceae and other gymnosperms were becoming increasingly prominent, though was a high proportion of minor taxa that had mesothermal-megathermal well-defined affinities. Limited pollen resolution has meant that it is uncertain to what extent the palynofloral similarities between the Murray Basin, as well as  other inland areas, and southern Queensland are artefacts. In Queensland the rainforests are likely to be megathermal and nonseasonal, in the Murray Basin they were more clearly mesothermal and seasonal.

During the Oligocene-Early Miocene the flora was strongly ecologically differentiated. Some plant communities were given a modern appearance by the first appearances of families such as the Asteraceae, Chenopodiaceae, Mimosacae and Poaceae.

According to Macphail macrofossils demonstrate that this is deceptive and he mention several points:

1. The actual diversity of flora is rather greater than the pollen and pores suggest, a situation that applies to the Cainozoic in general,
2. The flora had a much wider phytogeographical affinities than had been believed.

Records have been found in Tasmania of Acmopyle (Podocarpaceae) and Libocedrus (Cupressaceae). One discovery that has surprised many is that in the Oligocene many of the taxa that first appear in the fossil record at this time have affinities with South American taxa. All taxonomic groups are represented, such as Lophosoria (Filicopsida) Austrocedrus (Cupressaceae), Mutiseae (Asteraceae) and Embothrium (Proteaceae). Lophosoria and the two angiosperms appear to have migrated to Australia, though Macphail suggests the gymnosperms could have derived from the Gondwanan stock that Australia inherited. The taxa that migrated to Australia are assumed to have arrived via Antarctica, though Lophosoria was the only one of these migrants to have been found in Tertiary deposits in this region.

A strong contrast had been established between environments in the coast and the interior, though it was not strong enough to lead to the evolution sclerophyllous plant formations or of grasslands on the regional scale.

Sources and Further Reading

1. Macphail, M.K., in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press