Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Plant Communities Middle to Late Eocene

Nothofagus (Brassospora spp.) underwent an expansion that was widespread, though it varied locally, at this time, largely continuing a trend from megathermal to mesothermal rainforests. Macphail suggests it may be at least partly an artefact of the geological record that indicates there was an apparently rapid spread. In southern Queensland the Nypa mangrove palm  and other tropical mangrove taxa continued to be preset, though it disappeared from the Bassian region. They were also possibly present on the South Australian coast, their presence in this case being suggested to be made possibly by the warm shelf waters (Macphail, 1994).

The first appearance of modern, xerophytic genera at this time is consistent with the climatic trend. These included such plant types as the Dodonaea triquetra type and the Banksia serrata type. The 'open habitat' taxa that included sedges and rushes also underwent a time-transgressive expansion. Megathermal lineages within groups such as the Fabaceae (Caesalpinoidae) and the Malpighiaceae migrated into southern Australia, apparently being permitted to enter this region because of local conditions. As there is no known record of these megathermal groups in the central parts of the continent it has been suggested that they probably spread around the coast.

It has been suggested that the 'competitive ability and gregarious behaviour of Nothofagus' (Macphail, 1994) was strongly linked to the diversity of floras in the Middle-Late Eocene. Swamp habitats appear to be the only habitats of the coastal/lowland regions of southeastern Australia that Nothofagus did not invade and dominate, the result being that the rainforest communities were species-poor. This doesn't appear to have occurred in the coastal areas to the west and north that were possibly warmer where there are records of mesothermal-megathermal rainforest associations. The flora of the Murray Basin was very similar to that of the Queensland south coast, based on the pollen record, and within the limits of pollen resolution. There doesn't appear to be a reason to believe the source, rainforest vegetation, was identical in physiognomy or the species that were dominant.

In the interior of the continent not many consistent patterns have been established. An exception is that Nothofagus (Brassospora spp.) were included in a mosaic of rainforest associations in which the pollen dominance was shifting among Nothofagus and a number of other taxa such as Casuarinaceae, Cunoniaceae, gymnosperms and Myrtaceae.

There was continued diversification of both number of species and ecotypes among groups originating in Gondwana, such as Proteaceae and Nothofagus, Some of these were Proteacidites pachypolus and Nothofagus falcatus. N. falcatus made its first appearance in the fossil record of southeast Australia in the Middle Eocene, rapidly extending its range though eastern Australia and some areas of central Australia. During the Oligocene it reached the the northwest of Australia and New Zealand. During the Early Eocene P. pachypolus is found in environments that were influenced by saltwater, becoming widely established around coastal lowland swamps, and by the Late Eocene it is found in uplands and in the interior of the continent.

From the Early Eocene a trend had been established towards ecological diversification, as well as geographical provincialism, of the flora of Australia. An example of the provincialism is seen in the southwest sector in the high diversity of the Proteaceae.

Sources and Further Reading

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

 

 

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Last updated 22/10/2011

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading