Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Plant Communities – latest Eocene-earliest Oligocene

This was the time of the Terminal Eocene Event, a time of global cooling as Antarctica began to freeze over, making it a critical period in the climatic history of the Cainozoic. In southern Australia evidence has been found that it was also a critical time in biological history. Macphail has listed a number of points regarding the Australian flora of this time.

Of the tree and shrub taxa that were present in the mesothermal-megathermal rainforests of the Eocene significant numbers disappear from the fossil record at this time, including some that were considered typical of the mesothermal-megathermal forests. Of these plant groups Sphagnum spp. had been a constituent of the forests since the Masstrichtian but was extinct by the end of this time.

Nothofagus (Brassospora) became increasingly dominant palynologically in most areas where data is available, with few other taxa such as Casuarinaceae or a podocarp, and Araucariaceae less frequently. Floristic impoverishment has been shown by data from microfossils to be partly an artefact of deposition in specialised peat swamp environments, but Macphail considers the Nothofagus expansion did actually occurred outside the southeastern corner of the continent, also suggesting it can be connected to climatic trends beginning in the Early Eocene.

The resilience of the rainforest species of the Early Tertiary, and their responses to excursions of the climate are thought to have probably varied, resulting in plant community level time-transgressive changes, or possibly masked by local factors. The temperatures along the southern margin that were relatively high are believed to have allowed the the survival  of megathermal species in the flora of the Murray Basin, following their disappearance from the Bassian region to the east. Sedges and reeds were better represented in inland locations, which is another example.

It is not known if the transitional nature of the rainforest flora was the same in other parts of northwest Australia at this time. Macphail suggests that any changes in the dominance in communities in inland Australia could result from trends in precipitation as well as temperature. It is possibe that northeast coastal regions may have begun acting as refugia for the megathermal elements. It is emphatically indicated by the conditions in southeast Australia were most conducive to the dominance of Nothofagus (Brassospora), apparently restricting it to that region. A species that was able to cope with the conditions at lower latitudes in coastal as well as inland environments is Nothofagidites falcata. 

Sources and Further Reading

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