Tertiary Australia^{198, 202, 203}

For most of the Paleocene Australia was still joined to Antarctica although a rift valley was developing between them. Southern Australia was at 65°S but there is evidence that sea temperatures were of subtropical warmth so temperatures on land were probably warm also. Microfossils indicate widespread forests dominated by Podocarpaceae; Nothofagus was rare. Araucariaceae, Proteaceae, and Myrtaceae were also represented and in addition Anacolosa and possibly Cupania, genera now restricted to the tropics. Forests of this type were also present in Central Australia so the general climate was moist as well as warm. High rainfall, warm temperatures and low relief at this and later times resulted in many places in intensely leached infertile soils which have persisted to the present day.

At the beginning of the Eocene, final separation of Australia and Antarctica began. At first conditions continued to be warm and moist and further tropical genera were added to the forests such as Bombax (the kapoc genus), and the tropical mangrove palm Nipa. Conifers and Nothofagus were still poorly represented. From the middle Eocene the climate cooled, a number of the tropical genera disappeared from southern Australia, and pollen of N. brassii group increased in importance.

During the Oligocene the climate cooled further and the sparse fossil record indicates similar floras to the late Eocene although with some reduction in diversity. It is suggested that with the lower temperatures the climates would also have been drier and perhaps relatively arid in northern parts. An ice cap probably began to develop in Antarctica at this time.¹⁹⁸

In the early Miocene the climate became distinctly warmer and moister than it was in the Oligocene. The fossils suggest extensive moist forests over southern Australia with Nothofagus of the brassii group, conifers, Myrtaceae and Lauraceae prominent. In central Australia, localised forests of the same type occurred but so did open habitats as indicated by the record of Acacia, Casuarina and grasses. Temperatures dropped again by the late Miocene when Australia had reached its present latitudes and the Antarctic ice sheet had reached its present page 244dimensions. The rather limited evidence¹⁹⁸ suggests a retreat of rain forest with the cooler and drier climates.

The Pliocene also began with a warmer phase resulting in some rain forest expansion, but eventually the temperature declined leading to the severe glacial/interglacial oscillations of the Pleistocene. This finally led to the widespread disappearance of rain forest and expansion of plants adapted to arid conditions. These included Eucalyptus, Casuarina, Acacia and various Proteaceae. Most of the arid and semi-arid floral components in Australia are thought to be derived from families of early rain forest origin adapted to heavily leached infertile soils during the Tertiary.