Eocene monsoon forests in central Australia?

Abstract

The Australian Tertiary plant fossil record documents rainforests of a tropical to temperate character in south-eastern and south-western Australia for much of the Early Tertiary, and also shows the climatically mediated contraction of these rainforests in the mid to Late Tertiary. The fossil record of Australian monsoon forests, that is semi-evergreen to deciduous vine forests and woodlands of the wet-dry tropics, however, is poorly known. Phytogeographic analyses have suggested an immigrant origin for some floral elements of present day monsoon forests in northern Australia, while other elements appear to have a common history with the tropical rainforests sensu stricto and/or the sclerophyllous flora. Early Tertiary macrofloras in northern South Australia may provide some insight into the origins of Australian tropical monsoon forests. The Middle Eocene macrofloras of the Poole Creek palaeochannel, and the ?Eocene-Oligocene silcrete macrofloras of Stuart Creek, both in the vicinity of modern Lake Eyre South, have foliar physiognomic characteristics which distinguish them from both modern rainforest and Eocene-Oligocene floras from south-eastern Australia. Preliminary systematic work on these floras suggests the presence of: (1) elements not associated today with monsoon forests (principally 'rainforest' elements, e.g. Gymnostoma, cf. Lophostemon, cf. Athertonia, Podocarpaceae, ?Cunoniaceae); (2) elements typical of both monsoon forests and other tropical plant communities (e.g. cf. Eucalyptus, cf. Syzygium, and Elaeocarpaceae); (3) elements likely to be reflecting sclerophyllous communities (e.g. cf. Eucalyptus, Banksieae and other Proteaceae); and (4) elements more typically associated with, but not restricted to, monsoon forests (e.g. Brachychiton). The foliar physiognomic and floristic evidence is interpreted as indicating a mosaic of gallery or riverine rainforests, and interfluve sclerophyllous plant communities near Lake Eyre in the Early Tertiary; deciduous forest components are not clearly indicated. Palaeoclimatic analysis of the Eocene Poole Creek floras suggests that rainfall was seasonal in the Lake Eyre area in the Eocene; however, whether this seasonality reflects a monsoonal airflow is not clear.