PLEISTOCENE PLANT EXTINCTIONS IN SOUTH-EASTERN AUSTRALIA

Significant plant extinctions occurred in south-eastern Australia during the Early Pleistocene. 45% of angiosperm and conifer macrofossil species in the Early Pleistocene Regatta Point site (western Tasmania) are distinct from any modern species. A few could have evolved into modern species, but most are totally extinct. In contrast, only one of the 60 or so Middle or Late Pleistocene macrofossil species from that region is extinct. Ecological data suggest that other species went extinct during the Pleistocene. Fossil pollen and macrofossils from a site in Central Victoria, Stony Creek Basin, confirm that there were large species turnovers after the Plio-Pleistocene boundary (see Sniderman; this volume). Significant Pleistocene plant extinctions also occurred in New Zealand.

The detection of these extinctions depends on well-preserved macrofossils, which are uncommon. The extinctions are obscure in the fossil pollen record, because of pollen's poorer taxonomic resolution. Instead, some appear as range alterations of extant "species", genera or families.

The extinct species mostly have living relatives in temperate-subtropical rainforests. However some were sclerophyllous, others have montane relatives. The extinctions appear to have been climate driven. The most obvious potential culprit is glacial climates. The climate cycles appear to have increased amplitude abruptly later in the Early Pleistocene. Evidence at Stony Creek Basin implies a lack of glacial climates in the Earliest Pleistocene, but extensive glaciation of Tasmania did occur during the pre-Brunhes Early Pleistocene (perhaps contemporaneous with South American glaciation 1 - 1.2 Ma). How many species failed after surviving early glaciations remains to be seen.

Rainforest species that survived the extinctions suggest evolutionary stasis. Virtually all modern Tasmanian rainforest trees and shrubs were present in the Early Pleistocene with no perceptible change in morphology. One genus (Athrotaxis) shows signs of Pleistocene microevolution, and similar processes could explain species differences in a few other genera. In contrast, many non-rainforest genera (notably Eucalyptus) show strong signs of Pleistocene radiation. Thus, neither stasis nor rapid evolution models of Quaternary evolution seem to be the general case.