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al., 1991) from the Seymour Island area; recycled palynomorphs from around the Antarctic coast (Truswell, 1983); leaves (e.g., Birkenmajer and Zastawniak, 1989; Li and Shen, 1989), wood (Torres and LeMoigne, 1988), and palynomorphs (Stucklik, 1981; Lyra, 1986; Torres and Meon, 1990) in the South Shetland Islands; and palynomorphs in Middle Eocene ODP samples in the South Orkney region (Mohr, 1990), close to 60°S. Terrestrial dispersal corridors that linked Antarctica with the other austral land masses were severed by the Late Eocene, keeping Antarctic plant taxa from emigrating northward, and preventing active immigration to Antarctica.

Oligocene-Miocene-Pliocene

The Oligocene epoch, during which major tectonic and climate changes of lasting effect took place, saw a drastic reduction in vegetational diversity. For Early Oligocene Nothofagus-fern and fern bush floras on King George Island, Birkenmajer and Zastawniak (1989) suggest MAT of 11.7 to 15°C and rainfall between 1220 and 3225 mm. Also on King George Island, leaf floras of Oligocene-Miocene age suggest Nothofagus-podocarp conifer forest communities with possible MAT of 5 to 8°C and rainfall of 600 to 4300 mm (Birkenmajer and Zastawniak, 1989).

In the Maud Rise area (ODP 113 material), late Early Oligocene palynomorph assemblages contain no terrestrially derived spores or pollen (Mohr, 1990), implying possible ice cover or greatly reduced vegetation on adjacent East Antarctica.

In the Ross Sea area (close to 70°S paleolatitude), Late Oligocene material (from Deep-Sea Drilling Project (DSDP) site 270) indicates that a Nothofagus-dominated cold temperate forest, with Proteaceae, conifer (Podocarpus), Myrtaceae, and rare ferns still existed, despite evidence for sea-level glaciation. Also in this area, other drillholes penetrating Oligocene sediment (MSSTS-1, CIROS-1) recovered evidence of mixed Nothofagus-conifer forests, with other angiosperms also present (results summarized in Truswell, 1990). Mildenhall (1989, CIROS-1) suggested that the coastal fringes of the Ross Sea supported a Nothofagus forest with podocarps, Proteaceae, and other shrubby angiosperms. Upper Oligocene sediments in CIROS-1 contain a leaf (Hill, 1989) that resembles Nothofagus gunnii, an extant deciduous alpine Tasmanian species. Clay mineralogy, together with the fossil evidence, is consistent with a cool or cold temperate environment, and temperate rather than polar ice is indicated (Barrett et al., 1989).

There is at present no good information on Miocene Antarctic vegetation; however some elements of the earlier flora must have survived in sheltered coastal refugia during major glacial advances. Wood and pollen remains, albeit reflecting a stunted, very depauperate flora, have been found near the head of the Beardmore Glacier in Pliocene glacial deposits now at 1800- to 1990-m elevations (Webb et al., 1987; Harwood, 1988). The wood is from a shrubby Nothofagus similar to extant species in Tasmania (N. gunnii) and southernmost South America (Carlquist, 1987), and the palynomorph assemblage contains only a few taxa, among them Nothofagus (fusca group), a podocarp and pollen resembling herbaceous Labiateae or Polygonaceae (Askin and Markgraf, 1986).

COMPARISON OF NORTHERN AND SOUTHERN HIGH-LATITUDE VEGETATION

Cretaceous Evolutionary Trends

Southern Cretaceous floras are notable for their high latitude origins of major southern clades. In contrast, ancestral forms for all major northern clades (e.g., families) occur first at lower latitudes (Spicer et al., 1987). This contrast is due to differences in continental configurations and resulting differences in biotic stress.

The early (Cenomanian) diversity of angiosperm, and particularly platanoid leaf forms, in the northern latitudes was at the generic or lower level. Diversity at the family level was low. These trends are a function of evolutionary novelty, coupled with hybridization and polyploidy, giving rise to morphological intergradation (Spicer, 1986). Vast land areas surrounded the North Pole through the Late Cretaceous to Cenozoic, facilitating migrations to and from the polar latitudes. Thus, new taxa were exposed to broad selection pressures, suppressing development of polar specialists.

In the high southern latitudes, Antarctica remained in the polar position and was connected to lower-latitude land areas by relatively restricted dispersal corridors. The Antarctic plant communities were thus buffered to a certain extent from the competitive stress of invading taxa. Changing climatic conditions in this relatively isolated region would favor origin of specialists adapted to the changing environment, and low competitive stress would allow these specialists to become established. Major evolutionary innovation started in the Santonian and peaked in the Campanian-Maastrichtian. The increased rate of evolution coincides approximately with the end of the "Cretaceous quiet zone" (Harland et al., 1982) at the Santonian-Campanian transition and, more importantly, occurs during the Late Cretaceous cooling.

Northern and Southern Floras: Deciduous Versus Evergreen

In fossil floras, evergreen habit is recognized from the thick coriaceous nature of leaves and diagnostic (thick)



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