A Cretaceous (Aptian) flora from President Head, Snow Island, Antarctica

Cretaceous and Cenozoic vegetation of Antarctica integrating the fossil wood record

A compilation of data for Cretaceous and Cenozoic Antarctic fossil wood floras, predominantly from the James Ross Island Basin, provides a different perspective on floristic and vegetation change when compared with previous studies that have focused on leaf macrofossils or palynology. The wood record provides a filtered view of tree-forming elements within the vegetation, something that cannot be achieved from studies focusing on regional palynology or leaf floras. Four phases of vegetation development in the over-storey are recognized in the Cretaceous and Cenozoic of the Antarctic Peninsula based on the distribution and taxonomic composition of wood floras: Aptian-Albian coniferous forests; ?Cenomanian-Santonian mixed angiosperm forests; Campanian-Maastrichtian southern temperate forests; and Palaeocene-Eocene reduced diversity Nothofagus forests. Comparisons between the wood record and information derived from palynological and leaf floras have important implications for our understanding of the spatial composition of the vegetation. There is no doubt that climate change during the Cretaceous and Tertiary influenced the vegetational composition, but evolving palaeoenvironments in the Antarctic Peninsula region were probably of equal, if not greater, importance.

Cretaceous patterns of floristic change in the Antarctic Peninsula

Cretaceous radiation of angiosperms from low to high palaeolatitudes, coupled with the break-up of Gondwana, played a major role in establishing and maintaining biogeographic patterns across the southern hemisphere. Uncertainties in details of plate reconstructions provide conflicting hypotheses about area relationships of Gondwana fragments. This has led to a number of competing proposals concerning angiosperm migration across Gondwana. Central to this debate is the role of the Antarctic Peninsula, a region that is often envisaged as providing the main connection between east and west Gondwana. The initial radiation of angiosperms into the Antarctic Peninsula region, however, postdates appearances elsewhere in east Gondwana (e.g. Australia), strongly suggesting that the Antarctic Peninsula was not the main gateway, at least in the early stages of Gondwana radiation. A steep climatic gradient in this part of the world probably acted as an effective barrier to angiosperm radiation. The peak of floristic replacement coincides with the peak of Cretaceous warmth (Turonian) which in turn suggests that climatic warming acted as a forcing mechanism by pushing latitudinal belts of vegetation southwards. Once into the southern high latitudes angiosperms diversified, and as climates cooled during the Late Cretaceous a number of important groups seem to have their origins here. Recent investigations of Antarctic macro- and microfloras indicate progressive floristic replacement through the Cretaceous. Bryophytes, hepatophytes, bennettites and other seed plants all show a rapid decline in diversity. In contrast, ferns initially decline then recover, while conifers remain relatively stable. The ecological preferences of the replaced groups imply that angiosperms initially occupied areas of disturbance and were understorey colonizers, only later replacing fern thickets and becoming important in the overstorey. This pattern is consistent with those observed elsewhere through the Cretaceous.

Cretaceous (Late Albian) coniferales of Alexander Island, Antarctica. 2. Leaves, reproductive structures and roots

Coniferous foliage from the Albian of Alexander Island, Antarctica, is assigned to the Araucariaceae, Podocarpaceae, and Taxodiaceae based on attached or associated fertile remains. Araucarian foliage represented by Araucaria alexandrensis sp. nov. and A. chambersii sp. nov. is associated with ovulate cone scales described as Araucarites wollemiaformis sp. nov. and A. citadelbastionensis sp. nov., respectively. The Podocarpaceae is represented by Bellingshausium willeyii sp. nov. and the Taxodiaceae by Athrotaxites ungeri, both with attached cones. Sterile foliage is widespread belonging to the form genera Podozamites, Elatocladus, Brachyphyllum and Pagiophyllum. The conifers in this Albian southern high-latitude flora make up ca. 15% of the species diversity. Evidence from leaf litter distribution on palaeosols and leaf morphology suggest that the majority of conifers were large canopy-forming trees, although a few were probably small understorey shrubs.