Lower Cretaceous Land Plants

Australia: The Land Where Time Began

Lower Cretaceous Land Plants

The fossil record of land plants in Australia during the Lower Cretaceous has been examined in some detail by the use of macrofossils and Palynofloras – spores and pollen. Coniferous forests, Podocarps and araucarians, associated with open heathlands and river bank/coastal communities, that had a continent-wide distribution is suggested by the evidence available to have been typical of the vegetation of this time. Microfossils found in subsurface drillcores from the Otway Basin indicate a bennettitalean understorey, cycad-like plants. In the Algebukina Sandstone near Oodnadatta, South Australia, in the Eromanga Basin, of Berriasian-Valanginian age indicate a canopy of conifers with an understorey of seed-fern, Rienitsia variabilis, pentoxylaceans, Taeniopteris spatulata, cycads and bennettitaleans. In the Otway Basin and the Gippsland Basin there is evidence of some regional endemism in the floras of the Berriasian-Valanginian. In these locations there appear to have been a less diverse flora, though it seems to have been more densely forested than those of the more northerly parts of the continent. Examples are seen in the floodplains of the Eromanga Basin and the Surat Basin in northeastern Queensland from the Cretaceous. Plants recovered from these deposits include canopy elements such as Podocarp (Elatocladus) and araucarian conifers (Brachyphyllum), ginkgophytes (Ginkgoites australis). Included in the understorey were ferns (Phyllopteroides), pentoxylaceans (Taeniopteris), bennettitaleans and horsetails (Equisites).

In Western Australia macrofloras from the Cretaceous are poorly preserved, such as material from the Broome Sandstone that is fragmentary. Palynofloras that are well documented indicate that strong links existed between Australia and eastern Antarctica and Australia and India. In eastern Australia links are indicated between Australia and floras of western Antarctica.

Terrestrial floras were progressively restricted by continuingly rising sea levels during the Barremian-Aptian. In the Otway Basin and the Gippsland Basin, particularly the Koonwarra Fossil Beds, the rich macrofossil material indicates a vegetation of cool, humid montane forests. Taxa in the canopy of this vegetation included Podocarps (Bellarinea, Brachyphyllum, Podostrobus), araucarians (Araucaria) and Ginkgoites. In deposits such as the Bulldog Shale, a marine deposit, of South Australia drift wood has been found of Aptian age that has closely spaced growth rings, indicating a cool climate with marked seasonality. Ferns and pentoxylaceans such as Taeniopteris daintreei that is believed to have grown in large open areas, dominated the vegetation of the understorey. During the Aptian bennettitaleans and cycads were becoming increasingly rare, with horsetails, Equisetum, ferns, mosses, liverworts, as well as some early angiosperms, that are believed to have been low-growing herbaceous species, growing on the river banks and around the edges of lakes.

A major shift in the composition of the vegetation occurred around the time of the Aptian-Albian boundary that included an increase in diversity and distribution of angiosperms. In the pollen record magnolias are especially well represented. In the Styx and Burrum coal measures in northeastern Queensland they appear to be more diverse, the macrofossils of angiosperms occurring in deposits laid down in swampy palaeoenvironments. In these deposits there are ferns (Phyllites), horsetails, pentoxylaceans (Taeniopteris that was no longer present in contemporary floras further south), cycads, araucarian conifers and Ginkgoites are also found in these deposits.

New floral elements evolved during the Albian in the rift valley that had developed between Antarctica and Australia. It has been suggested this burst of evolution may have been triggered by the combination of the expansion of floodplain environments and the onset of warmer climatic conditions. At this time Phyllopteroides, an osmundaceous fern, and taxodiaceaean conifers (Geinitzia) are first found in the fossil record. In the canopy the modern ginkgophyte genus Ginkgo replaced Ginkgoites. In the Eromanga Basin of the late Albian coastal areas around the inland sea were vegetated by araucarian/podocarp forests, as indicated by wood found in marine units such as the Toolebuc Formation and cycads (Nilssonia macronatum), and in the heathland and boggy areas, ferns, liverworts and mosses.

Sources & Further reading