Triassic Vegetation of Australia

Australia: The Land Where Time Began

Triassic Vegetation of Australia

According to the authors¹ the vegetation of Australia during the Triassic was distinctly provincial. Climatic factors such as monsoons and extreme seasonality, resulting from the symmetrical location of Pangaea on the equator, differentiated the floras of southern Gondwana and northern Laurasia. In the Sydney Basin there are assemblages that have been well-documented that provide a good fossil record of Australia during the Lower-Middle Triassic. Similarly, there is a good macrofossil record of the Late Triassic in the coal measures of the Ipswich Basin, and in South Australia, the Telford Basin.

According to the authors¹ the general composition of these floras was very similar to contemporary communities in other parts of Gondwana, with Dicroidium a dominant low-growing seed fern being present in all assemblages from lowland heath to dry upland woodlands. In the Triassic there were also other plants that were common in the assemblages of Australia such as voltziacean, and primitive podocarpaceous conifers

Also during the Triassic the widespread Glossopteris Flora was gradually replaced by the Dicroidium flora which included ferns, ginkgophytes, cycadophytes (cycads and related forms), conifers, equicetaleans (horsetails), lycophytes (clubmosses) as understorey plants and other seed-ferns. This vegetation was already showing adaptations to arid conditions, thickened cuticles and reduced leaf area, some becoming needles or spikes, this vegetation in places like Australia was to become very important as the continent dried out.

In the Sydney Basin macrofloras have been found from the earliest Triassic (Olenekian-Landinian). There is a diverse range of arborescent lycophytes such as Pleuromeia, seed ferns (Dicroidium) and equicetaleans as well as Voltziopsis a rare voltziacean and the podocarp (Podocarpus) conifers. This vegetation was present in such environments as coastal wetlands, a reason they have been considered indicators of a warm humid climate. It is believed the plants such as Pleuromeia, a key element in such communities at this time, were probably plants that were mangrove-like, growing along water courses and in waterlogged swampy areas, forming monodominant thickets that are partly submerged for at least part of each day. The authors¹ suggest Dicroidium-dominated heath zones probably grew around the margins of lakes, the conifers being restricted to the upland areas that were well-drained.

It has been found, based on the evidence from macrofossils, spores and pollen, that similar floras, such as Dicroidium and Pleuromeia also occurred in Western Australia, and in Queensland, in sub-arid areas in eastern parts of the state. Vegetative types from the inland areas, as seen in the material from the Arcadia Formation and the Blina Shale, indicate a flora that is more seasonal, mainly confined to established watercourses, summer monsoonal rainfall being a strong influence. This indicates there was some variability.

Uniform arid-zone floras evolved across the continent in the Middle Triassic. Some arid-adapted Dicroidium species arose that had adaptations such as thickened leaf cuticles and reduced spiny leaves, both leading to lower water loss in the hot, dry climate of such environments. Based on contemporary macrofloras found in the Hawkesbury Sandstone, especially at the Beacon Hill Quarry at Brookvale near Sydney, that evidence has been preserved of understorey foliage. Included among these fossils are (Dicroidium) seed ferns, ferns (Rienitsia spatulata), equicetaleans (Neocalamites hoerenis), ginkgophytes and cycadophytes. The main canopy elements were podocarp conifers, Rissikia, a form that survived until the Jurassic.

In the Late Triassic the macrofloras across Gondwana were broadly similar. There were predominantly seed ferns, volzialean conifers, ginkgophytes, pleuromeian lycophytes and ferns. The Telford Basin (Leigh Creek Coal Measures) and Ipswich Basin have all produced evidence of these vegetative types being present, seed ferns (Dicroidium) osmundacean ferns, ginkgophytes (Ginkgoites), podocarp conifers (Rissikia) and the bennettitalean Otozamites (a distinctive Mesozoic cycadophyte).

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