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Floral Modifications at the Permian-Triassic Boundary in Australia
Australian Late Permian plant macrofossil associations are of the familiar Gondwana type, in which Glossopteris is a ubiquitous and predominant element. Glossopteris has never been found in Australian strata for which a Triassic age is unequivocally indicated by other fossil evidence. Spore and pollen assemblages from Australian Upper Permian sediments are among the most diverse encountered anywhere in the geological column. They are interpreted as representing part of a mature floral ecosystem, stabilized at least throughout the area of the present continent.
In Western Australia virtually none of the distinctive plant microfossil taxa persists into the Triassic. Here early Scythian microfloras are characterized by lack of diversity and enormous numbers of small, spinose acritarchs. The microfloral break is less sharp at the base of the Triassic in the Sydney Basin, although the trend towards lower diversity above the boundary is still apparent. Although Australian Early Triassic plant microfossil assemblages differ notably from those of the Late Permian, they contain few new forms, apart from the lycopsid genus Aratrisporites. Almost all the well-characterized Scythian elements are known, usually as minor components, from Late Permian microfloras in Australia or other parts of the world. By late Scythian times the Dicroidium-flora had become established throughout the continent and persisted until the Early Jurassic.
Floral modifications at the Permian-Triassic boundary are spectacular; if not as complete and abrupt as those among certain invertebrate groups. Like other regional and local upheavals in the vegetation of the past they are concomitant with a period of shallow marine transgression, following extensive continental emergence and erosion. Destruction of the mature and delicately balanced Late Permian continental ecosystem is related to rapid transgression of the early Scythian sea, over extensive areas of marginal lowland. This collapse initiated a new floral succession marked in its early stages by lack of diversity and high proportions of lycopods. Plant microfossil assemblages indicate gradually increasing maturity of the flora throughout the Scythian, and the attainment of essential stability by the beginning of the Middle Triassic.
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