The discovery of Griesbachian ammonoid faunas in Western Australia, and the expansion of palynological and paleontological studies have, in recent years, provided a firmer biostratigraphic basis for establishing the Permian-Triassic boundary in Australia. Present evidence suggests that Permian and basal Triassic strata are separated, in all Australian basins, by a sedimentary or erosional hiatus. The magnitude of this is difficult to estimate, because of correlation difficulties within the predominantly continental successions that characterize the Australian Upper Permian.

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

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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. It may be concluded, therefore, that the close of Permian time was marked by widespread extinction of many plant taxa and relatively rapid migration of others. There is no palynological evidence implying accelerated evolutionary diversification of the land floras. 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. Increase in the total oceanic area is suggested as the primary mechanism effecting major transformations of the land floras.

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