Australia: The Land Where Time Began

A biography of the Australian continent 

In the earliest part of the Permian the glacial phase continued from the Carboniferous. It is not certain whether the ice sheets were continuous, as they are in Antarctica at the present, or whether there were a number of centres of glaciation, which are thought to have possibly changed over time, with unglaciated land between the centres. The evidence from South Africa and South America suggesting that the glaciers in those continents are older than those in Australia, which is thought to support the notion that there were a number of centres of glaciation, not a single solid mass of  ice. Whichever is the case, it seems likely that there would have been refugia in which at least some of the flora and fauna could survive the worst of the climatic conditions. At the present time it is believed that ice caps have covered Antarctica for about 15 million years, but fossil wood has been found in the rocks of the Serious Formation dated to 5 Ma, indicating that forests could survive the polar conditions, presumably at times when the polar climate ameliorated for long enough for forests to become established. It is believed that this probably also occurred during the glacial period in the Late Carboniferous and Early Permian.

Further evidence that ice and forest co-exist is seen in the interbedding of glacial and coal deposits from the Early Permian of Gondwana. At the present time forests grow at the foot of glaciers in South America and New Zealand.

Evidence has been found of aridification in parts of the Euramerican landmasses that were in the warm to hot zone on the equator at this time. Australia has always had periods of increased aridity during ice ages in the Northern Hemisphere.

By the close of the Permian, Brachiopods were in steep decline, never to recover, as did crinoids, nearly all the types from the Palaeozoic had become extinct. There was also a continued decline of the Nautiloid Cephalopods, but Ammonites continued to diversify. Trilobites and Eurypterids became extinct, and Horseshoe Crabs declined. On the land, insects were flourishing, increasing both in numbers and diversity.

Australia

After the end of the glacial phase the climate took some time to warm, remaining cool to cold throughout the Early Permian. Glaciers persisted on the high country in the east of the continent. As the climate warmed a rich, diverse flora evolved in the coal swamps, that persisted in many areas until late in the Permian, though the conditions are believed to have been probably warm-temperate. Throughout the Permian the climate was very seasonal. This strong seasonality is indicated by the pronounced annual rings found in the petrified wood from the time.

Epicontinental seas covered large parts of South Australia and Victoria, with a branch on the join between Antarctica and Australia separating the mainland from Tasmania, early in the Permian. In western Australia the sea covered part of the coast and the Canning Basin. The distribution of sediments accumulating in the basins during the Permian were largely controlled by major sea level changes and tectonic activity.

There are believed to have been 2 major sealevel rises during the Permian. The first was caused by the melting of the ice cap, resulting in the flooding of the Carnarvon Basin and Canning  Basin in Western Australia, and the Sydney Basin and Bowen Basins in the east of in the continent. The sealevel rise in the Middle Permian, when the Sydney and Bowen Basins were again flooded, the last major marine incursion on the east coast, was caused by local subsidence. In the Late Permian there was a major global sealevel rise. The east coast basins were not subjected to marine incursions at this time, the main effect being the impounding of run-off water in the major basins forming vast floodplains. The Cooper, Galilee, Bowen and Sydney Basins were all influenced by inward-draining, silting regimes.

Sediments were accumulating in the central zone of the Westralian Depression and the Canning Basin in Western Australia. Deltas in the Bonaparte Gulf supplied the sediments from which the reservoir rocks of the natural gas accumulations formed. Volcanic activity occurred from the Sydney Basin north to Townsville in Queensland. Tectonics in the New England area folded rock strata, intruded granites and uplifted the area as a result of tectonic plate interactions in the Yarrol-New England provinces. The activity in these areas was the last of the phase of activity of the Tasman Orogeny. By this time most of the crust of the Australian landmass had been stabilised. With the increased weight of the accumulating sediments the basins of eastern Australia continued to subside.

At this time the Westralian Depression became increasingly pronounced, assuming a dog-leg shape. In the Mesozoic the Westralian Depression became a rift zone as Gondwana began to split up.

On the eastern margin of Gondwana adjacent to Australia, a sequence of sediments of marine and volcanic origin were deposited in a deep, subsiding trough. Subsequent uplift of these rocks were to become part of the New Zealand landmass. At a thickness of 20,000 m, they are the most complete sequence of Permian rocks known in the world.

When the ice sheets finally retreated the bare land was quickly occupied following a burst of rapid evolution as the flora and fauna filled the new niches that became available. When the climate had warmed to the point of becoming cool-temperate, the Glossopteris Flora diversified, becoming the predominant flora of Gondwana. For some time there were still severe winters in which the darkness lasted for months, but the climate gradually improved. There were now large inward-flowing drainage systems in which swamps developed. The vast coal deposits of Australia were formed in these Permian swamps that formed in the cool-temperate conditions. It is believed that the Glossopterids were probably ancestral to several plant groups appearing in the fossil record towards the end of the Period that had adapted to swampy habitats with a cool climate. They evolved root systems that incorporated blocks of aerating tissue to allow them to survive in waterlogged mud . Evolution within this order led along several lines to the more efficient and better adapted Cycadophytes, Southern Conifers and Seed-ferns that replaced the Glossopterids by the end of the Permian. It is believed that the combination of diminished habitat and appearance of new plant groups led to the virtual disappearance of the Glossopterids by the end of the Permian, when the cool-temperate coal swamps had almost all gone. This suggests it was probably environmental change that drove the evolution of new forms that were better adapted to the changed environmental conditions. Glossopterids had first appeared at a time of retreating ice sheets when the climate was still cold, growing at the edges of glaciers, flourishing while the climate remained cool, disappearing as they gave rise to new forms better adapted to the new conditions. Glossopterids were the diagnostic plant fossils of the Permian in the Southern Hemisphere. They were deciduous so their leaves are very common in fossil deposits from the Permian

The Glossopteris Flora was a rich, diverse flora with many Clubmosses, Tree-ferns, Ferns, Ginkgophytes, Horsetails and Cordaites. By late in the Permian the Cordaites conifers (except the Southern Conifers) had evolved from the Cordaites, which dropped out of the fossil record in a similar manner to the Glossopterids. By the latter part of the Permian Conifers, Ginkgophytes and Cycadophytes, all plants that no longer required to live in or near water, had the covered the hillsides and places that didn't hold standing water. Plants had evolved by this time that could occupy nearly all the niches that are occupied by modern plants. To reach this stage of plant evolution had taken about 150 million years.

At the Permian-Triassic boundary the climate abruptly changed to one that was hot and humid, that required a different type of vegetation from the types that had existed in the Permian. Araucarian and Podocarp conifers flourished, and Clubmosses that were mangrove-like, small and woody vegetated the deltas and estuaries. It was the time that the first forked-frond Seed-ferns appear in the fossil record.

The marine fossils from this time also have changed to suit the changing environment. The faunas were of a high latitude, impoverished type at the start of the Permian. Eurydesma bivalves grew abundantly on the rocky seabed along coastlines, being adapted to the conditions in environments where they were subjected to high-energy wave action.

Later in the Permian the seas were warmer and many shallow-water environments resulted from a rising sealevel. The fauna responded by evolving rapidly to fill the new and changed niches now available, leading to a diverse rich, fauna. 

By the Middle Permian there were a very large numbers of species, though by the end of the Period only about 10 % survived co cross the Permian-Triassic boundary. Huge areas of the shallow water environments were lost at the end of the Permian. This loss is believed to be at least partly responsible for the dramatic disappearance of about 90 % of marine species from the fossil record that had existed prior to the change in sealevel. It was at this time that Pangaea formed, so there has been speculation that the resulting reduced coastal areas around the continents that combined at this time contributed to the great decline in numbers of marine faunas that occurred over a short length of time. As tectonic plates collided there must have been a big increase in volcanic activity and changing climate, both from the larger continental expanses and the associated uplift that occurs when continents collide. With so much volcanic activity that probably occurred at this time it is thought that there would probably have been acid rain some degree of acidification of the seas, that is believed to have occurred at the close of the Cretaceous, the Terminal Cretaceous Event. The vegetation on the land didn't show the same cataclysmic changes as occurred in the ocean, only progressive changes to suit the new environment.

The fossil record of plants shows a different pattern of response to the intermittent mass extinction events from that of the fauna. They tend to adapt progressively to the changes that result in mass extinctions of the associated fauna. It is believed this may be the result of their high ability to hybridise and produce polyploids and diploids that meant plant populations usually contain a number of variants at any one time that are often preadapted and available for selection that allows them to adapt readily to changes that take place in their environment. The Permian Marine Collapse, as the mass extinction event at the close of the Permian is called, had little effect on the terrestrial flora present at the time. The Southern Conifers, Mangrove-like Clubmosses and Cycadophytes were already well established prior to the event that led to the collapse of the fauna populations. The is not known visible change in the land plant flora between the close of the Permian and the opening of the Triassic. The same occurred at the close of the Terminal Cretaceous Event, land plant communities simply progressed in their normal manner, adapting to new conditions as they change. The vegetation was already changing rapidly towards the modern type flora before the start of the mass extinction that so badly affected the fauna.

The amphibians were still the dominant land vertebrates during the Permian, but the reptiles were catching up. The fossil record from this time is much sparser than that in the other continents making up Gondwana. In the Sydney Basin, amphibians that looked very much like overgrown salamanders were present in the coal swamps and fresh-water lakes, probably eating fish. The tracks of reptiles have been found in the Coalcliff Sandstone from the Late Permian. At this time the first known mammal-like reptiles had evolved in other parts of the world at this time.

Several deposits in Australia contain rich insect faunas from the Permian. The Insect Beds at Belmont in New South Wales had the right conditions for fossilisation in great detail. At this locality the beds containing the fossils were formed when volcanic ash settled on a quiet pond, burying any flying or floating insects that were present in ash that is so fine-grained that the rock formed from it preserves minute details of the entombed insects.

The dominant land vertebrates now were labyrinthodont amphibians and, to a lesser extent, reptiles.

This was also the period that saw the greatest mass extinction ever, when life on Earth came closer to total annihilation than it has come so far, before or since. The Permian Mass Extinction saw the demise of 90-95 % of marine species. and 70 % of terrestrial families of animals.

During the Permian Gondwana in the south and Laurasia in the north collided to become one giant landmass, the formation of the supercontinent Pangaea. 

Sources & Further reading

Mary E. White, The Nature of Hidden Worlds, Reed, 1993

Permian Fauna

Permian Mass Extinction