The Devonian Period

Australia: The Land Where Time Began

Devonian Australia 408-360 Ma

In the Northern Hemisphere, the landmasses were a number of separate blocks. Gondwana was in the Southern Hemisphere, the only part sill on the equator was Australia, on an outer edge as it always was throughout it association with Gondwana, like a peninsula. The climates of the world were warm to hot, and there were no polar ice caps. Worldwide many evaporites were formed, indicating widespread aridity.

The Devonian is the period when colonisation of the land by plants and animals got underway around the margins of the water. By the end of the period plants, such as giant club mosses and giant horsetails, had colonised the margins of the water and were evolving the adaptations necessary to spread to the land away from the margins of water bodies. During this period fossil arthropods, spiders, mites, millipedes, scorpions, etc. begin to appear.

Plants remained small and low-growing, reproducing by the production if spores, during the first half of the Devonian. Some time around the Middle Devonian plants evolved the support and conducting tissues that allowed them to grow taller. By the end of the Devonian some had evolved to the level of reproducing by seeds. At this time the flora was characterised by Giant Clubmosses and Giant Horsetails, the swamps they lived in being densely vegetated and teeming with animal life. Some of the creatures from this time are known as fossils, spiders, mites, millipedes and scorpions. Early amphibians inhabited these forests and lungfish swam in the water. The abundant fish from this time were very diverse, and coral reefs were formed, one of which is known from the northern margin of the Canning Basin in Western Australia.

Australia

Australia lay between 15^o N and 15^o S, making it a tropical country during the Early to Middle Devonian. By the Late Devonian it had moved to be between 30^o and 45^o S, and its orientation had changed slightly, at which point the climate would have cooled.

Australia was apparently warm and tropical during the Devonian, as evidenced by the fish and invertebrate diversity, large areas of carbonate deposits in the Lachlan Fold Belt of south-eastern Australia and north Queensland and the Canning Basin in Western Australia, oolitic limestones in the Wee Jasper area of New South Wales.

Volcanic activity continued along the eastern margin of the continent, adding new stabilised land to the eastern edge, and by the end of the Devonian the coastline was substantially as it is at present. Sediments collected on a large delta in the Adelaide Basin during the Early Devonian that formed the reservoir rocks of the Gilmour Natural Gas Field, the oldest known hydrocarbons found in eastern Australia. At this time near Melbourne and Bathurst, deep ocean troughs were accumulating sediments, in eastern Victoria and around Canberra there were shallow seas with reefs and a rich, diverse shallow water fauna populating a shelf that extended north through the New England area all the way to North Queensland.

The eastern and western coasts were again linked by a broad band of epicontinental sea, the successor to the Larapinta Sea, during the Early Devonian, that had an abundance of shallow water environments. In the western part of this sea was a major incursion into the Canning Basin. An orogeny began to affect the eastern half of Australia during the Middle Devonian that caused the seas in the eastern part of the epicontinental sea to retreat to the east, exposing more land on the eastern edge of the continent. The westward retreat of the remainder of the epicontinental sea was caused by concurrent elevation of central Australia. A small incursion in the western Canning Basin was all that remained by the Late Devonian. From the Middle Devonian the west coast basins underwent regional subsidence that resulted in marine transgressions into the Carnarvon and Bonaparte Basins.

As the raised central rocks eroded the sediments banked up against the emerging mountains along the eastern margin where the north-south trending mountainous region was being created by the Tabberabberan Orogeny. The high country that resulted was a watershed, the east flowing river systems carrying large amounts of sediment to the floodplains on the coast. These rivers teemed with freshwater fish.

By the Late Devonian, aridification was widespread throughout the world, as it was in Australia. After the marine incursions retreated from the western part of the Canning Basin in Western Australia the area became desert. The widespread aridification in Australia is indicated by the presence of wind-blown sediments over large areas of the continent. Towards the end of the Devonian, the climate again became hot and humid.

Plants were still herbaceous and low-growing, and still tied to the water in the Early Devonian. The Baragwanathia Flora of Victoria in the Silurian persisted into the Early Devonian. In the Early Devonian some of the Rhyniophytes are of the advanced Trimerophyte type, intermediate forms ancestral to Seed Plants. The land plants had evolved the necessary support and conducting tissues to grow into tall trees by the Middle Devonian. This was the time Giant Clubmosses and Giant Horsetails formed forests in Australian swamps. Lepidosigillaria appeared in the fossil record of Australia at this time. lepidodenron

The characteristic plant of the Late Devonian was a Giant Clubmoss, Leptophloem australe, that had rhombic leaf base patterns on its trunk. Species of Lepidodendron Giant Clubmosses had appeared by the end of the Devonian. There were also early Gymnosperms (Seed Plants), but they were not common. The evolution of reproduction by seeds allowed the plants that evolved it to move further from water.

The Devonian has been called the Age of Fishes, a time when marine and freshwater fish flourished, including lungfish, Placoderms from central and southeast Australia that show affinities with those from southern China. Based on this evidence it has been suggested that during the Devonian large areas of land that were destined to become part of Asia were probably attached to Gondwana.

Footprints of Labyrinthodont amphibians have been found at Genoa River in Victoria, and a jaw has been found in the Clognan Shale southwest of Forbes in New South Wales. The only other deposits know to contain tetrapods from the Devonian are in Greenland. These amphibians from the Late Devonian are signs of the invasion of the land by Vertebrates.

In the oceans there was abundant marine life. It is at this time that Ammonites make their appearance, their coiled shells are of types that are similar to those found in Europe, North Africa and North America from the same time. In the Garra Formation from central-western New South Wales there are Receptaculitids, a creature that had a complex structure with a hollow spherical form, were very common. There were also Brachiopods, Corals, Bivalves, Gastropods, Coelenterates, Sponges, Echinoderms and Conodonts. By the Devonian the Graptolites had almost disappeared from the fossil record and the Trilobites were in steep decline.

Stromatoporoids formed massive reefs late in the Devonian, and in the Canning Basin, corals and algae flourished. In deposits from the fore-reef and back-reef deposits there were abundant rugose and tabulate Corals. Gogo fish that lived near the reefs are found in the Gogo Formation near Fitzroy Crossing in Western Australia. The fish fauna at this site included Lungfish, Placoderms and early ray-finned fish. Sharks were common around the world at this time, some being found near Bunga Head, near Eden in New South Wales.