The Silurian Period

Australia: The Land Where Time Began

Australian Silurian

Early to Mid Silurian 436-420 mya - Late Silurian Period 420-408 mya

Australia was straddling the equator.

The Australian part of Gondwana was still on the equator, with the rest of Gondwana in the Southern Hemisphere, the South Pole being situated on North Africa. North America, Scandinavia, and some of Europe were combined and straddled on the equator. In the Northern hemisphere there were a number of blocks that had yet to collide to form Asia.

Following the ice age on the boundary between the Ordovician and Silurian, sea levels rose globally. Most of the glaciation had occurred in north Africa, and when the glaciers retreated they were replaced by a shallow epicontinental sea. The ice age sediments were overlain with organically rich sediments. Glacial sediments are highly porous, forming ideal reservoir rocks. It was the combination of the marine source rocks and the glacial reservoir rocks that allowed the accumulation that was to become the North African oilfields.

Life was still confined to water.

Australia

During this period Australia was still situated on the equator, so the climate would have been hot. Now life began its move onto land. The earliest footprints on land belong to a Eurypterid, and are found on a fossilised beach in the Kalbarri National Park in Western Australia.

During the earliest part of the Period the glacial phase was at its most intense, and as the continent was still on the equator, the climate was probably cool, but not glaciated. Sea levels reached their lowest level at the start of the Period, rising again after the ice melted. In the Middle Silurian, the global climate was warm to hot, and there were no polar ice caps. Evidence from all the margins of Australia supports the suggestion that the oceans were warm at this time.

Late in the Ordovician, the Alice Springs Orogeny had begun a round of mountain-building. Mountain-building occurred along the eastern seaboard, as the Lachlan Orogeny continued to uplift the land. Among the strata uplifted during this orogeny were marine limestones formed by the reefs that flourished in the shallow seas of the marine incursions, soon to be eroded during the tectonic phase. Stromatoporoids, not true corals, formed some of the reefs. Remnants of these extensive limestones can be seen in the Bungonia and Jenolan Caves in New South Wales and caves at Chillagoe in Queensland. The caves being formed by the subsequent weathering, with stalactites and stalagmites formed by the dripping water.

Along the southeast margin of the continent, a complex fracture system developed, leading to subsidence of the Darling and Adavale Basins. A series of depressions and troughs developed on the northeastern margin. In the southeastern sector of the continent, a complex pattern of throughs, shelves and elongated islands formed offshore from a fairly narrow shelf, their positions often changing over time. This was followed by a period of volcanic activity in which volcanic rocks were intruded in this area. Sometimes, in shallow water, they formed islands, sometimes cooling below the surface to form granite basoliths. In the deeper troughs sedimentation continued. Sandstones, shales and limestones were deposited in shelf areas. Trilobites, Molluscs and rugose and tabulate corals are found in the limestones.

Graptolites, found in the previous Period, remained abundant, and there were also stromatoporoids (coralline organisms that formed reefs), Bryozoa, crinoids, starfish, ostracods. It was at this time that the Eurypterids (sea scorpions) reached their peak, being abundant and growing to large sizes. Tracks of these animals have been found on rock ledges in Western Australia, and are the first known tracks of an animal made on land. It is believed they were probably foraging along the tide line long before plants moved onto land to pave the way for the first animals to move out of the water permanently. There were also horseshoe crabs, sponges and annelids. Being soft-bodied animals, annelids don't fossilise, but their burrows and tubes are found as trace fossils at places like the Tumblagooda Sandstone Formation in Western Australia.

Late Silurian 420-408 Ma

It was during the Late Silurian that life finally emerged onto land. It is believed the main change that had taken place that allowed this to happen is that the atmospheric oxygen levels reached a high enough level to form ozone in the upper atmosphere. Now that much of the harmful radiation from the Sun could be blocked from reaching the land surface it became possible for living organisms to live on the land, previously they couldn't leave the water because the high levels of UV radiation reaching the surface would have been fatal. Approaching the end of the Silurian it is believed the land invasion began with an algal scum, together with bacteria and fungi, formed on the margins of the water around places like swamps, where the edges of the land would remain moist permanently.

In the world of the Late Silurian the climate was hot and humid around the margins of swamps the green scum could evolve into the more typical land plants, but the earliest forms to evolve are unknown. They gradually spread away from the waters edge and began forming soil, which increased the water holding capacity of the sand, making it easier for more advanced forms to evolve, as well as slowing erosion. It is thought these very first land plants may have been formed of a symbiosis between algae, fungi and bacteria. In the Rhynie Flora of Scotland, some of the earliest known vascular plants, preserved well enough for them to be known in detail, have been found with Mycorrhizal fungi growing in their tissues, a relationship that is still in existence with modern plants.

It was at this time that the first known footprints on land were formed. They were made by large arthropods, Eurypterids, that looked like large scorpions, as they moved across the wet mud. They were probably one of the first animal groups to move onto the land. There are members of nearly all classes of the Arthropoda living on land, making the Arthropoda a very successful group on land. The arthropods were pre-adapted for a life on land, with their chitinous exoskeleton to prevent drying out so they could live on land secure in their own personal microclimate. The chitin armour of their legs also supported the body on land and their respiratory system was easily adapted to air breathing. Some arthropods, such as scorpions, have a very high resistance to radiation, so maybe this allowed ancestral groups with this feature to move on to the land earlier than competitors.

Australia

Sedimentary deposits, such as carbonates, were formed in the new marine environments as the seas encroached on the western margin of the continent during the Late Silurian. That there were periods of aridity at this time is shown by the presence of evaporites that formed when embayments dried out. The position of Australia between the equator and 30^o N indicates it had a hot climate. The presence of coral reefs shows that the oceans were warm. The formation of elongated islands and troughs along the eastern seaboard that began in the parts of the Silurian continued. The sediments that formed between the mainland and these islands formed a continuous rock from the Silurian to the Devonian. The Baragwanathia Flora of Victoria is found in parts of these sequences deposited during the Late Silurian and Early Devonian. It is one of the most remarkable early land floras known in the world. The part of the sequence containing the Baragwanathia Flora was dated by the presence of a specific Graptolite.

Baragwanathia is a lycopod (Clubmoss). It resembles its living descendants Lycopodium squarrosum in its reproduction by spores in sporangia that it has at the base of some of its elongated leaves. It is surprising to many that it has such a high level of organisation at such an early date. Lycopods descended from Zosterophylls, some of which occur with the Baragwanathia. Rhynophytes, considered ancestral to ferns, horsetails and all seed plants, are also present in the Baragwanathia Flora.

Silurian trilobites, Bowning, New South Wales

Silurian corals, Hume Limestone Member of the Silurian sequence at Yass

Silurian chain coral, Liscombe Pools, New South Wales

Late Silurian crinoid stems, Quedong, New South Wales

Late Silurian chain coral, Forbes, New South Wales

470 my old Arandaspis The earliest relatively complete fossil fish found so far.

420 myo - Late Silurian club moss, Victoria

Sources & Further reading