Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Australia - 540-490 Ma

Australia was in the Northern Hemisphere during the Cambrian, with part of the west Australian coastline being on the Equator, the remainder of the continent extending between 0^o and 30^o north. Sea levels were high globally. An embayment into central Australia increased the area of shallow marine environments late in the the Period. Bottom-dwelling (benthic) marine invertebrates were dominant among a invertebrate marine life.

This period is characterised by the first appearance of shells and exoskeletons. The ancestors of all modern phyla appeared at this time. The most characteristic animal of this period was the trilobite. The food chain at this time was based on plants, but the algae were the only phyla of the plant kingdom that had evolved by this time. At this time Australia was situated in the northern hemisphere between 0⁰ and 30⁰ N, with part of the coastline of Western Australia on the equator. Globally, it was a time of high sea levels. As a result of a marine transgression water covered a large area of central Australia. The life in this shallow, warm sea was dominated by marine benthic organisms.

A high proportion of the world's land surface was situated in low and middle latitudes, indicating that the climate would probably have been hot on most landmasses, with possible areas of aridity. It has been suggested there could have possibly have been a large temperature gradient, at least for some parts of the Cambrian, based on some evidence of glaciation in North America. This also suggests the seas may have been cool. As life was still confined to the water at this time, the water temperature would have a big effect on it. Because the only evidence for glaciation over such a long period is fragmentary, some believe there were possibly more glacial periods, the evidence for which has yet to be found.

High ground was mostly restricted to the southern parts of the Northern Territory and the Kimberleys of Western Australia. Locally, some areas were affected by aridity, with some embayments of the epicontinental sea sometimes being cut off from the main part of the sea, which resulted in evaporation of the water and the deposition of evaporites such as phosphorites, gypsum, as well as other evaporite deposits.

In the north of the Northern Territory, and in the region of the Great Victoria Desert in Western Australia, volcanic activity occurred that spread vast sheets of basalt across a very large area. The Deccan Traps in modern India are a good example of the basaltic flows that occurred in Cambrian Australia, and are of a similar scale. In New South Wales, Victoria and Tasmania, volcanism on a smaller scale occurred.

The sea over the area around the Gulf of Carpentaria and the central areas of the continent retreated as a result of folding and uplift of the zone through Adelaide and northward, the Delamerian Orogeny, that occurred during the Late Cambrian. Plant life from this time, being restricted to algae, that doesn't leave fossils, is known only from the remains of those that secrete lime in the formation of reefs. In the seas around Australia at this time the main reef forming organisms were Archaeocyathids, forms that research indicates were probably early sponges. They were previously believed to have disappeared from the fossil record at the close of the Cambrian, but it is now believed they evolved into sponges of a more modern type.

A diverse invertebrate fauna was dominated by trilobites, mainly the type living on the sea bed and often burrowing, and brachiopods (lamp-shells). Available niches were filled soon after the appearance of creatures that where substantial enough to leave fossils. The ancestors of all living phyla were soon present, including burrowers, free-swimmers and sedentary organisms. Gastropods and bivalves were present from the Early Cambrian, starting off very small, increasing in size throughout the Cambrian. By the end of the Period, the first large predators, cephalopods, had appeared.

Other Cambrian faunas included Echinoderms and Conodonts. The first protistans, Foraminifera and Radiolaria, to appear in the fossil record are found in the Cambrian. It is obvious from the complexity of the animals from this time that they must have been evolving for some time before they acquired enough hard parts to leave recognisable fossils.

Ostracods and Bryozoans appear for the first time in the Late Cambrian.

• - Early Cambrian Beatle Creek Formation in the Mt Isa district, western Queensland
• - Granite Tors in the Mt Isa District, western Queensland

565 mya - Early Cambrian fossil stromatolites at Eurowie Creek in the Georgina Basin of central Australia. 

540 mya - Mid-Cambrian marine shell fossils, Woolomin near Tamworth, New South Wales

Sources & Further reading

1. Mary E. White, The Nature of Hidden Worlds, Reed, 1993
2. Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994

Links

1. http://www.peripatus.gen.nz/Paleontology/Cambrian.html
2. Weird Wonders Lived Past the Cambrian