Australia: The Land Where Time Began

A biography of the Australian continent 

Ediacara Hills, South Australia                                                                                                                            Last updated 27/01/2012

The Ediacaran fossil assemblage is named after the Ediacara Hills where they were found, part of the Flinders Ranges in South Australia. Since their discovery, components of the Ediacaran fauna have been found around the world from about the same age, though the fossils in the Ediacara Hills are still the most complete assemblage so far found. The deposit containing the Ediacaran Fauna is a coarse-grained sandstone, the Pound Quartzite.

In 1946, an Australian mining geologist named Reginald C. Sprigg was exploring a range of mountains north of the city of Adelaide, South Australia, known as the Ediacara Hills. Serendipitously, he found fossilized imprints of what were apparently soft-bodied organisms, preserved mostly on the undersides of slabs of quartzite and sandstone. Most were round, disc-shaped forms that Sprigg dubbed "medusoids" from their seeming similarity to jellyfish. Others, however, resembled worms, arthropods, or even stranger things.

Initially, Sprigg thought that these fossils might be Cambrian in age, but later work established that these fossils are in fact Late Precambrian. These were not the first Precambrian soft-bodied fossils to be found and described -- scattered reports of them had appeared in the scientific literature as far back as the mid-nineteenth century. However, it was the first diverse and well- preserved assemblage of such fossils to be studied in detail, and it helped spark a surge of interest in Precambrian palaeontology. The Ediacara Hills gave a name to the entire "Ediacara biota" of the late Precambrian. Appropriately, the name "Ediacara" comes from an Aboriginal language expression meaning "veinlike spring of water" -- the "spring," perhaps, from which complex animals have arisen.

The main ridge forming rocks of the central and northern sections of the Flinders Ranges, extend about 300 km north-south and about 50-80 km in the east west direction. Many fossil bearing sites have been found in the Ediacara Member. The Pound Subgroup (the Pound Sandstone) is comprised of 2 formations, Bonney Sandstone, that is red, and the Rawnsley Quartzite, that is lighter coloured.

In the Chace Range area the sedimentary layers proved to have been buckled into a vertical position. This allowed the study of the edges from aerial photos. It was found that the coarse sandstone had valleys eroded in them that had later filled with finer sediment that contained the fossils.

Stratigraphy and taphonomy of the Ediacara biota4

Precambrian fossils have also been found throughout the Heysen Range in the south, in places such as Parachilna Gorge, Brachina Gorge, Bunyeroo Gorge and Mayo Gorge, and the eastern end of the Chace Range. In the Ediacara Member of the Rawnsley Quartzite, 500 m (1,640 ft) below the earliest Cambrian deposits in this area, the Ediacaran fossils have been found in a stratigraphic range of at most 110 m (360 ft). the Rawnsley Quartzite is part of the Pound Supergroup, which was named from Wilpena Pound, an eroded syncline  that forms a circle of quartzite cliffs that face outwards in a natural fortification. A series of siltstones and sandstones deposited under pelagic to intertidal conditions comprise the Ediacara Member. It is implied that sediment was being delivered from a continental edge into deep water, sometimes by turbidity flows and on occasion as a delta where the water was shallowed to sub-tidal and intertidal levels. There are some storm horizons present, the fossils being deposited in the shallow levels around the storm wave base.

Thin clay films representing gentle deposition out of suspension between the sandstone layers aided in preservation of the fossils, the sandstone layers representing more energetic flow and deposition of sediment, that the authors suggest possibly occurred during storm events. The interpretation of the morphology of the fossils was made possible by the clay acting to some extent a a glue that cohered the sands beneath and fossils on the sea bed, moulding fine detail of the fossils. Preservation of the bodies of dead animals would also have been aided by a microbial mat on the sea bed that enclosed the bodies in a closed environment, evidence of which can be seen on some of the rippled surfaces.

The fossils are generally preserved in a squashed state as they were soft bodied. During diagenesis the layers of clay compact to a considerable extent resulting in relief being provided by the sandstones. An external mould of the upper surface of a fossil is occasionally preserved as an impression on the base of the sandstone. A cast sometimes can be seen as a positive relief on the sandstone base that formed when the animal's body decays or collapses, sand filling the space previously occupied by the body. A counterpart mould can be formed if the clay layers are thin, the cast projecting into the underlying soft sand. Conversely a counterpart cast can also form. The result can be ventral and dorsal structures superimposed on one another. Resistant internal organs, such as gonads, of possible medusoids, can be preferentially moulded in some organisms that had thin outer walls.

No organic material remains of these fossils, all of them being casts or moulds. In the field they are best viewed in low angle light, and in the lab by the light from a lamp at a low angle. A better view of some fossil material can be achieved by silicone rubber casts of fossils preserved as moulds and vice versa.

 

David Attenborough visited the Ediacaran fossil site site in the series First Life.

Emu Bay, Kangaroo Island

Sources & Further reading

  1. The Rise of Animals: Evolution and Diversification of the Kingdom Anamalia, Mikhail A. Fedonkin, James G. Gehling, Kathleen Grey, Guy M. Narbonne, Patricia Vickers-Rich, The Johns Hopkins University Press, Baltimore.
  2. Penny Van Oosterzee, The Centre - The Natural history of Australia's Desert Regions, Reed Australia, 1993
  3. Mary E. White, The Nature of Hidden Worlds, Reed, 1993
  4. Selden, Paul, Nudds, John, 2004, Evolution of Fossil Ecosystems, Manson Publishing

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Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 27/01/2012 

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading