Ediacara fauna

▪ paleontology

 unique assemblage of soft-bodied organisms preserved worldwide as fossil impressions in sandstone from the Proterozoic Eon at the close of Precambrian (Precambrian time) time. These fauna represent an important landmark in the evolution of life on Earth: they immediately predate the explosion of life-forms at the beginning of the Cambrian Period 542 million years ago. The Ediacara fauna, named for the Ediacara Hills of South Australia in which they were discovered in 1946, were the first metazoans (animals made up of more than one type of cell) that required atmospheric oxygen for their growth. These animals were the precursors of organisms with skeletons, the appearance of which marked the end of Precambrian time and the beginning of the Phanerozoic Eon. The discovery of the Ediacara fauna demonstrated that a far more complex level of evolution had been achieved during Precambrian time than had been previously thought.

      The fossil impressions of the Ediacara fauna have a wide variety of shapes, ranging from circular discs (made up of internal radial arrangements, concentric ribbed structures, or combinations) and amorphous masses to plantlike fronds. The disc-shaped impressions are commonly a few centimetres across, though rare specimens reach 20 cm (almost 8 inches) in diameter. The frond-shaped impressions can attain lengths of about 1 metre (3 feet). Accompanying all types of impressions are “trace fossils”—irregularly shaped, winding burrows on and beneath the surface of the sandstone beds.

      The Ediacara impressions were derived from soft-bodied organisms similar to modern-day jellyfish, lichen, soft corals (coral), sea anemones (sea anemone), sea pens, annelid worms, and seaweed, as well as some organisms unlike any that are known today. They lived on or near the surface of coarse-grained sediments in the shallow continental shelf or on the deep continental slope of late Precambrian continental margins. Ediacara remains occur in rocks ranging in age from approximately 600 million to 542 million years old; the most-complex forms occur in the last 20 million years of this interval. The oldest radiometrically dated assemblage of Ediacaran organisms in the world, found in the Avalon Zone of Newfoundland, has an age of 565 million years.

      It had long been thought that the Ediacara fauna became entirely extinct at the end of the Precambrian, most likely due to heavy grazing by early skeletal animals. However, more recently, it was thought that environmental events such as changes in sea level played a greater role in the extinction of many Ediacaran organisms. Yet, recent discoveries have led to the current view that a few Ediacara-type organisms continued into the Cambrian. Moreover, some calcareous shelly fossils and sponge spicules have been found in Ediacara-age sediments, indicating that there was some degree of diachronous transition between the Precambrian soft-bodied organisms and the organisms with skeletons in the Cambrian.

      Most of the Ediacara fauna are found immediately above tillites (tillite) (glacial beds derived from ice sheets) that were widespread in the late Precambrian. Though it has been suggested that development of the Ediacara organisms was aided by the improvement of climate after the ice ages, a few occurrences of the Ediacara fauna are located between two glacial tillite beds, and some in West Africa and northwestern Canada have been found immediately below a layer of tillites. It is more likely that the origin of the Ediacara fauna was related to a global rise in the atmospheric oxygen level, which triggered a burst of development in these primitive metazoan animals toward the end of Precambrian time.

      Fossils of Ediacara organisms have been discovered in some 30 localities over five continents, including seven sites in North America. The principal occurrence is in South Australia's Ediacara Hills, which are part of the Flinders Range (Flinders Ranges) and are located 650 km (about 400 miles) north of Adelaide. More than 60 species have been defined from the fossils contained in the Pound Quartzite formation at this site. Other important sites are in central England (Charnwood Forest in Leicestershire), southeastern Newfoundland, northwestern Canada (the Mackenzie and Wernecke mountains), Namibia, Iran, Ukraine, the White Sea, the Urals, northern Siberia, the Yangtze valley in China, and several localities in central Australia.

Brian Frederick Windley

 

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