Ediacara fauna

Ediacara fauna, unique assemblage of soft-bodied organisms preserved worldwide as fossil impressions in sandstone from the Ediacaran Period (approximately 635 million to 541 million years ago)—the final interval of both the Proterozoic Eon (2.5 billion to 541 million years ago) and Precambrian time (4.6 billion to 541 million years ago). Traditionally, these fauna have come to represent an important development in the evolution of life on Earth, because they immediately predate the explosion of life-forms at the beginning of the Cambrian Period 541 million years ago. Some scientists have suggested that 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 (541 million years ago to the present). 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.

Characteristics of Ediacara fossils

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 be as long as 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.

Some impressions, such as those of the rangeomorph Fractofusus found in late Ediacaran rocks in Newfoundland, have given paleontologists clues on how sophisticated reproduction had become by that point in Earth’s history. Fossil evidence suggests that Fractofusus reproduced asexually and was capable of switching between budding (a process where a new individual develops from a generative point on its parent) and releasing waterborne daughter propagules that were clones of the parent.

The Ediacara impressions were derived from soft-bodied organisms similar to modern-day jellyfish, lichen, soft corals, sea anemones, 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 from approximately 600 million to 541 million years ago; the most-complex forms occur in the last 20 million years of this interval. One of the oldest radiometrically dated assemblages of Ediacaran organisms in the world occurs in the Avalon Zone of Newfoundland and has an age of 565 million years.

Discoveries and analysis

The primacy of the Ediacara fauna as a definitive stage in metazoan evolution has been complicated by fossil discoveries that date to before the start of the Ediacaran Period. Spongelike fossils with metazoan characteristics, as well as chemicals that are likely precursors to those produced by modern sponges, have been found in rocks dating from 760 million to 650 million years ago. This evidence strongly suggests that the Ediacara fauna were not the first metazoans.

It had long been thought that the Ediacara fauna became entirely extinct at the end of the Precambrian, most likely because of 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 overlap between the Precambrian soft-bodied organisms and the organisms with skeletons in the Cambrian.

Most of the Ediacara fauna are found immediately above tillites (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.

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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, where more than 1,500 well-preserved specimens have been collected. The Ediacara Hills are part of the Flinders Range and are located 650 km (about 400 miles) north of Adelaide. More than 60 species representing about 30 genera 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.