Ediacaran biota
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The Ediacaran biota (also known as Ediacaran fauna(s), Vendian biota, Vendian forms, Vendian fauna(s), Vendobionta or Vendozoa) are a group of ancient lifeforms that are found in rocks of the Ediacaran Period, a bit older than the Cambrian faunas that represent the oldest (shelled) fossils of classical paleontology.
[edit] Locations
The name comes from the Ediacara Hills (derived from an Indigenous Australian term for a place near water) of South Australia, thus being the only geologic period to have a name originating in the southern hemisphere. In the Ediacara Hills, on the western margin of the Flinders Ranges, peculiar Precambrian fossils were found by the geologist Reg Sprigg in 1946, and studied by Martin Glaessner starting in the 1950s. Glaessner at first thought that the creatures were primitive versions of animals such as corals, sea-pens, and worms better known from later times. In later decades, many more Precambrian fossils have been found in the Flinders Ranges of South Australia. Related fossils have been found in dozens of outcrops on all continents, and collectively these have come to be known as the Ediacaran biota. Especially important deposits have been found in the White Sea area of Russia, in southwestern Africa, in northwestern Canada, and in south eastern Newfoundland.
The original descriptions came from the Ediacara biota of South Australia. Similar fossil assemblages found in Namibia had been previously described, but their great age had not been appreciated. Similar fossils were found later in Brazil, Antarctica, Newfoundland, the Canadian Maritimes, North Carolina, England, Canada's Northwest Territories, the western United States, Scandinavia, the White Sea, Siberia, and Ural region of Russia, Poland, and elsewhere.
[edit] Morphology
These now extinct forms are generally segmented or frondlike with no visible organs other than holdfasts in some varieties. Many of these fossils are difficult to interpret. They are probably entirely late Precambrian, although several supposed Ediacara-like forms have been identified from the Cambrian, but many of these have since been redescribed as trace fossils or pseudofossils. Many believe that some or all of the Ediacarans are precursors to one or more modern phyla that arose in the Cambrian.
Movement traces are known for some organisms, such as Dickinsonia, Kimberella, and Yorgia. These, and similar fossils, are often interpreted as being ancestral to Phanerozoic phyla - especially the arthropods. It is commonly noted how Spriggina and Parvancorina resemble trilobites. Some frond-like fossils, such as Charniodiscus and Charnia were attached to the seafloor by discoidal holdfasts. They include frond-like forms (e.g. the rangeomorphs), disks with various ornamentations, what appear to be air mattress-like forms, and other unlikely shapes. They were originally thought to be simple precursors of more modern forms, and a few elements of the fauna still look like possible precursors of such later forms as arthropods and molluscs. But many appear to belong to some evolutionary sidetrack. It has been proposed that they constitute an ancient phylum, the Vendobionta, that largely died out just before the beginning of the Cambrian. Another commonly cited possibility is that the frond-like fossils belong to the phylum cnidaria and are related to modern sea pens.
Well known Ediacaran forms include Arkarua, Charnia, Dickinsonia, Ediacaria, Marywadea, Onega, Yorgia and Pteridinium. The full list runs to 100 or more taxa. Some of those named are rare but interesting for one reason or another. Others are widely distributed.
As time has passed, assemblages of the Ediacara biota have, if anything, become more rather than less enigmatic.
- The earliest Ediacaran fossils, 575 million years ago, were fronds attached to the seafloor by discs.
- Frond-like fossils such as Charniodiscus resemble living seapens.
- The Rangeomorphs, show fractal branching (meaning that the branches are self-similar).
- Various disc-like fossils appear to have been creatures like sea-anemones (Mawsonites, Hiemalora and Inaria) and sponges (Palaeophragmodictya).
- One of the largest and most distinctive Ediacara animals was a flattened, oval-shaped and segmented worm-like form called Dickinsonia that could grow to a metre or more long.
- Arkarua, a tiny circular disc with five, evenly spaced points, suggesting the form of an extinct echinoderm called an edrioaster.
- About 560 million years ago, trace fossils like worm burrows appear in the fossil record together with small body fossils that have bilateral symmetry. A few of these fossils Kimberella, Parvancorina and Spriggina seem to be possible early examples of molluscs and arthropods.
Many of the best known Ediacaran creatures appear to be immobile bags, annulate disks, fronds, and air-mattress-like shapes that have no obvious relationship to later forms. They generally lack typical features of digestive tracts such as mouths, intestines (or processing chambers) and an anus. Legs and jointed limbs are also absent. Some suggest that without significant predators at the time, there is no need for fast processing of nutrients or quick ingestion, and thus digestion was mostly via absorption. The eventual introduction of significant predators may be what triggered the Cambrian explosion, according to this view.[citation needed]
There is considerable controversy about the nature of many Ediacaran forms, with some having been classified in as many as six kingdoms.
[edit] In context
The Ediacara biota is occasionally referred to as the Vendian biota but this has been used more rarely in recent times. This usage echoes the former name Vendian, by which the Ediacaran Period was known in Russia and some other parts of the world before the official naming of the period in 2004. Modern usage tends toward using Ediacaran to describe the full faunal range including algae, sponges, and all other life forms of the late Precambrian.
The term "Vendobionta", which is also used, is not a description of the fauna, but rather the name of a separate kingdom where German palaeontologist Dolf Seilacher put many of the fossils. This has been extremely controversial, and has not gained widespread acceptance.
The Ediacaran fossils are the oldest definite multicellular fossils, but there are even older fossils known. Well-dated fossils of bacteria are found in cherts 3460 million years and probable bacterial mats known back to 3600 million years. 3800 million year old graphite in metasediments from Western Greenland is thought to be of organic origin. Many very old proposed fossils such as Eozoon have subsequently been rejected as naturally occurring pseudo-fossils. Coiled organic tubes with serial partitions, up to a meter in length and a few mm wide, called Grypania first discovered in 2000 million year old rocks of Lake Superior, are the oldest known large fossil organisms. They may be algae or giant bacteria. Chain of bead fossils called Horodyskia occur in 1000-1300 million year old rocks in Montana, Western Australia and Tasmania. The oldest current candidates for early multicelled life are 1000 million-year old burrows from India and Australia, and 700 million-year old worm impressions from China. The first known surface burrows, that are common and clearly the traces of moving animals, occur in Ediacara fossil assemblages from rocks about 560 million years before the present.
[edit] External links
[edit] List of Ediacaran Genera
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[edit] References
- McMenamin, Mark A. S. The Garden of Ediacara New York: Columbia University Press, 1998. ISBN 0231105592
