Ediacaran biota

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Ediacaran or Vendian biota, fauna, or forms (also Vendobionta or Vendozoa) is a group of ancient lifeforms found in rocks of the Ediacaran Period that predate by 10–40 million years (Ma) the Cambrian faunas that represent the oldest (shelled) fossils of classical paleontology.

Contents 1 Locations 2 Notable Discoveries 3 Morphology 4 In context 5 External links 6 List of known Ediacaran genera 7 See Also 8 References

[edit] Locations

Ediacaran biota take their name from the first discovery in 1946 by Reg Sprigg a mineral geologist exploring the Ediacara Hills at the northern margin of the Flinders Ranges in South Australia. Whilst similar fossil assemblages from Nama, Namibia had been previously described, their great age had not been appreciated. They have subsequently been attributed to the Ediacaran period, approximately 635 to 542 million years ago, just before the Cambrian.

Similar fossils have since been found in Brazil, Newfoundland (Mistaken Point), the Canadian Maritimes, North Carolina, England (Charnwood), Canada's Northwest Territories, the western United States, Scandinavia, the White Sea, Siberia, and Ural region of Russia, Poland, and elsewhere.

[edit] Notable Discoveries

In 1946, the geologist Reg Sprigg found peculiar Precambrian fossils on the western margin of the Flinders Ranges in the Ediacaran hills. ^[1] They were first 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. Since the Ediacaran deposits occur in coarse sandstone, details below about a millimeter cannot be preserved. Guy Narbonne's fortuitous discovery of fossils under a fine ash bed at Mistaken Point, Newfoundland^[2] were preserved in fine muds, allowing details a hundred times finer to be resolved. This closer examination allowed the previous interpretation to be quashed: the developmental pattern of the individuals is not seen in any animals.

[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, pseudofossils, or microbial structures.

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. Ediacara organisms include frond-like forms (e.g. the rangeomorphs), disks with various ornamentations, what appear to be air mattress-like forms, and other unlikely shapes. Some frond-like fossils, such as Charniodiscus and Charnia were attached to the seafloor by discoidal holdfasts. 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 Ediacara 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 superficially resemble living seapens, but grow by inflation or insertion at the tip, not basal insertion as modern sea pens.

Various disc-like fossils superficially resemble 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, suggests 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. However, these are not considered part of the Ediacaran Biota in its strict sense..

Many of the best known Ediacaran creatures appear to be immobile bags, annulate disks, fronds, large single-cells, 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.^[3]

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 biological 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 burrow-like forms 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

• Online exhibit at U. of California Museum of Paleontology
• The Ediacaran Biota page @ Paleos

[edit] List of known Ediacaran genera

Albumares Anfesta Annulusichnus Andiva Arborea Archaeaspinus Archaeichnum Arkarua Aspidella Ausia Baikalina Beltanella Beltanelliformis Beltanelloides Bomakella kelleri Bonata Bradgatia Charnia Charniodiscus Chondroplon Circulichnis Cochlichnus Conomedusites Cyclomedusa Dickinsonia Ediacaria Eoporpita Ernietta Glaessneria Glaessnerina Gordia Hagenetta Harlaniella Helminthoidichnites Hiemalora Inaria Intrites Irridinitus Ivesia or Ivesheadia Jampolium Kimberella Kuibisia Lorenzinites Madigania Marywadea Mawsonites Medusina Medusinites Namacalathus

Namalia Namapoikia Nasepia Nemiana Neonereites Nimbia Onega Ovatoscutum Palaeohelminthoida Palaeopascichnus Paliella Paracharnia Paramedusium Parvancorina Persimedusites Phyllozoon Planolites Planomedusites Praecambridium Protodipleurosoma Protolyella Protoniobia Pseudorhizostomites Pseudorhopilema Pteridinium Rangea Redkinia Rugoconites Sekwia Sellaulichnus Skinnera Spatangopsis Spriggia Spriggina Swartpuntia Tateana Tentaculato Thectardis Tirasiana Tribrachidium Triforillonia Vendella Vendia Vendomia Wigwamiella Windermeria Yelovichnus Yorgia

[edit] See Also

• List of fossil sites (with link directory)

[edit] References

1. ^ Sprigg, Reg (1947). "Early Cambrian (?) Jellyfishes from the Flinders Ranges, South Australia". Transactions of the Royal Society of South Australia 71: 212-224. 
2. ^ Template:Cite journal first=Guy
3. ^ Gehling, James G.; Guy M. Narbonne and Michael M. Anderson (1999). "The First Named Ediacaran Body Fossil, Aspidella terranovica". PALAEONTOLOGY 43: 429. Retrieved on 11 February 2007. 

• McMenamin, Mark A. S. The Garden of Ediacara New York: Columbia University Press, 1998. ISBN 0-231-10559-2