Vendian biota
From Freepedia
The Vendian biota (also known as Ediacaran 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 (shelly) fossils of classical paleontology.
The original descriptions came from the Ediacaran faunas of South Australia. It later turned out that similar faunas found in Namibia had been previously described, but their great age had not been appreciated. Subsequent discoveries have found similar fossils 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 other places.
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 some possible Vendian forms have been identified in the Cambrian. Many believe that some or all of the Vendians are precursors to one or more modern phyla that arose in the Cambrian.
The Vendian "animals" (assuming that they were animals) are probably too large and complex to be single-celled, and seem to be without exception diploblastic. They universally lack preserved mouths, any organs, or appendages. Symmetries may be two-, three-, four-, or even five-fold. Movement traces are known for some organisms, such as Dickinsonia, Kimberella, and Yorgia. Some appear to be or have 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 most 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.
Several supposed Vendian-like forms have been identified from the Cambrian, however many of these have since been redescribed as trace fossils or pseudofossils.
Well known Vendian forms include Arkarua, Charnia, Dickinsonia, Ediacaria, Marywadea, Onegia, 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.
