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Ediacaran
From Wikipedia, the free encyclopedia
The Ediacaran Period (IPA: /ˌiːdiˈækərən/, named after the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic Era, just preceding the Cambrian Period of the Paleozoic Era. Its status as an official geological period was ratified in March 2004 by the International Union of Geological Sciences (IUGS) and announced on May 13, 2004, the first new geological period declared in 120 years.[1][2] The type section is in the Flinders Ranges in South Australia. It overlaps, but is shorter than the Vendian period, a name that was earlier proposed in Russia.
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Although the Ediacaran Period does contain soft bodied fossils, it is unusual in comparison to later periods because its beginning is not defined by a change in the fossil record. Rather, the beginning is defined at the base of a chemically distinctive carbonate layer, referred to as a "cap carbonate", because it caps glacial deposits and indicates a sudden climatic change at the end of an ice age. This bed is characterized by an unusual depletion of 13C, and is considered by many scientists to be of global extent, although this is controversial.
No dating has been possible at the type section of the Ediacaran Period in South Australia. Therefore the age range of 635 to 542 million years before the present is based on correlations to other countries where dating has been possible. The base age of approximately 635 million years ago is based on U-Pb (uranium-lead) isochron dating from Namibia.[3] Applying this age to the base of the Ediacaran assumes that individual cap carbonates are synchronous around the world and that the correct cap carbonate layers have been correlated between Australian and Namibia. This is controversial because an age of about 580 million years has been obtained in association with glacial rocks in Tasmania which some scientists tentatively correlate with those just beneath the Ediacaran rocks of the Flinders Ranges.[4] The age of the top is the same as the widely recognised age for the base of the Cambrian Period.[2]
The animal fossil record from this period is sparse, possibly because animals had yet to evolve hard shells, which make for easier fossilization. The Ediacaran biota include the oldest definite multicellular organisms with tissues, and the most common types resemble segmented worms, fronds, disks, or immobile bags. They bear little resemblance to modern lifeforms, and their relationship even with the later lifeforms of the Cambrian explosion is difficult to interpret. More than 100 genera have been described, and well known forms include Arkarua, Charnia, Dickinsonia, Ediacaria, Marywadea, Onega, Pteridinium, and Yorgia.
- List of fossil sites (with link directory)
- ^ Knoll, A.H.; Walter, M.R., Narbonne, G.M., Christie-blick, N. (2004). "A new period for the geologic time scale". Science(Washington) 305 (5684): 621-622.
- ^ a b Ogg, J.G. (2004). "Status of Divisions of the International Geologic Time Scale". Lethaia 37 (2): 183-199. Retrieved on 2007-05-05.
- ^ Hoffmann, K.H.; Condon, D.J., Bowring, S.A., Crowley, J.L. (2004-09-01). "U-Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: Constraints on Marinoan glaciation". Geology 32 (9): 817-820. doi:10.1130/G20519.1.
- ^ Calver, C.R.; Black, L.P., Everard, J.L., Seymour, D.B. (2004-10-01). "U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania". Geology 32 (10): 893-896. doi:10.1130/G20713.1.
- ^ Cite error 8; No text given.
- ^ Hofmann, H.J.; Narbonne, G.M., Aitken, J.D. (1990). "Ediacaran remains from intertillite beds in northwestern Canada". Geology 18 (12): 1199-1202.
- "Geological time gets a new period: Geologists have added a new period to their official calendar of Earth's history—the first in 120 years", BBC, 17 May, 2004.
- Ediacaran Period. GeoWhen Database. Retrieved on January 5, 2006.
- Introduction to the Vendian Period
- Introduction to the Ediacaran Fauna
- transcript - Catalyst (Australian Broadcasting Corporation)
- Mistaken Point Fauna : The Discovery
| Proterozoic eon | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Paleoproterozoic era | Mesoproterozoic era | Neoproterozoic era | |||||||
| Siderian | Rhyacian | Orosirian | Statherian | Calymmian | Ectasian | Stenian | Tonian | Cryogenian | Ediacaran |
