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Ediacaran and VendianEdit

Ediacaran period overlaps, but is shorter than the Vendian period, a name that was earlier, in 1952, proposed by Russian geologist/paleontologist Boris Sokolov. The Vendian concept was formed stratigraphically top-down, and the lower boundary of the Cambrian became the upper boundary of the Vendian.[1][2] Paleontological substantiation of this boundary was worked out separately for the siliciclastic basin (base of the Baltic Stage of the Eastern-European Platform [3]) and for the carbonate basin (base of the Tommotian Stage of the Siberian Platform [4]). Lower boundary of the Vendian was suggested to be defined at the base of the Varanger (Laplandian) tillites.[2][5]

The Vendian in its type area consists of large subdivisions such as Laplandian, Redkino, Kotlin and Rovno Regional stages with the globally traceable subdivisions and their boundaries, including its lower one. The Redkino, Kotlin and Rovno regional stages have been substantiated in the type area of the Vendian on the basis of the abundant organic-walled microfossils, megascopic algae, metazoan body fossils and ichnofossils.[2][6] The lower boundary of the Vendian could have a biostratigraphic substantiation as well taking into consideration the worldwide occurrence of the Pertatataka assemblage of giant acantomorph acritarchs.[5]

Borders of the EdiacaranEdit

The Ediacaran Period (ca. 635-542 Ma) represents the time from the end of global Marinoan glaciation to the first appearance worldwide of somewhat complicated trace fossils (Trichophycus pedum). [7]

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 (this bed is characterized by an unusual depletion of 13C), referred to as a "cap carbonate", because it caps glacial deposits and indicates a sudden climatic change at the end of the Marinoan ice age. The lower boundary GSSP of the Ediacaran is at the base of the cap carbonate (Nuccaleena Formation), immediately above the Elatina diamictite in the Enorama Creek section, Flinders Ranges, South Australia.

The GSSP of the upper boundary of the Ediacaran is the lower boundary of the Cambrian on the SE coast of Newfoundland approved by the International Commission on Stratigraphy as a preferred alternative to the base of the Tommotian Stage in Siberia which was selected on the basis of the ichnofossils Treptichnus pedum. In the history of stratigraphy it was the first case of usage of bioturbations for the System boundary definition.

Nevertheless, the definitions of the lower and upper boundaries of the Ediacaran on the basis of chemostratigraphy and ichnofossils are disputable.[5][8]

Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation) and usually siliciclastic sediments replace laterally the cap carbonates in a rather short distance and cap carbonates do not occur above every tillite elsewhere in the world. The C-isotope chemostratigraphic characteristics obtained for contemporaneous cap carbonates in different parts of the world may be variable in a wide range owing to different degrees of secondary alteration of carbonates, dissimilar criteria used for selection of the least altered samples, and, as far as the C-isotope data are concerned, due to primary lateral variations of δ l3Ccarb in the upper layer of the ocean. [5][9]

As to the Treptichnus pedum, a reference ichnofossil for the lower boundary of the Cambrian, its usage for the stratigraphic detection of this boundary is always risky because of the occurrence of very similar trace fossils belonging to the Treptichnids group well below the T. pedum in Namibia, Spain and Newfoundland, and possibly, in the west of USA. The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.[5][10]

DatingEdit

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[11] and China.[12] 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.[13] The age of the top is the same as the widely recognised age for the base of the Cambrian Period[14] 542± 0.3 Ma (million years ago).[15]

BiotaEdit

Main article: Ediacaran biota

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.

In addition, there are unverified claims of "footprints" - tracks made by legged organisms thought to resemble arthropods or legged worms. The significance of these finds awaits their publication.[16]

See alsoEdit

ReferencesEdit

  1. ^ B. M. Sokolov (1952). "On the age of the old sedimentary cover of the Russian Platform". Izvestiya Akademii Nauk SSSR, Seriya eologicheskaya 5: 21–31. 
  2. ^ a b c Sokolov, B.S. (1997). "Essays on the Advent of the Vendian System." 153 pp. KMK Scientific Press, Moscow. (in Russian)
  3. ^ Sokolov B. S. (1965) "Abstracts of All-Union Symposium on Paleontology of the Precambrian and Early Cambrian." Nauka, Novosibirsk.
  4. ^ Rozanov, A.Y., Missarzhevskij, V.V., Volkova, N.A., Voronova, L.G., Krylov, I.N., Keller, B.M., Korolyuk, I.K., Lendzion, K., Michniak, R., Pykhova, N.G., and Sidorov, A.D. (1969). "The Tommotian Stage and the problem of the lower boundary of the Cambrian". Trudy Geologičeskogo Instituta AN SSSR 206: 1–380. 
  5. ^ a b c d e M.A. Fedonkin, B.S. Sokolov, M.A. Semikhatov, N.M.Chumakov (2007). "Vendian versus Ediacaran: priorities, contents, prospectives." In: "The Rise and Fall of the Vendian (Ediacaran) Biota. Origin of the Modern Biosphere. Transactions of the International Conference on the IGCP Project 493, August 20-31, 2007, Moscow." Moscow: GEOS.
  6. ^ B. S. Sokolov, M. A. Fedonkin (1984). "The Vendian as the Terminal System of the Precambrian". Episodes 7 (1): 12–19. http://www.episodes.org/backissues/71/ARTICLES--12.pdf. 
  7. ^ A. Knoll, M. Walter, G. Narbonne, and N. Christie-Blick (2004) "The Ediacaran Period: A New Addition to the Geologic Time Scale." Submitted on Behalf of the Terminal Proterozoic Subcommission of the International Commission on Stratigraphy.
  8. ^ Comments By B. S. Sokolov, M. A. Semikhatov, And M. A. Fedonkin. (2004) Appendix 2 in: "The Ediacaran Period: A New Addition to the Geologic Time Scale." Submitted on Behalf of the Terminal Proterozoic Subcommission of the International Commission on Stratigraphy. pp. 32-34
  9. ^ Bristow, T.F. and Kennedy, M.J. (2008). "Carbon isotope excursions and the oxidant budget of the Ediacaran atmosphere and ocean". Geology 36 (11): 863–866. doi:10.1130/G24968A.1. http://taylorandfrancis.metapress.com/index/PUNM6BQLNW1BJMFN.pdf. Retrieved on 2007-05-05. 
  10. ^ A. Ragozina, D. Dorjnamjaa, A. Krayushkin, E. Serezhnikova (2008). "Treptichnus pedum and the Vendian-Cambrian boundary". 33 Intern. Geol. Congr. August 6- 14, 2008, Oslo, Norway. Abstracts. Section HPF 07 Rise and fall of the Ediacaran (Vendian) biota. P. 183.
  11. ^ 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. http://geology.geoscienceworld.org/cgi/content/abstract/32/9/817. 
  12. ^ Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., and Jin, Y. (1 April 2005). "U-Pb Ages from the Neoproterozoic Doushantuo Formation, China" (abstract). Science 308 (5718): 95–98. doi:10.1126/science.1107765. PMID 15731406. http://www.sciencemag.org/cgi/content/abstract/308/5718/95. 
  13. ^ 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. http://geology.geoscienceworld.org/cgi/content/abstract/32/10/893. 
  14. ^ Ogg, J.G. (2004). "Status of Divisions of the International Geologic Time Scale". Lethaia 37 (2): 183–199. doi:10.1080/00241160410006492. http://taylorandfrancis.metapress.com/index/PUNM6BQLNW1BJMFN.pdf. Retrieved on 2007-05-05. 
  15. ^ Amthor, J. E.; and others (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman". Geology 31: pp 431–434. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2 10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2 . 
  16. ^ Earliest Animal Footprints Ever Found -- Discovered in Nevada Newswise, Retrieved on October 5, 2008. Reporting on a poster session at the Geological Society of America


External linksEdit

Preceded by Archean eon 2.5 Ga - Proterozoic eon - 542 Ma Followed by Phanerozoic eon
2.6 Ga - Paleoproterozoic era - 1.6 Ga 1.6 Ga - Mesoproterozoic era - 1.0 Ga 1.0 Ga - Neoproterozoic era - 542 Ma
Siderian Rhyacian Orosirian Statherian Calymmian Ectasian Stenian Tonian Cryogenian Ediacaran

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