Ediacaran

The Ediacaran Period (pronounced /ˌiːdiˈækərən/; named after the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon. Its status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS), making it the first new geological period declared in 120 years.^[5][6][7]

Although the Period takes its name from the Ediacara Hills where geologist Reg Sprigg first discovered fossils of the eponymous biota in 1946, the type section is located in the Brachina Gorge^[8] in the Flinders Ranges of South Australia.

Ediacaran and Vendian

The 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.^[9][10] Paleontological substantiation of this boundary was worked out separately for the siliciclastic basin (base of the Baltic Stage of the Eastern-European Platform ^[11]) and for the carbonate basin (base of the Tommotian Stage of the Siberian Platform ^[12]). The lower boundary of the Vendian was suggested to be defined at the base of the Varanger (Laplandian) tillites.^[10][13]

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.^[10][14] 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.^[13]

Borders of the Ediacaran

The 'golden spike' (bronze disk in the lower section of the image) or 'type section' of the Global Boundary Stratotype Section and Point (GSSP) for the base of Ediacaran period. To read the disk, click through to the full image.

The Ediacaran Period (ca. 635-542 Mya) represents the time from the end of global Marinoan glaciation to the first appearance worldwide of somewhat complicated trace fossils (Treptichnus pedum).^[5]

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, Brachina Gorge, 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.^[13][15]

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 ^l3C_carb in the upper layer of the ocean.^[13][16] Furthermore, Oman presents in its stratigraphic record a large negative carbon isotope excursion, within the Shuram^[17] Formation that is clearly away from any glacial evidence^[18] strongly questioning systematic association of negative δ ^l3C_carb excursion and glacial events^[19]. 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.^[13][20]

Dating

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^[21] and China.^[22] 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.^[23] The age of the top is the same as the widely recognised age for the base of the Cambrian Period^[24] 542± 0.3 Mya (million years ago).^[25]

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.^[26]

See also

• List of fossil sites (with link directory)

References

External links

• "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". London: BBC. 2004-05-17. http://news.bbc.co.uk/1/hi/sci/tech/3721481.stm. 
• "Ediacaran Period". GeoWhen Database. http://www.stratigraphy.org/bak/geowhen/stages/Ediacaran.html. Retrieved January 5, 2006. 
• Introduction to the Vendian Period
• Introduction to the Ediacaran Fauna
• transcript – Catalyst (Australian Broadcasting Corporation)
• Mistaken Point Fauna: The Discovery