The Ediacaran Period overlaps but is shorter than theVendian Period (650 to 543 million years ago), a name that was earlier, in 1952, proposed by Russian geologist andpaleontologistBoris Sokolov. The Vendian concept was formed stratigraphically top-down, and the lower boundary of the Cambrian became the upper boundary of the Vendian.[12][13]
The Vendian in its type area consists of large subdivisions such as Laplandian,Redkino,Kotlin andRovno 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-walledmicrofossils, megascopic algae,metazoan body fossils andichnofossils.[13][17]
The lower boundary of the Vendian could have abiostratigraphic substantiation as well taking into consideration the worldwide occurrence of the Pertatataka assemblage of giant acanthomorphacritarchs.[16]
The 'golden spike' (bronze disk in the lower section of the image) or 'type section' of theGlobal Boundary Stratotype Section and Point (GSSP) for the base of the Ediacaran SystemThe 'golden spike' marking the GSSP
The Ediacaran Period (c. 635–538.8 Mya) represents the time from the end of globalMarinoan glaciation to the first appearance worldwide of somewhat complicated trace fossils (Treptichnus pedum (Seilacher, 1955)).[7]
Although the Ediacaran Period does contain soft-bodiedfossils, 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 distinctivecarbonate layer that is referred to as a "cap carbonate", because it caps glacial deposits.
TheGSSP 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 theTommotian Stage inSiberia which was selected on the basis of the ichnofossilTreptichnus pedum (Seilacher, 1955). 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 andichnofossils are disputable.[16][18]
Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation)[vague] and usually siliciclastic sediments laterally replace the cap carbonates in a rather short distance but cap carbonates do not occur above every tillite elsewhere[clarification needed] 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 δl3Ccarb in the upper layer of the ocean.[16][19]
Furthermore,Oman presents in its stratigraphic record a large negative carbon isotope excursion, within the Shuram[20] Formation that is clearly away from any glacial evidence[21] strongly questioning systematic association of negative δl3Ccarb excursion and glacial events.[22] Also, theShuram excursion is prolonged and is estimated to last for ~9.0 Myrs.[23]
As to theTreptichnus 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 level ofT. pedum inNamibia,Spain andNewfoundland, and possibly, in thewestern United States. The stratigraphic range ofT. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.[16][24]
Outcrops of micaschists and marble of theGroupe de Canaveilles [fr] (Ediacaran, c. 580 Ma) in the eastern Pyrenees (commune of Fontpédrouse, France)[25]Groupe de Canaveilles exposure (Piton de Castell Vidre) in the eastern Pyrenees (commune of Llo, France). 1 - Very silicious schist. 2 - Marble. 3 - Limestone and dolomite. 4 (& inset) - "Barégiennes" (thin, deformed, alternating calcareous/siliceous beds).[26]
The Ediacaran Period is not yet formally subdivided, but a proposed scheme[27] recognises an Upper Ediacaran whose base corresponds with theGaskiers glaciation, a Terminal Ediacaran Stage starting around550 million years ago, a preceding stage beginning around 575 Ma with the earliest widespreadEdiacaran biota fossils; two proposed schemes differ on whether the lower strata should be divided into an Early and Middle Ediacaran or not, because it is not clear whether the Shuram excursion (which would divide the Early and Middle) is a separate event from the Gaskiers, or whether the two events are correlated.
Thedating of the rock type section of the Ediacaran Period in South Australia has proven uncertain due to lack of overlying igneous material. Therefore, the age range of 635 to 538.8 million years is based oncorrelations to other countries where dating has been possible. The base age of approximately 635 million years is based onU–Pb (uranium–lead) andRe–Os (rhenium–osmium) dating from Africa, China, North America, and Tasmania.[28][29][30][31][32]
The fossil record from most of the Ediacaran Period is sparse, as more easily fossilized hard-shelled animals did not evolve until the latest Ediacaran. The Ediacaran biota include the oldest definitemulticellular organisms (with specialized tissues), the most common types of which resemble segmented worms, fronds, disks, or immobile bags. Among largely undisputed animals,Auroralumina andHaootia werecnidarians, whileYilingia represented the motilebilaterians.[34][35][36]Sponges recognisable as such also appeared, at latest, during the terminal Ediacaran, includingHelicolocellus, a likely non-biomineralizing sponge.[37][38]
Other than these definitive animals, most members of the Ediacaran biota bear little resemblance to modern lifeforms, and theirrelationship with even the immediately following lifeforms of theCambrian explosion is rather difficult to interpret.[39][40] More than 100genera have been described, and well known forms includeArkarua,Charnia,Dickinsonia,Ediacaria,Marywadea,Cephalonega,Pteridinium, andYorgia. However, despite the overall enigmaticness of most Ediacaran organisms, some fossils identifiable as hard-shelled agglutinatedforaminifera (which are not classified as animals) are known from latest Ediacaran sediments of western Siberia.[41]
Four different biotic intervals are known in the Ediacaran, each being characterised by the prominence of a unique ecology and faunal assemblage. The first spanned from 635 to around 575 Ma and was dominated by acritarchs known aslarge ornamented Ediacaran microfossils.[42] The second spanned from around 575 to 560 Ma and was characterised by the Avalon biota. The third spanned from 560 to 550 Ma; its biota has been dubbed the White Sea biota due to many fossils from this time being found along the coasts of theWhite Sea. The fourth lasted from 550 to 539 Ma and is known as the interval of the Nama biotic assemblage.[43]
There is evidence fora mass extinction during this period from early animals changing the environment,[44] dating to the same time as the transition between the White Sea and the Nama-type biotas.[45][46] Alternatively, this mass extinction has also been theorised to have been the result of ananoxic event.[43]
The relative proximity of the Moon at this time meant thattides were stronger and more rapid than they are now. The day was 21.9 ± 0.4 hours, and there were 13.1 ± 0.1 synodic months/year and 400 ± 7 solar days/year.[47]
^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.
^abcSokolov, B.S. (1997). "Essays on the Advent of the Vendian System." 153 pp.KMK Scientific Press, Moscow. (in Russian)
^Sokolov B. S. (1965) "Abstracts of All-Union Symposium on Paleontology of the Precambrian and Early Cambrian." Nauka, Novosibirsk.
^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. & 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.
^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
^Le Guerroué, E.; Allen, P. A.; Cozzi, A. (2006). "Chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in Earth history: The Neoproterozoic Shuram Formation (Nafun Group, Oman)".Precambrian Research.146 (1–2):68–92.Bibcode:2006PreR..146...68L.doi:10.1016/j.precamres.2006.01.007.
^Gong, Zheng; Kodama, Kenneth; Li, Yong-Xiang (2017). "Rock magnetic cyclostratigraphy of the Doushantuo Formation, South China and its implications for the duration of the Shuram carbon isotope excursion".Precambrian Research.289:62–74.Bibcode:2017PreR..289...62G.doi:10.1016/j.precamres.2016.12.002.
^A. Ragozina, D. Dorjnamjaa, A. Krayushkin, E. Serezhnikova (2008). "Treptichnus pedum and the Vendian-Cambrian boundaryArchived 4 October 2011 at theWayback Machine". 33 Intern. Geol. Congr. August 6–14, 2008, Oslo, Norway. Abstracts. Section HPF 07 Rise and fall of the Ediacaran (Vendian) biota. P. 183.
^B. Laumonier et al.,Notice explicative de la feuille Prats-de-Mollo-La-preste (1099) à 1/50 000, BRGM Éditions, Orléans, 2015, pages 22-23 (bC1 - « ...niveau de marbres calcaires et dolomitiques M1...(dans les) hautes vallées de la Ribérole et de la Carança M1 est formé de 2 ou 3 niveaux décamétriques très continus. »,ficheinfoterre.brgm.fr.
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