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The Ediacaran ( EE-dee-AK-ər-ən, ED-ee-) is a geological period of the Neoproterozoic era that spans 96 million years from the end of the Cryogenian period at 635 Mya to the beginning of the Cambrian period at 538.8 Mya. It is the last period of the Proterozoic eon as well as the last of the so-called "Precambrian supereon", before the beginning of the subsequent Cambrian period marks the start of the Phanerozoic eon, where recognizable fossil evidence of life becomes common. The Ediacaran period is named after the Ediacara Hills of South Australia, where trace fossils of a diverse community of previously unrecognized lifeforms (later named the Ediacaran biota) were first discovered by geologist Reg Sprigg in 1946. 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. Although the period took namesake from the Ediacara Hills in the Nilpena Ediacara National Park, the type section is actually located in the bed of the Enorama Creek within the Brachina Gorge in the Ikara-Flinders Ranges National Park, at 31°19′53.8″S 138°38′0.1″E, approximately 55 km (34 mi) southeast of the Ediacara Hills fossil site. The Ediacaran marks the first widespread appearance of complex multicellular fauna following the end of the Cryogenian global glaciation known as the Snowball Earth. The relatively sudden evolutionary radiation event, known as the Avalon Explosion, is represented by now-extinct, relatively simple soft-bodied animal phyla such as Proarticulata (bilaterians with simple articulation, e.g. Dickinsonia and Spriggina), Petalonamae (sea pen-like animals, e.g. Charnia), Aspidella (radial-shaped animals, e.g. Cyclomedusa) and Trilobozoa (animals with tri-radial symmetry, e.g. Tribrachidium). Most of these organisms appeared during or after the Avalon explosion 575 million years ago and died out during the End-Ediacaran extinction event 539 million years ago. Forerunners of some modern animal phyla also appeared during this period, including cnidarians and early bilaterians such as Xenacoelomorpha, as well as mollusc-like Kimberella. Hard-bodied organisms with mineralized shells or endoskeletons, which can be fossilized and preserved, were yet to evolve and would not appear until the superseding Cambrian Explosion some 35 million years later. The supercontinent Pannotia formed and broke apart by the end of the period. The Ediacaran also witnessed several glaciation events, such as the Gaskiers and Baykonurian glaciations. The Shuram excursion also occurred during this period, but its glacial origin is unlikely. Ediacaran and Vendian The Ediacaran Period overlaps but is shorter than the Vendian Period (650 to 543 million years ago), a name that was earlier, in 1952, proposed by Russian geologist and 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. Paleontological substantiation of this boundary was worked out separately for the siliciclastic basin (base of the Baltic Stage of the Eastern European Platform) and for the carbonate basin (base of the Tommotian stage of the Siberian Platform). The lower boundary of the Vendian was suggested to be defined at the base of the Varanger (Laplandian) tillites. 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. 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 acanthomorph acritarchs. Upper and lower boundaries The Ediacaran Period (c. 635–538.8 Mya) represents the time from the end of global Marinoan glaciation to the first appearance worldwide of somewhat complicated trace fossils (Treptichnus pedum (Seilacher, 1955)). 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 that is referred to as a "cap carbonate", because it caps glacial deposits. This bed is characterized by an unusual depletion of 13C that indicates a sudden climatic change at the end of the Marinoan ice age. The lower global boundary stratotype section (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 ichnofossil Treptichnus 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 and ichnofossils are disputable. Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation) and usually siliciclastic sediments laterally replace the cap carbonates in a rather short distance but 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. Furthermore, Oman presents in its stratigraphic record a large negative carbon isotope excursion, within the Shuram Formation that is clearly away from any glacial evidence strongly questioning systematic association of negative δ l3Ccarb excursion and glacial events. Also, the Shuram excursion is prolonged and is estimated to last for ~9.0 Myrs. 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 level of T. pedum in Namibia, Spain and Newfoundland,.... Discover the B C Tweedt popular books. Find the top 100 most popular B C Tweedt books.