The Ediacaran period (/ˌiːdiˈækərən,ˌɛdi-/EE-dee-AK-ər-ən, ED-ee-)[3] is a geological period of the Neoproterozoicera 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.[4] It is the last period of the Proterozoiceon 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.[5] 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.[6][7][8] 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[9] within the Brachina Gorge[10] in the Ikara-Flinders Ranges National Park, at 31°19′53.8″S138°38′0.1″E / 31.331611°S 138.633361°E, approximately 55 km (34 mi) southeast of the Ediacara Hills fossil site. / -31.331611; 138.633361
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.
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.[11][12]
Paleontological substantiation of this boundary was worked out separately for the siliciclastic basin (base of the Baltic Stage of the Eastern European Platform[13]) and for the carbonate basin (base of the Tommotian stage of the Siberian Platform).[14]The lower boundary of the Vendian was suggested to be defined at the base of the Varanger (Laplandian) tillites.[12][15]