| Ediacaran period 630 - 542 million years ago |
|
| Mean atmospheric O2 content over period duration | ca. 8 Vol %[1] (40 % of modern level) |
| Mean atmospheric CO2 content over period duration | ca. 4500 ppm[2] (16 times pre-industrial level) |
The Ediacaran Period (IPA: /ˌiːdiˈækərən/, named after the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic Era, immediately preceding the Cambrian Period of the Paleozoic Era. 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.[3][4] The type section is in the Flinders Ranges in South Australia. It overlaps, but is shorter than the Vendian period, a name that was earlier proposed by Boris Sokolov of Russia. In 1952, Boris Sokolov proposed the name Vendian to label a discrete system of siliciclastic rocks.
Contents |
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, referred to as a "cap carbonate", because it caps glacial deposits and indicates a sudden climatic change at the end of an ice age. This bed is characterized by an unusual depletion of 13C, but most large isotopic excursions in the Ediacaran period are too large to represent a global signal, and must reflect local changes.[5]
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.[6] and China.[7] 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.[8] The age of the top is the same as the widely recognised age for the base of the Cambrian Period[4] 542± 0.3 Ma (million years ago).[9]
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.[10]
| Proterozoic eon | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Paleoproterozoic era | Mesoproterozoic era | Neoproterozoic era | |||||||
| Siderian | Rhyacian | Orosirian | Statherian | Calymmian | Ectasian | Stenian | Tonian | Cryogenian | Ediacaran |