The Ediacaran period is the last geologic period of the Neoproterozoic age, just before the Cambrian. It ranges from approximately 620 to 542 million years before the present. Historically this name has been variously used by researchers, but its status as an official geological period was ratified in March 2004 by the International Union of Geological Sciences (IUGS) and announced on May 13, 2004, the first new such period declared in 120 years.

The period is unusual because its beginning is not defined by a change in the fossil record. Fossils of the soft-bodied organisms of that time are very scarce. Rather, the beginning is defined by the appearance of a new texturally and chemically distinctive carbonate layer that indicates a climatic change. There is an unusual depletion of ¹³C that marks the end of the global ice ages of the preceding Cryogenian period. The date of the boundary remains to be determined precisely but is between 599 and 636 million years ago [1] (http://www.sciencemag.org/cgi/content/full/305/5684/621).

The name comes from the Ediacara (occasionally 'Ediacarian') Hills of South Australia where peculiar pre-Cambrian fossils were found by the geologist Reg Sprigg in 1946, and studied by Martin Glaessner starting in the 1950s. Glaessner initially thought the creatures to be primitive versions of animals such as corals, sea-pens and worms that were better known from later times. In subsequent decades, many more pre-Cambrian fossils have been found in South Australia. Additional fossils have been found in dozens of outcrops on all continents, and collectively these have come to be known as the Ediacaran fauna. Especially important deposits have been found in the White Sea area of Russia, in southwestern Africa, in northwestern Canada, and in eastern Newfoundland.

As time has passed, the Ediacaran faunas have, if anything, become more rather than less enigmatic. A few fossils such as Kimberella, Bomakellia, and Xenusion seem to be possible precursors to Cambrian forms. Close to the Cambrian boundary, worm tracks, and various hard shelled forms - especially a collection of small forms known as the 'small shelly fauna' also appear to be probable precursors of Cambrian life. On the other hand, many of the best known Ediacaran creatures appear to be immobile blobs, disks, fronds, and mattress-like shapes that have no obvious relationship to later forms. There is considerable controversy about the nature of many Ediacaran forms.

The Ediacaran faunas are sometimes referred to as "Vendian Faunas". Modern usage tends toward using "Ediacaran" to describe the full faunal range including algae, sponges, and all other life forms of the late preCambrian. The term "Vendian" is then reserved by some for a diverse collection of multilobed, softbodied, sessile forms that appear as fronds, disks, and other shapes. These are termed "Vendazoa" and have been proposed as a separate phylum or kingdom.

There are even older fossils known. Well dated fossils of bacteria are found in cherts as old as 3460 million years and probable bacterial mats known back to 3600 million years. 3800 million year old graphite in metasediments from Western Greenland is thought to be of organic origin. Many very old proposed fossils such as Eozoon have subsequently been rejected as naturally occurring pseudo-fossils. The oldest current candidates for early multicelled life are 2000 million year old tracks from West Texas, 1000 million year old tracks from India and Australia, and 700 million year old worm impressions from China. (There is considerable debate as to whether any of these may be traces of "bags of cells" -- confederations of single-celled creatures moving in concert -- or even inorganic geological anomalies.)

External links

• BBC science news article: "Geological time gets a new period". (http://news.bbc.co.uk/1/hi/sci/tech/3721481.stm) Geologists have added a new period to their official calendar of Earth's history—the first in 120 years.