Ediacaran

From Freepedia

 id:eon  value:rgb(1,0.7,1)  legend:Proterozoic_eon  # light purple
 id:era  value:rgb(1,1,0.7)  legend:Neoproterozoic_era   # light yellow
 id:period  value:rgb(1,0.7,0.7)  legend:Ediacaran_period  # light red
 id:black  value:black

 bar:eon
 bar:era
 bar:period

 width:80 align:center textcolor:black mark:(line,black)

 bar:eon color:eon

 from:542  till:end  width:100  mark:none
 at:660  mark:none  text:Proterozoic~(542-2500)

 bar:era color:era

 from:542  till:650  width:100  mark:none
 at:640  mark:none  text:Neoproterozoic~(542-1000)

 bar:period color:period

 from:542  till:630  text:Ediacaran~(542-630)

 tabs:(25-left)

 pos:(15,350)
 text:"Preceded by:"
 text:^Cryogenian period

 pos:(15,125)
 text:"Followed by:"
 text:^Phanerozoic eon,
 text:^Paleozoic era and
 text:^Cambrian period

The Ediacaran period is the last geological period of the Neoproterozoic Era, just before the Cambrian. It ranges from approximately 635 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. Unusual soft bodied fossils do occur in the Ediacaran Period, but these are limited to the latter parts of the Period, after about 580 million years ago. Rather, the beginning is defined by the appearance of a new texturally and chemically distinctive carbonate layer that indicates a climatic change (the end of a global ice age). 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 is fairly well constrained at 635 million years ago based on U-Pb (uranium-lead) dates from Namibia and China.

The name comes from the Ediacara (occasionally 'Ediacarian', derived from an Indigenous Australian term for place near water) Hills of South Australia, thus being the only geologic period to have a name originating in the southern hemisphere. It was in the Ediacara Hills that peculiar Precambrian 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 Precambrian 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 biota. 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 biotas have, if anything, become more rather than less enigmatic. A few fossils such as Kimberella seem to be possible precursors to Cambrian forms. After about 560 million years, trace fossils like worm burrows indicate the presence of true animals. On the other hand, many of the best known Ediacaran creatures appear to be immobile blobs, disks, fronds, and air mattress-like shapes that have no obvious relationship to later forms. There is considerable controversy about the nature of many Ediacaran forms, with some having been classified in as many as six different kingdoms.

The Ediacaran biotas is occasionally referred to as the Vendian biota but this has been used more rarely in recent times. This usage echoes the former name Vendian, by which the Ediacaran period was known in many parts of the world prior to the official naming of the period in 2004. Modern usage tends toward using "Ediacaran" to describe the full faunal range including algae, sponges, and all other life forms of the late Precambrian. A term Vendobionta, which is also used, is not a description of the fauna, but rather the name of a separate Kingdom into which many of the fossils were placed by German palaeontologist Dolf Seilacher. This has been extremely controversial, and has not gained widespread acceptance.

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, though most geological opinion tends to the latter. The Ediacaran biota is accepted as the earliest record of animal life by most palaeontologists).

External links

• BBC science news article: "Geological time gets a new period". Geologists have added a new period to their official calendar of Earth's history—the first in 120 years.