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Last Updated: Monday, 17 May, 2004, 23:09 GMT 00:09 UK
Geological time gets a new period
Ediacaran fossil, Jim Ogg
Ediacaran organisms appear after a series of ice ages that covered the Earth
Geologists have added a new period to their official calendar of Earth's history - the first in 120 years.

The Ediacaran Period covers some 50 million years of ancient time on our planet from 600 million years ago to about 542 million years ago.

It officially becomes part of the Neoproterozoic, when multi-celled life forms started to take hold on Earth.

However, Russian geologists are unhappy their own title - the Vendian - which was coined in 1952, was not chosen.

The decision was taken after a fifteen-year long period of consideration by expert geologists.

"There's always been a recognition that the last part of the Precambrian is a special time before the first shelled animals, when there are these weird, mesh-like creatures of uncertain affinity," Professor Jim Ogg, secretary-general of the International Commission on Stratigraphy (ICS), told BBC News Online.

Infographic, BBC
"Now it's an official part of the timescale."

'Snowball' Earth

The Ediacaran begins at the end of the last ice age of the Snowball Earth, or Cryogenian Period, a term given to a series of glaciations that covered most of our planet between 850-630 or 600 million years ago.

One theory proposes that these climate shocks triggered the evolution of complex, multi-celled life.

Professor Ogg said many of the new life forms that appeared in the Ediacaran seem to be simple organisms, probably related to present-day sponges.

"They appear to be lying flat on the seafloor and people think they may have had photosynthetic symbiosis much like corals do today," he explained.

"These organisms were probably ripped to shreds when the first predators came along. That probably happened in the Cambrian Period."

The proposal had to pass three balloting stages, first by the members of the ICS's Terminal Proterozoic Period subcommission (which was set up specifically to consider the Ediacaran question), then by the ICS itself and finally by the International Union of Geological Sciences (IUGS) which ratified the definition in March.

At each stage, the vote had to be passed by two-thirds of the voting members.

However, Russian geologists are likely to continue to call the period by its alternative name: the Vendian.

In 1952, the Russian geologist Boris Sokolov coined the term Vendian for a system of sedimentary rocks in the former Soviet Union.

Enorama Hills section, Ogg
Enorama Creek, Australia, was chosen as the global bench mark for Ediacaran rocks
The two Russian members of the Terminal Proterozoic Period subcommission and Dr Sokolov submitted a formal comment expressing their disappointment at the decision to choose the Ediacaran over the Vendian.

"This decision ignores both the priority of the name Vendian and a long tradition to use this term in the international geological literature," Sokolov, Mikhail Semikhatov and Mikhail Fedonkin wrote in their comment.

The name Ediacaran takes its name from the Ediacara Hills in the Flinders mountain range of south Australia. The name is of Australian Aboriginal origin and refers to a place where water is present.

The Enorama Creek section of Flinders was designated the "boundary stratotype" for the Ediacaran by the Terminal Proterozoic Period subcommission.

A boundary stratotype is a rock sequence and level that is defined and used as the standard comparison for all other rock sequences of its age.

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