From: Andy Soos, ENN
Published October 19, 2010 01:20 PM

The Edicaran Age

The discovery of blocks of gravel which sank to the bottom of the sea trapped in ancient icebergs has sparked a new understanding of a bizarre group of creatures. The research, published in the Australian Journal of Earth Sciences, has also forced a rethink of the conditions that existed more than 500 million years ago. 580 million years ago, the ancient oceans were flooded with enough oxygen that the way in which life was constructed was completely changed. This moment was the birth of multi-cellular organisms, and shortly preceded the burst of biological diversification called the Cambrian explosion. Recent evidence indicates that this was the last in a series of similar increases in oxygen availability that changed the world's climate and ecological conditions.

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The Ediacaran Period named after the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period. Its status as an official geological period was ratified in 2004 by the International Union of Geological Sciences, making it the first new geological period declared in 120 years.

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 multi-cellular organisms with tissues, and the most common types resemble segmented worms, fronds, disks, or immobile bags. They bear little resemblance to modern lifeforms.

Associate Professor Victor Gostin and colleagues at the University of Adelaide found evidence of ancient icebergs mixed in with volcanic rocks which were spewed out when an asteroid hit the earth between 635-542 million years ago. The impact area is now known as Acraman Crater which is a deeply eroded impact crater in the Gawler Ranges of South Australia. Its location is marked by Lake Acraman, a circular ephemeral playa lake about 12 miles in diameter. The site is north of Adelaire in South Australia.

Gostin, who first discovered rocky traces of the asteroid "ejecta" almost 30 years ago, said the asteroid impact occurred during a period of extreme cold.

This contradicts previous ideas which suggest the impact of the asteroid actually precipitated a period of glaciations, Gostin says.

He said evidence of the ancient icebergs is embedded in the fine grained shale found several hundred miles from Lake Acraman.

"Icebergs carry coarse debris, boulders and grains," said Gostin. "As the iceberg melts it is dragged to the bottom by the weight of the debris, then the ice melts and the surrounding mud eventually buries the debris, creating this sort of fossilized iceblock.

"So when you find coarse boulders embedded in fine deposits, something must have rafted it there -- in this case, an iceberg," he added.

In a commentary to be published in the The Australian Geologist, Professor Malcolm Walter of the University of New South Wales says the research resolves issues around the timing and record of the glaciations, the impact and the rise of the Ediacaran biota.

He describes the effect of the icy climate and asteroid impact as a "double whammy" that paradoxically led to the "flourishing of exotic plankton and the first macroscopic animals".

"Did the Acraman impact and coincidentally coeval glaciation help trigger one of the great biological revolutions in earth history? Time will tell," writes Walter.

The Ediacaran fauna were a major change in earth life forms. An asteroid impact may have helped to trigger this change in evolution by a sudden change in climate conditions.

For further information: http://news.discovery.com/animals/early-life-iceblocks.html#mkcpgn=rssnws1

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