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635 Mya
First multi-cellular animals

Prior to this period, all known life consisted of unicellular creatures – bacteria and their relatives.The first appearance of genuine multi-cellular creatures opened up potential pathways to all more advanced life as we know it.

In 1946 Geologist Reg Sprigg found fossils of what he called ’jellyfish’ in the Ediacara Hills in S Australia. The rocks were thought to be of younger Cambrian Age. Similar fossils had been found in Namibia and Scotland, but all were thought to be in younger rocks. It was not until 1956 that fossil evidence was found in England by some schoolchildren in definitely Pre-Cambrian rocks was it realized that there was life prior to the development of animals with hard parts. Subsequently many more of these type and age fossils have been found in a number of areas including Newfoundland and Russia.

Spriggina

Dickinsonia

Charnia

It is not entirely clear to what kingdom these belong, and many suggestions have been made. They have no hard parts and possibly some were just sacs that absorbed nutrient from the sea. Some were definitely confined to lying on or moving over the sea floor, but others seem to have been standing up anchored to the sea floor.

The original finds were all in sandstones, and it is difficult to work out how such soft bodied creatures could be fossilized in such a way in that environment. Most fossils of soft bodied creatures are in fine- grained muds and shales, so how could this preservation have occurred?

From close investigation of the rocks containing the Ediacaran fossils it is thought that there were ‘biomats’ layers of bacterial growth in the areas. The creatures could have been “grazing” on these bacterial mats. When they died, the mats could have protected the creature and enabled the soft parts to be preserved.

They do not appear to be the progenitors of subsequent animals and plants, initially, some were thought to be related to jelly fish and Sea Pens, part of the Cnidaria Phylum but this is now not thought to be the case. Some resembled small, inflated mattresses.

There are no known correlatives or descendants. There are a number of theories why they died out totally. This could have been due to changes in the Sea water, for instance causing the bacterial biomats to disappear. Perhaps the evolving of other animals with whom they could not compete. Animals with hard parts that could consume the soft bodies are found in the rocks above the Ediacaran. This could also have applied to the biomats which could have been removed by more aggressive grazing. This could have led to destabilization of the substrate which would remove the anchors of the upright creatures. Whatever the reasons, there is little or no evidence of Ediacaran fauna subsequently.

Cnidarian Sea Pen

It remains a fact that for around 85%of the Earth’s history there were only single celled life forms.

635 Mya First multi-cellular animals

Further reading:

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635 Mya
First multi-cellular animals