EDIACARAN  BIOTA

One of 'Darwin's Dilemmas' was the apparent absence of any complex life prior to the Cambrian Explosion. How could complex life suddenly appear in the fossil record? Since the discovery of Charnia many pre-Cambrian fossils have been found around the world and although specific lineages are yet to be agreed upon the Ediacaran biota is clearly an earlier more primitive form of life.

A Charnia fossil was discovered in 1957 in the English Charnwood Forest within rock that was unambiguously laid down before the Cambrian period. It was the first acknowledged pre-Cambrian lifeform and it reinvigorated pre-Cambrian research leading to the recognition of the Ediacaran Period and its unique biota.

Charnia was a sessile organism that existed in the deep ocean away from sunlight which means that it couldn't have been a form of photosynthesizing plant.

It had a fractal anatomy, built up of simple reoccurring patterns of hydrostatic cellular units that were not symmetrical but offset along the bilateral line. This anatomical structure is known as glide reflection and predates the bilateral symmetry that is common to all modern creatures.

It is not known how Charnia acquired food from the environment but as it had no gut it is thought to have absorbed nutrients directly through the cell wall.

Funisia may appear to belong to the plant kingdom but it is considered an entirely different extinct lifeform that didn't require photosynthesis.

It had a simple concatenated anatomy consisting of a simple chain of hydrostatic cellular units and it grew up to 300mm in height.

Although sexual reproduction is thought to have evolved at an earlier stage in the history of life on earth, Funisia represents the first evidence of this. Its fossil remains are found as clustered groups of individuals of the same age. This is strikingly similar to the dispersal patterns of modern day corals that release drifts of reproductive cells (gametes) into the surrounding ocean. It is difficult to account for this form of dispersal other than by sexual reproduction.

Dickinsonia is one of the most distinctive creatures of this period with its flattened, bilaterally segmented, disc shaped body.

It was first recorded in 1947 by Reg Sprigg in the Flinders Ranges of South Australia but has since been found around the world and varied greatly in size from a few millimetres up to one metre in length.

Fossil remains suggest that Dickinsonia could move around the ocean floor to feed on microbial mats. As no digestive tracts have been clearly identified it is thought that nutrients were absorbed directly through body contact.

It has been speculated that the extinction of Dickinsonia and other Ediacaran biota was brought about by the evolution of burrowing creatures that effectively killed off the microbial mats that the Ediacaran fauna fed upon. However the reasons for the large scale Ediacaran extinction are still being debated.

Spriggina is of great interest to palaeontologists as it seems to display a number of key evolutionary traits. These include bilateral symmetry, a 'head' and 'tail', self-locomotion and even the possible suggestion of a mouth and light sensitive buds on its horseshoe shaped head.

Some analysis suggests that Spriggina may be related to arthropods such as trilobites but the fossils of this period are so faint that it is hard to draw any conclusive observations.

An ongoing problem for palaeontology is to work out how the differing lifeforms of the Ediacaran and Cambrian periods are related.

Kimberella had a pronounced shell-like exterior however it would have been non-mineralized and flexible.

Feeding marks have been found in association with Kimberella fossils that suggest it used a proboscis type appendage, similar to a modern snail's radula to gather and eat nutrients. This in turn suggests the development of a rudimentary gut system.

It is thought to have moved around on a muscular 'foot' in the same way as a modern snail. Although much about Kimberella is suggestive of the mollusc phylum, this association is still being debated.

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