A million fish dating

These "modern" organisms appear relatively quickly in the geological time scale, and their abrupt appearance is often described as the "Cambrian explosion" however, bear in mind that the fossil record of the "explosion" is spread over about 30 million years.

The oldest fossil evidence of multicellular animals, or , is burrows that appear to have been made by smooth, wormlike organisms.

Such trace fossils have been found in rocks from China, Canada, and India, but they tell us little about the animals that made them apart from their basic shape.

Although many of the Ediacaran organisms have been compared to modern-day jellyfish or worms, they have also been described as resembling a mattress, with tough outer walls around fluid-filled internal cavities - rather like a sponge.

As a group, Ediacaran animals had a flat, quilted appearance and many showed radial symmetry.

This is because, in the absence of a protective ozone layer, the land was bathed in lethal levels of UV radiation.

Once photosynthesis had raised atmospheric oxygen levels high enough, the ozone layer formed, meaning that it was then possible for living things to venture onto the land.

The Ediacaran animals Between 620 and 550 million years ago (during the Vendian Period) relatively large, complex, soft-bodied multicellular animals appear in the fossil record for the first time.

While found in several localities around the world, this particular group of animals is generally known as the Ediacaran fauna, after the site in Australia where they were first discovered.

Preservation of soft-bodied organisms is rare, and in this case seems to have occurred when the animals were rapidly buried in a mudslide down into deep, anaerobic waters, where there was little bacterial decay.

Prior to the discovery of this fossil assemblage, early in the 20th century, there was no evidence of soft-bodied animals from the Cambrian (remember that this is before the Ediacaran fauna were found).

They also give insights into how evolution might have progressed relatively early in the history of multicellular animals, and in fact some authors view the Cambrian as a period of extreme "experimentation" and diversity. The cause of the proliferation of animal forms in the Cambrian is a matter of considerable debate among scientists.

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