When do we see the major animal groups first appear in the fossil record?

Recent discoveries show that the first animals
appear in the fossil record
at about 580 million years ago – a strange looking group of
animals called the Ediacarans. But then, from 540 through to 530 million years
ago, there’s a period of remarkable
evolutionary diversification when most of the animal body plans that
we see at the modern day first appear. That includes the five arms of an
echinoderm, such as a starfish, or the forerunner of the stiffening rod that runs
down the body of a vertebrate and becomes
the spine in later vertebrates. Scientist have looked for a single cause
for this major evolutionary event, but it’s much more likely that it’s the complex
interplay of a series of geological, chemical
and biological processes that was perhaps triggered by a major sea level
rise at the base of the Cambrian period.

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