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Originally Posted by AndrewJ
That's what I thought: that the spread of life to land was only possible once a protective ozone layer had been established around 500 mya.
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Where does this come from?
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Regarding the Cambrian expolsion, was it not that a tipping point of sophistication had been reached whereafter animals tended to have skeletons that could form fossils?
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yes - the widespread emergence of animals with hard parts, which are more easily fossilised ...