Ch 12 Proterozoic Eon
Proterozoic Eon
Proterozoic Eon
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Neoproterozoic life<br />
• Thus, by the end of <strong>Proterozoic</strong> <strong>Eon</strong>, at least four<br />
major groups of animals had arisen<br />
– Segmented worms, arthropods, cnidarians, mollusks<br />
• What triggered the initial radiation of animals?<br />
– Critical build-up of atmospheric O 2 ?<br />
– Evolution of key adaptive features such as muscle cells<br />
or nerve cells?<br />
• Answer is still unknown!<br />
Earth History, <strong>Ch</strong>. <strong>12</strong> 20
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