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#silurian period #geologic time scale #fossils #paleontology #ancient sea life #paleozoic era

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#nautilus #ammonite #fossil #ancient sea life #spiral #prehistoric #paleontology #living fossil

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#cambrian explosion #rapid diversification #ancient sea life #marine life explosion #burgess shale

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#ammonite fossil #fossil shell #spiral fossil #invertebrate fossil #marine remains #ancient sea life