Pteraspidomorpha
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Early Ordovician
Arandaspida
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Highland Boundary Fault
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Tremadocian
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Orthocerida
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Boothia Peninsula
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Xiphosura
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Horseshoe crab
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Brittle star
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McMurdo Dry Valleys
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Chiton
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Cape Breton Highlands National Park
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Bivalvia
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Coral reef
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Limestone
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Geologic time scale
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Bryozoa
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Algae
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Lower Ordovician
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