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The continent underwater
In the Ordovician, around 450 million years ago, North America sat on the equator and was largely covered by a shallow, warm inland sea. The Cincinnati Arch yields some of the world's best-studied Ordovician faunas, and its beds have been a reference point for evolutionary palaeoecology since the nineteenth century. The period ends in the Ordovician mass extinction, the second largest in Earth's history, tied to a short but severe glaciation while Gondwana sat over the south pole.
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