During the Ordovician, there were no landmasses in the northern hemisphere, which was entirely covered by the global ocean of Panthalassa. The southern continents were fused into the supercontinent Gondwanaland, which started the period abutting the equator, but drifted steadily southward. To the west of Gondwana, between the equator and the Tropic of Capricorn, were strung the island continents of Siberia, Baltica and Laurentia. The smaller continent of Avalonia calved from Gondwana and drifted north towards Laurentia and Baltica, where the three landmasses encircled the Lapetus Sea. In the early Ordovician, atmospheric carbon dioxide was around ten times present levels, causing an extreme greenhouse effect, but the drift of Gondwana towards the southern pole caused glacier formation which ultimately reversed the effect and produced cooling. The Ordovician was bracketed by major extinction events but, in between, life flourished with brachiopods, crinoids and cephalopods coexisting with the first fish.
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