The Ordovician (485 - 444 million years ago) was charactized by the great radiation of lifeforms originated in the previous Cambrian period. Invertebrates dominated the ecosystem, with enourmous cephalopods and arthropods drifting along the oceans while Trilobites and Gastropods covered the sea floor. Life outside the water was yet not possible. Although it was mostly a warm time, the Ordovician ended in the first mass extinction caused by a glaciation.
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