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Silurian period
The Silurian period was the time when reefs got their act together, grew really big and created a completely new type of ecosystem for marine life. Silurian reefs weren't built by the same types of coral around now, but by a host of tabulate and rugose corals, crinoids and sponges. As the Ordovician ice ages ended, sea levels rose, making the Silurian a period of extensive seas. Bony fish made their first appearance.