 Widespread reef building started in this period and continued into the Devonian period when rising water temperatures are thought to have bleached out the coral by killing their photo symbionts.The Llandoverian epoch ended with the Ireviken event which killed off 50% of trilobite species, and 80% of the global conodont species.The end of the Ordovician–Silurian extinction event occurred when melting glaciers caused the sea level to rise and eventually stabilize.