Ordovician trilobites were successful at exploiting new environments, notably reefs. The Ordovician mass extinction did not leave the trilobites unscathed; some distinctive and previously successful forms such as the Telephinidae and Agnostida became extinct. The Ordovician marks the last great diversification period amongst the trilobites: very few entirely new patterns of organisation arose post-Ordovician. Later evolution in trilobites was largely a matter of variations upon the Ordovician themes. By the Ordovician mass extinction, vigorous trilobite radiation has stopped and gradual decline is foreshadowed.