The second-oldest period in the Paleozoic Era. The Ordovician is remarkable because not only did one of the most significant Phanerozoic radiations of marine life take place (early Middle Ordovician), but also one of the two or three most severe extinctions of marine life occurred (Late Ordovician). The early Middle Ordovician radiation of life included the initial colonization of land. These first terrestrial organisms were nonvascular plants. Vascular plants appeared in terrestrial settings shortly afterward.