Global environmental change and continental glaciation induce a mass extinction which impacts all marine animals. As the Ordovician Period closes, over 50 percent of species world-wide decline and then vanish. The mass deaths open new niches for benthic (bottom-dwelling) and planktic (free-floating) marine life. New species and new groups of organisms which depend on these primary producers evolve. Researchers estimate biodiversity did not fully recover from this extinction for 25 million years.
Phytoplankton (photosynthesizing protoctists) bloom in colder resource-rich waters. They, in turn, enrich the food web. It is a period of plenty in the seas: plenty of room and plenty to eat.