One of the most significant marine keystone species made its debut in the Triassic seas: stony corals, the supreme builders of reefs. They did not start that way. The earliest stony corals were solitary and not as architecturally inclined.
These polyps did not radiate until the Jurassic, becoming the dominant reef-builders in all the world’s oceans from then. By this time these corals had become industriously colonial. The spur to their success appears to have been symbiotically teaming up with algae.
Stony coral were not the only contributors to oceanic reefs. Calcified seaweeds and sponges also did their part.