Are a group of mollusks that became more abundant and important parts of reefs.

Bivalvia

Explanation:

Bivalvia is the group of mollusks that is one of the important parts of coral reefs. The mollusks belonging Bivalvia flourished during the Mesozoic era. Rudistids are the bivalve mollusks that served as a major part of coral reefs and gave rise to Rudistid reefs. After the mass extinction that occurred in the Mesozoic era, bivalves again served as one of the dominant reef-forming organisms

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