Crustose coralline algae (CCA) play a key role in the consolidation of many modern tropical coral reefs but how about the past? The authors found a strong correlation between the presence of CCA and the formation of true coral reefs throughout the last 150 million years. Repeated breakdowns in the potential capacity of CCA spurred reef development and were associated with sea level, ocean temperature, CO2 concentration, CCA species diversity, and/or the evolution of major herbivore groups.