The rise and fall of the world's greatest marine biodiversity hotspot.

Habitat condition and area shape global species distributions, with shallow-water reefs hosting a disproportional share of marine biodiversity. Although reef area is a well-established predictor of marine species richness, its historical context is less well understood. Here, we show that the rise of tropical marine biodiversity is closely tied to reef expansion in space and time. During the Early/Mid Miocene (23 to 11.6 million years ago), Indo-Pacific reefs reached unprecedented size and thickness, surpassing any reef systems in the past 66 million years. These massive reefs, likely driven by unique environmental, biotic, and tectonic conditions, fostered the expanding diversity and functional evolution of marine fish and coral assemblages. Our findings underscore the importance of historical reef contexts and the implications of ongoing reef losses for tropical marine biodiversity.