Modularity explains large-scale reef booms in Earth’s history

Drivers of reef decline are well known both today and in the geological past. Considerably less is known about the preconditions for a pantropical expansion of coral reefs. The geological record of reef building is characterised by considerably long intervals with very limited reef expansion and geologically brief (< 20 million years) episodes of prolific, pantropical reef growth. Here, we propose a new "co-occurrence hypothesis" (COH), which posits that reefs thrive when fast-growing hypercalcifiers co-occur with encrusting organisms such as calcifying microbes or coralline algae to construct wave-resistant structures. While there is little evidence of the effect of abiotic drivers on reef proliferation, we find that positive co-occurrence patterns are significantly more common in reefal as compared to non-reefal communities, suggesting that biological interactions are more relevant in reefs than in non-reefs. Supporting COH, we also show that reefs after the end-Permian mass extinction became more modular in nature with limited membership in reef assemblages during reef booms than in typical periods of reef growth (background intervals). Modularity in reefs may have led to the stabilisation of reef ecosystems, giving them the ability to recover from small perturbations, promoting reefal carbonate accretion and prolific reef growth.