Nature, Published online: 05 March 2025; doi:10.1038/d41586-025-00650-xNitrogen-isotope analysis provides geochemical evidence that, similar to modern corals, some extinct coral groups had a symbiotic association with photosynthetic microorganisms. This photosymbiosis might have contributed to the high productivity of ancient reefs in low-nutrient environments and puts long-term ecological research on ancient reefs into a fresh perspective.