Stromatoporoid skeletal innovation enabled the Silurian origin of modern reef architecture

High-relief reef margins define modern tropical carbonate systems, yet their evolutionary origin remains debated. Environmental hypotheses attribute the Silurian (ca. 427 Ma) emergence of steep reef walls to climate change but cannot explain why stromatoporoid sponges, present since the Ordovician, failed to construct comparable relief under similarly favorable conditions. Here, I show that this architectural revolution was primarily controlled by evolutionary innovation in stromatoporoid skeletal structure. By distinguishing basal from derived stromatoporoid orders, I demonstrate that derived stromatoporoid composition correlates strongly with reef thickness, while environmental factors are negligible. The transition followed the radiation of derived orders with rigid, load-bearing skeletons. This biotic innovation created the architectural template for rimmed platforms repeatedly reoccupied throughout the Phanerozoic.