Decline in Mesozoic reef-building sponges explained by silicon limitation

Several unrelated clades of siliceous sponges proliferated on the shelves of the Jurassic Tethys Sea, becoming prominent builders in reefs and near-shore mounds1,2,3,4. Many of these builders are characterized by massive, rock-like skeletons made of spicules with a characteristic terminal hypersilicification4,5. Such hypertrophied spicules are generically known as desmas, irrespective of their phylogenetic origin5. Desma-bearing sponges virtually disappeared from reefs and other neritic environments during the Cretaceous and the Early Tertiary1,2,4,6, but have subsisted in relict populations in deeper, bathyal waters5,7,8. The causes of the decline and bathymetric shift of these sponges remain obscure. Here we show experimentally that the concentration of silicic acid in seawater modulates the phenotypic expression of the various spicule types genetically available in a sponge species. We also show that the concentration of this nutrient in Recent surface waters is insufficient for this species to secrete its desmas. These findings indicate that silicon limitation, probably aggravated in shallow waters by the diatom burst around the Cretaceous–Tertiary boundary9,10, may have forced neritic sponges with desmas to either lighten their skeletons or move to deeper, silicon-rich environments.