Evolutionary changes in herbivore abundance, diversity, and ability to excavate calcareous substrata occurred in three major herbivore groups. Herbivory today is more intense than at any time. Today, calcareous red algal crusts (i.e., corallines) are a conspicuous element of shallow marine communities. Increased grazing intensity may have given the corallines an advantage over the herbivore-susceptible solenopores. Four anatomical characters unique to corallines may have aided their rapid expansion into shallow reef environments and allowed them to withstand the more intense levels of grazing that evolved. Today corallines are the only algal form to thrive under intensive herbivory. The extinction of solenopores and the adaptive radiation of corallines parallel a gradient of escalating herbivory over time. Herbivores capable of excavating calcareous substrate are capable of preventing stands of large leathery macrophytes. Large-scale increases in herbivory may have caused reduction of algal biomass in benthic communities. Algal forests, especially in the Paleozoic and early Mesozoic, may be a new variable in reconstructing paleoenvironments.--Modified journal abstract.