Two end-member models are proposed to explain marine biotic responses to greenhouse conditions. Global warming and increasing sea level may: (1) promote dispersal of marine species, leading to larger geographic ranges and decreased speciation and biodiversity, or (2) form isolated epicontinental basins that host endemic radiations, leading to smaller geographic ranges and increased speciation and biodiversity. The Cenomanian–Turonian (C–T) interval, marked by greenhouse warming, sea level rise, ocean anoxia, and biotic turnover, presents an opportunity...