Extreme greenhouse warmth triggered the two major floristic turnover events of the past 120 million years.

Over the past 120 million years, flowering plants (angiosperms) became ecologically dominant in most terrestrial biomes, displacing other land plants. However, this spectacular transition was delayed until the early Cenozoic, tens of millions of years after major angiosperm groups emerged. One suggested trigger for the transition is the extreme warmth of early Cenozoic climates, but there are perceptions that important steps in angiosperm evolution also occurred under cool climates. To evaluate how angiosperms and other land plants responded to past climates, we implemented sampling-standardized analyses of compilations of spore-pollen fossil records from high and low palaeolatitudes. Here we show that floristic turnover (elevated rates of first and last appearances) of all land plant groups was concentrated within brief mid-Cretaceous and early Cenozoic episodes of extreme global warmth, synchronized across high and low latitudes. Our results suggest a general pattern of climate-plant dynamics through geological time, in which land plant diversity is ultimately controlled by global climate state.