Greenhouse Heat Waves Rewrote the World’s Flora Twice in 120 Million Years

Flowering plants rule the land today, blanketing nearly every terrestrial biome from tropical rainforest to temperate woodland, yet the story of how they seized that dominance has long been one of evolutionary biology’s most stubborn puzzles. Major angiosperm groups had already emerged tens of millions of years before they began to displace ferns, conifers and other older lineages, and the reasons for that long delay have been fiercely debated. Now a new analysis of the fossil pollen and spore record, published in Nature Plants by J. M. Kale Sniderman of the University of Melbourne, Gregory J. Jordan of the University of Tasmania and Chris Mays of the Natural History Museum Vienna and University College Cork, argues that the answer lies not in the plants alone but in the temperature of the planet itself. The team’s conclusion is striking: the two great pulses of floristic turnover in the past 120 million years both coincided with brief episodes of extreme global greenhouse warmth.

The researchers tackled the problem with an unusually rigorous statistical approach. Rather than counting fossils at face value, which risks mistaking gaps in sampling for genuine biological change, they compiled extensive spore-pollen datasets from two regions with exceptionally well-studied records: southeastern Australia, which sat at high southern palaeolatitudes through the Cretaceous and Cenozoic, and northeastern Africa, which lay in the low-latitude tropics. They then applied sampling standardization, including shareholder quorum subsampling, a technique that equalizes the completeness of samples across time bins so that diversity and turnover estimates reflect biological reality rather than how much rock has been drilled or how intensively a basin has been explored. Per-capita origination and extinction rates were calculated for three broad groups of land plants: angiosperms, gymnosperms and free-sporing plants such as ferns and their relatives.

The signal that emerged was remarkably consistent across both hemispheres and both latitudinal belts. Elevated rates of first and last appearances, the statistical fingerprint of floristic turnover, were not spread evenly through the last 120 million years. Instead, they were concentrated within two short windows of extreme global warmth: one in the mid-Cretaceous, when sea surface temperatures in the south of the ancient Australian region climbed to extraordinary levels, and one in the early Cenozoic, spanning the Paleocene-Eocene Thermal Maximum and the early Eocene Climatic Optimum. Crucially, the turnover was synchronized between high and low latitudes, suggesting a global driver rather than a regional one.

The mid-Cretaceous episode is particularly revealing. During the Turonian stage, roughly 90 million years ago, the Earth experienced what palaeoclimatologists call the Cretaceous Hot Greenhouse, with ocean temperatures far above modern values. The new analysis shows that southeastern Australian land plants recorded a peak of originations at exactly this time, followed by a dip during the cooler Coniacian, and then renewed warming and renewed origination between about 84 and 82 million years ago. The team compared their plant diversification curves with sea surface temperature reconstructions derived from oxygen isotope ratios in planktonic foraminifera, using a Bayesian calibration approach applied to deep-sea cores from the Falkland Plateau. The temperature and diversification peaks lined up closely, a pairing that is difficult to dismiss as coincidence.

The early Cenozoic pulse tells a parallel story. After the end-Cretaceous extinction removed much of the standing flora, the extreme warmth of the Paleocene-Eocene Thermal Maximum around 56 million years ago and the subsequent early Eocene Climatic Optimum coincided with another surge of first appearances across all land plant groups. This is the interval during which angiosperms finally consolidated their ecological dominance, and the new data suggest that the transition was not a slow, steady march but a burst tied to climate state. The finding challenges a persistent perception in the field that some of the most important steps in angiosperm evolution occurred under cool climates; the fossil record, as analyzed here, points instead to heat as the accelerant.

Technically, the study leaned on a battery of quantitative tools. Diversity was estimated using coverage-based extrapolation metrics, turnover was quantified with second-for-third per-capita rates standardized by sampling, and the relationships between diversification and potential drivers were tested with generalized linear models. In the northeastern African dataset, where the number of time bins was small, the authors deliberately restricted themselves to single-predictor models rather than risk overfitting with multi-predictor model selection, a methodological caution that speaks to the care taken throughout. They also tested whether the diversification of one plant group could be predicted by the richness of another, probing the long-standing hypothesis that angiosperms simply outcompeted older lineages. The climate variables consistently outperformed biotic ones as predictors of change.

The broader implication is a general pattern of climate-plant dynamics through geological time: land plant diversity, at least as recorded in these spore-pollen archives, is ultimately controlled by the global climate state rather than by internal biological dynamics alone. That framing echoes and extends earlier work. Previous studies had linked the rise of angiosperms to the decline of conifers during global cooling, documented the transformation of Neotropical rainforests after the end-Cretaceous extinction, and shown that insect herbivory and plant diversity tracked paleotemperature through millions of years in the Eocene of North America. What the new analysis adds is a unified, sampling-standardized demonstration that the biggest floristic upheavals of the entire interval were pulsed, synchronous and warm-driven, affecting gymnosperms and free-sporing plants just as strongly as the rising angiosperms.

Why would extreme warmth trigger turnover rather than stability? The authors’ data do not settle the mechanism, but several possibilities fit the evidence. Greenhouse climates bring not only higher temperatures but intensified hydrological cycles, with wetter subtropics and altered rainfall regimes that can open new ecological space. Physiological work has shown that early angiosperms possessed leaf hydraulic systems capable of a surge in photosynthetic capacity under warm, moist conditions, giving them a competitive edge precisely when the climate swung hot. Warm episodes may also have loosened the grip of established vegetation, allowing lineages to disperse, establish and diversify across continents. Whatever the precise chain of cause and effect, the synchrony of the signal from polar latitudes to the tropics implies that the whole terrestrial biosphere was responding to the same planetary thermostat.

The study also carries an uncomfortable contemporary resonance. The warm episodes that drove past floral revolutions unfolded over hundreds of thousands to millions of years, yet they reshaped the composition of forests and the evolutionary trajectories of entire lineages. Today’s greenhouse warming is proceeding orders of magnitude faster, and the palaeobotanical record suggests that vegetation does not simply track temperature passively; it reorganizes, with waves of extinctions and originations that can permanently rewrite which plants dominate the landscape. Understanding the dynamics of those ancient turnovers, and the thresholds at which gradual change tips into rapid reorganization, may help scientists anticipate how modern floras will respond to the climate of the coming centuries.

For now, the paper stands as a reframing of one of palaeontology’s grand narratives. The triumph of the flowering plants, long told as a tale of inherent evolutionary superiority unfolding over deep time, appears instead to have been gated by the thermal state of the Earth, with two brief furnace episodes acting as the hinges on which the modern flora swung into place. The datasets and R code behind the analysis are publicly available, allowing other researchers to test the pattern in additional regions and with independent climate reconstructions. If the correlation holds up, textbooks may need a new chapter heading for the Cretaceous and early Cenozoic: not merely the age of angiosperms, but the age of angiosperms made possible by greenhouse heat.

Subject of Research: Climate-driven floristic turnover and the rise of flowering plants over the past 120 million years

Article Title: Extreme greenhouse warmth triggered the two major floristic turnover events of the past 120 million years

Article References: Extreme greenhouse warmth triggered the two major floristic turnover events of the past 120 million years. (n.d.). https://doi.org/10.1038/s41477-026-02429-y

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02429-y

Keywords: angiosperms, floristic turnover, greenhouse climate, Cretaceous, early Cenozoic, palynology, paleoclimate, plant evolution, diversification, Paleocene-Eocene Thermal Maximum, fossil record, biogeography