The flowering plants that dominate modern vegetation possess leaf gas exchange potentials that far exceed those of all other living or extinct plants. The great divide in maximal ability to exchange CO 2 for water between leaves of nonangiosperms and angiosperms forms the mechanistic foundation for speculation about how angiosperms drove sweeping ecological and biogeochemical change during the Cretaceous. However, there is no empirical evidence that angiosperms evolved highly photosynthetically active leaves during the Cretaceous. Using vein density (D V ) measurements of fossil angiosperm leaves, we show that the leaf hydraulic capacities of angiosperms escalated severalfold during the Cretaceous. During the first 30 million years of angiosperm leaf evolution, angiosperm leaves exhibited uniformly low vein D V that overlapped the D V range of dominant Early Cretaceous ferns and gymnosperms. Fossil angiosperm vein densities reveal a subsequent biphasic increase in D V . During the first mid-Cretaceous surge, angiosperm D V first surpassed the upper bound of D V limits for nonangiosperms. However, the upper limits of D V typical of modern megathermal rainforest trees first appear during a second wave of increased D V during the Cretaceous-Tertiary transition. Thus, our findings provide fossil evidence for the hypothesis that significant ecosystem change brought about by angiosperms lagged behind the Early Cretaceous taxonomic diversification of angiosperms.angiosperm evolution | plant evolution | transpiration | tropical rainforest | venation P hotosynthesis and transpiration by leaves fundamentally influence the cycling of carbon and water in the terrestrial realm. Consequently, evolutionary changes in the rates at which leaves exchange water for carbon bear on the origin and maintenance of biodiversity by varying the size and resource stoichiometry of the primary productivity base. How leaves exchange gases also shape climate and atmospheric gas composition by changing the amounts of water vapor and carbon in the atmosphere (1-3). Recent evidence has suggested that the evolution of flowering plants involved a sharp rise in the capacity of leaves to transport water and extract CO 2 from the atmosphere (4, 5).The evolution of unrivaled CO 2 uptake and transpirational output by angiosperm leaves form the mechanistic cornerstone for a multitude of hypotheses citing angiosperms as agents of expansive ecosystem change during the Cretaceous (6-8). These hypotheses include (i) intensified mineral weathering by angiosperms that decreased global atmospheric CO 2 concentration; (ii) heightened transpirational input to the atmosphere that increased regional rainfall and favored the spread and diversity of tropical rainforest vegetation; (iii) the nearly complete competitive exclusion by angiosperms of diverse gymnosperms and ferns from high-productivity sites worldwide; and (iv) the spread of novel fire regimes that entrained a positive feedback on angiosperm takeover (3,4,6,(9)(10)(11)(12)(13)(14)(15).

The majority of environments are dominated by flowering plants today, but it is uncertain how this dominance originated. This increase in angiosperm diversity happened during the Cretaceous period (ca. 145-65 Ma) and led to replacement and often extinction of gymnosperms and ferns. We propose a scenario for the rise to dominance of the angiosperms from the Barremian (ca. 130 Ma) to the Campanian (ca. 84 Ma) based on the European megafossil plant record. These megafossil data demonstrate that angiosperms migrated into new environments in three phases: (i) Barremian (ca. 130-125 Ma) freshwater lake-related wetlands; (ii) Aptian-Albian (ca. 125-100 Ma) understory floodplains (excluding levees and back swamps); and (iii) Cenomanian-Campanian (ca. 100-84 Ma) natural levees, back swamps, and coastal swamps. This scenario allows for the measured evolution of angiosperms in time and space synthesizing changes in the physical environment with concomitant changes in the biological environment. This view of angiosperm radiation in three phases reconciles previous scenarios based on the North American record. The Cretaceous plant record that can be observed in Europe is exceptional in many ways. (i) Angiosperms are well preserved from the Barremian to the Maastrichtian (ca. 65 Ma). (ii) Deposits are well constrained and dated stratigraphically. (iii) They encompass a full range of environments. (iv) European paleobotany provides many detailed studies of Cretaceous floras for analysis. These factors make a robust dataset for the study of angiosperm evolution from the Barremian to the Campanian that can be traced through various ecosystems and related to other plant groups occupying the same niches.

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During the late Early Cretaceous (Barremian-Albian), angiosperms showed a stepwise widening of their ecological range, being recorded first during the Barremian as aquatic plant mega-remains and at the Cenomanian onwards occurred in all the environments.

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During the Mid-Cretaceous, European floras were characterized by (a) coastal gymnosperms, (b) highly diversified fluvial angiosperms and (c) the first European brackish water-related angiosperm.

Morphology and anatomy of a fossil monocotyledon from the late Early Cretaceous and extant monocots are compared. Anatomy was examined based on publications, while leaf morphology, especially the venation, required new observations on fresh and herbarium material. Spixiarum kipea gen. et sp. nov. belongs most likely to Araceae, and may be sister to Orontioideae or is even part of this tribe. Consequently, proto‐Araceae were most likely present during the Early Cretaceous in South America. The occurrence of Spixiarum in South America indicates a north Gondwana origin for Orontioideae and and thus may indicate a Gondwanan origin for proto‐Araceae. Sedimentological and taphonomic context indicate that Spixiarum had probably a helophytic ecology similar to living Orontioideae and formed possibly the aquatic vegetation of the Crato Lake in association with the Nymphaeales Pluricarpellatia peltata and Jaguariba wiersemana. Early Cretaceous monocotyledon remains have been rarely recorded. It is debatable if their scarceness is a sign of low diversity or may be due to taphonomic/ecologic reasons.