When and Why Nature Gained Angiosperms

Abstract

Flowering plants, the angiosperms, are the most diverse group of plants on our planet. Today, they dominate most vegetation types, but their origin continues to remain a mystery. However, we continue to gain knowledge about their early evolution and history. It seems increasingly probable that their origin is associated with climatic and environmental changes in tropical areas and was coeval with the breakup of the supercontinent Gondwana. The first angiosperms appeared in the fossil record about 135 million years ago based on the occurrence of their rare pollen grains in fossil assemblages of North Gondwana and southwest Europe. Their evolution may be associated with climate perturbation and an overall change in wet- land to mesophytic habitats, as this group is adapted to tolerate a seasonally dry climate. Soon after the first early angiosperms in the late Valanginian, higher angiosperms, the eudicots, are part of the fossil record of Africa. These initial flowering plants had small inconspicuous flowers and small fruits, and were most probably of small growth stature, likely herbs and shrubs. After angiosperms colo- nized mineral soils across the landscape, they expanded their habitats to aquatic environments and evolved strate- gies for their rapid dispersal in these settings. By the mid- Cretaceous (90–100 Ma), angiosperms conquered higher latitudes in both hemispheres and expanded into various tropical to warm temperate (= subtropical) environments. Chloranthoids, laurels, and plane trees experienced their heyday. In the Late Cretaceous, core∗ and higher eudicots evolved rapidly, and nearly all extant angiosperm families appeared by the end of the Cretaceous. Angiosperm clades developed a physiology capable of overcoming drought conditions by the Cenomanian. However, their expansion and colonization of mesophytic upland habitats only took place in the Late Cretaceous. Seasonally dry habitats, such as savannas, were inhabited by angiosperms in the Late Cretaceous, with the first evidence of graminoids.