Environmental considerations for angiosperm origination

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Mono/ Polyphyletic
origins

Terrestrial/ Aquatic
origins

Euanthial/
palaeoherb?

Purported Triassic
angiosperms

Mid Cretaceous
radiation

Environmental
considerations

Coevolution with
insects

Coevolution with
dinosaurs

Why did the angiosperms diversify so rapidly from the mid-Cretaceous onwards? Were there some significant geological events then that might have given the angiosperms the upper hand over the gymnosperms? There certainly were dramatic changes in continental configuration by plate tectonic movements at that time.

There was rapid plate spreading 124-83 million years ago when the supercontinent Gondwana divided into Africa and South America. Oceanic crust production increased by fifty to a hundred percent. The event was termed a ‘superplumeepisode’ where there was extensive heat and material upwelling from the core-mantle boundary. This had great repercussions on both atmospheric composition and sea levels. Increased volcanic activity as a result of the upwelling considerably increased greenhouse gases, especially CO2, the levels being 4-5 times those today. The greenhouse effect raised temperatures by as much as 7.7°C, the deep ocean water warming by 15°C. As a result, the polar ice melted, and sea level rose 100m. Water covered much of the continents, both as a result of the polar ice melting and the upwelling. Coral reefs developed closer to the poles than today by 1500 kilometres.

5globes.gif It is possible that these changes brought about modifications in the plant communities, allowing the angiosperms to flourish. Angiosperms are adapted to survive drought, in ways that gymnosperms could not. The innovations include tough leathery leaves reduced in size which would reduce water loss, and vessel member cells in the stems more efficient at conducting water, thus reducing desiccation. The second tough resistant seed coat protecting the embryo would stop them from drying out too quickly, which combined with their rapid life cycle would permit them to become established and reproduce rapidly .