Abominable mystery no more

Sweet Bay (Magnolia virginiana)

In his time Charles Darwin struggled to understand a fossil record which revealed an explosion of flowering plants during the Cretaceous period, around 100 million years ago.

Before the Cretaceous, the Earth’s vegetation consisted mainly of conifer and fern-type plants known as gymnosperms. These were replaced with flowering plants, or angiosperms, and in the tropics the process was relatively rapid. By the end of the Cretaceous, flowering plants had established themselves throughout the world, and gymnosperms survived only in the far north.

How this vegetative conquest of the Earth could occur baffled Darwin, who in a letter to his friend Joseph Hooker referred to it as an “abominable mystery”. The sudden transition to a diversity of fossil flowering plants between the early and late Cretaceous conflicted with Darwin’s view that the emergence of new species could only take place gradually.

According to Wageningen University ecologists Frank Berendse and Marten Scheffer, flowering plants were able to change the environment during the Cretaceous period to suit their own needs. The world at the time was dominated by gymnosperms, which kept the soil so nutrient-poor with their poorly degradable plant litter that flowering plants struggled to establish themselves. But at certain locations where gymnosperms were temporarily wiped out by floods, fires, storms and the like, angiosperms could increase in number so that they were capable of improving their conditions with their easily degradable litter.

A positive feedback mechanism was thus created in which flowering plants could increase rapidly and replace gymnosperms in much of world. This improved the edibility of flowering plants, opening the way to the further evolution of plant eaters, including mammals.

Further reading

Frank Berendse and Marten Scheffer, “The angiosperm radiation revisited, an ecological explanation for Darwin’s ‘abominable mystery'”, Ecology Letters (2009)