While relatively few angiosperms were established at the beginning of the Cretaceous, by the Late Cretaceous flowering plants accounted for possibly half of the plant diversity. With their amazingly rapid growth rates and relatively short reproduction periods, these plants were predestined for success. Encoded in their genetic makeup was the ability to radiate into a variety of habitats, from bogs and marshes to stone crannies, mountaintops, and tree branches.
In his book on dinosaurs,52 Robert Bakker felt that plant-eating dinosaurs could have "invented" flowering plants. He concluded that in contrast to the Late Jurassic browsers that fed on foliage in the canopy and subcanopy layers, Cretaceous dinosaurs were predominately grazers that indiscriminately clipped the flora to near-ground levels. Angiosperms, which grow and reproduce quickly, recovered from this clear-cutting faster than gymnosperms, thus giving them a competitive advantage that eventually led to their dominance. However, this theory has met with criticism69 and Paul Barrett and Katherine Willis rejected Bakker's theory on the grounds that angiosperms were "neither sufficiently abundant nor widespread to have been a major component of dinosaur diets during the Early Cretaceous." It is also unlikely that most Cretaceous dinosaurs grazed the vegetation to the ground, and seedlings of both angiosperms and gym-nosperms would have survived. Also, Bakker compared widespread dinosaur herbivory to that of mammals grazing on grass lands, and there is no evidence that such habitats occurred at any time in the Cretaceous.
Early Cretaceous herbivorous insects definitely played a significant role in angiosperm evolution through their feeding on archaic gymnosperms. Occurring in all strata, insect damage would have been found from the roots to the tips of the leaves. By stripping the spreading cycad, conifer, and fern foliage, devouring the stands of horsetails and carpets of club moss, and avoiding the angiosperms, they weeded out the competition, and their selective feeding worked in favor of the flowering plants. So if herbivory in the Cretaceous facilitated the diversification of flowering plants, the insects could certainly take most of the credit.
When insects with appetites for angiosperms appeared on the scene around the mid-Cretaceous, they defoliated the limbs, bored into stems, and fed on flowers and fruits. Subterranean creatures such as termites, beetle grubs, fly maggots, caterpillars, and aphids ate away on the roots and underground stems.65 By selective feeding on specific genera and species of angiosperms, Cretaceous insects were determining which lineages of flowering plants would be here today.
Another activity, however, put insects far ahead of dinosaurs as the driving force behind angiosperm evolution, and it had nothing to do with herbivory but everything to do with plant reproduction. Insects became the prime transporters of genetic material from one flower to another, replacing the chance encounters of wind pollination with the personal transportation of gametes.
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