To create a little flower is the labour of ages. ~ English poet William Blake
280 MYA the proliferation of a tremendous variety of seed plants was well underway. The next milestone in plant evolution came with pollen and flowering plants (angiosperms) in the mid-Triassic, some 245 MYA. This was a divergence from simpler seed-bearing plants.
As always, geology and climate combined to create conditions conducive to more sophisticated plant life. Angiosperms may have got their edge by developing drought-resistant features and rapid reproduction. The warm and moist climate then helped ensure success, though their proliferation took time. Angiosperms diversified enormously during the early Cretaceous, around the same time that many new types of dinosaurs became prominent.
The adaptive advantage of animal pollination is being able to cross-fertilize using a relatively small amount of pollen compared to the copious quantities needed when relying upon the wind, as gymnosperms do. The evolutionary outgrowths were billboards advertising fine dining: flowers.
Insects drove angiosperm speciation, as plants aimed at captive audiences for their delicacies. Until their co-option insects had been a pest to plants. By the time angiosperms arose, the 6-legged creatures were well-known to flora. It was a matter of wiles to turn select insects from foe into ally.
Plants underwent rapid adaptations from interacting with both insect pollinators and predators. Insects too underwent changes to take better advantage of the herbal bounty, whatever feeding off a plant meant.
Attempted insect pollination was not always a success. Several groups of flowering plants went back to the wind, some investing instead in defense. Plants in the nettle family are exemplary. Mulberry family members gave up on pollinators, but some then gave it another go. Certain figs managed a mutualism with little wasps. (While most fig wasp species act as pollinators, some simply feed off the plant. Insects as allies is tricky.)
Though insects drove diversity, dinosaurs also made a dent. High-browsing herbivores, such as sauropods and stegosaurs, were the norm 160 MYA, dieting on a wide range of conifer tissue. This put pressure on the canopies of mature trees and egged on development of plant defenses. During this time, tender saplings went relatively ungrazed. Their relatively modest physical presence gave angiosperms an edge.
Flowering plants revolutionized terrestrial ecosystems. They have a broader range of growth forms than all other plant groups – from giant trees to tiny annual herbs – and can produce nutrient-rich tissues at a faster rate than other plants. So, when they started dominating ecosystems, they allowed for a wider variety of life modes and also for much higher ‘packing’ of species with similar ecological roles, especially in tropical forests. ~ Swedish botanist and paleobiologist Caroline Strömberg