The base of the food chain on land is plants. They are responsible for taking sunlight, carbon dioxide, and water and combining them to produce glucose and oxygen (in other words, the reverse of the aerobic respiration equation).
Plants at the dawn of the age of dinosaurs were very different from those of the modern world: not so much of what was there as what wasn't. There was no grass, no grain, no fruit, no flowers. But by the end of the Mesozoic, these were present.
Primitive plants are spore plants. Spore plants are a paraphyletic grade: some are more closely related to seed plants than are others. Spore plants first colonized land in the early Paleozoic Era.
They reproduce by releasing spores, which settle onto moist surfaces, grow into plants that produce sex cells which meet in the thin film of water, and join to produce a new spore-producing plant. (Note that this is somewhat analogous to amphibian-grade tetrapods: plants that live their life on land, but need to put their sex cells in water to reproduce.)
Various sorts of spore plants exist:
All of these were present in the Mesozoic. For most of the Mesozoic the dominant ground cover was ferns, and tree ferns were fairly important trees in the Triassic and Jurassic.
Spore plants lack true wood (tree ferns cheat by having many stalks growing right next to each other for support), nor do they have complex root systems.
Those traits, however, ARE present in the seed plants. Seed plants first appear in the mid-Paleozoic Era, and become the dominant land plants in the Permian Period of the Paleozoic Era.
They reproduce by relasing male sex cells (carried in pollen) which land on female sex organs, join with female sex cells, produce a fertilized seed, which can then be released from the plant to land in the soil and germinate on its own. (This is analogous to the amniotic egg in tetrapods, allowing plants to colonize further inland into drier regions.)
All non-flowering seed plants are traditionally grouped into a paraphyletic grade, the "gymnosperms": some gymnosperms are more closely related to flowering plants than to other gymnosperms. Various sorts of gymnosperms exist:
As today, gymnosperms were an important group of land plants in the Mesozoic. In fact, they were even more common then! Ginkgoes, dawn redwoods, cycads, and bennettitaleans (all rare or extinct today) were major parts of the flora, and major sorts of dinosaur food.
Some gymnosperms (conifers, cycads) wrap their seeds in a fleshy coating, and some (bennitataleans, cycads) had specialized structures around their female sex organs to attract insects. But only in the next group do we get true fruit and true flowers.
Flowering plants, called the angiosperms or anthophytes are the major clade of modern plants. With rare exceptions, if you have eaten a plant, it was an angiosperm. Angiosperms are a clade within the seed plants. Their mode of reproduction is to develop a specialized set of both male and female sex organs within a ; pollinators are lured to the flower, pick up pollen, have pollen rub off on the flower of another plant, where they fertilize the female sex cells, and a seed is made. That seed is covered by a coating of fleshy or nutty tissue: the fruit.
The basic angiosperm life cycle hinges on co-evolution with animals:
If angiosperms evolved flowers and fruit in the Cretaceous, who were their target audiences?
So thank the insects for flowers, and thank the dinosaurs for fruit.
The rise of the angiosperms occurs about the same time that low-browsing herbivorous dinosaurs (ankylosaurs, iguanodontians, rebbachisaurids) become dominant over medium (stegosaur) and high (typical sauropod) browsers. Are these changes linked? Although angiosperms were present in the Early Cretaceous, they seem to have been relatively rare then, and unlikely to have been a major food source for these groups at first. But it may be that increase in low-browsing forms favored the spread of herb-sized angiosperms.
By the Late Cretaceous many modern clades of angiosperms were present (mangolias, rose-relatives, maples, etc.). Also during this time the first grass appears. Grasses include not only the stuff that grows in lawns and meadows, but all the grain-producing plants (wheat, barley, etc.), as well as bamboo. Their flowers are extremely small, and they are often wind-pollinated rather than by the help of insects.
Grasses grow from the base of the leaf rather than the tip. They often have little bits of silica in them to persuade herbivores not to eat them. Recent discoveries in both Laurasia and Gondwana: they latter were found in titanosaur sauropod coprolites! So at least some Cretaceous dinosaurs were grass eaters. However, grasses seem to have been relatively rare in the Mesozoic, and did not form grasslands until much later. Ground cover in the later Mesozoic was a mixture of ferns and herbaceous angiosperms. So as far as we know, no dinosaur other than birds ever wandered in prairies or savannahs: these appear much later, long after the end of the Mesozoic.
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