Grasses belong to the angiosperms, also known as the flowering plants. Angiosperms first appeared during the late Triassic Period (230 to 195 million years ago), and became common during the Cretaceous Period (140 to 65 million years ago). The rise of angiosperms during the Mesozoic Era (230 to 65 million years ago) may have influenced the feeding strategies of some dinosaurs, this may be the cause of the radiation of ornithiscian herbivores in the Late Cretaceous Period. More information about the relationship of dinosaurs and plants.
Grasses did not develop until the late Paleocene Epoch (65 to 55 million years ago), and were not well-developed until the Oligocene Epoch (36 to 22.5 million years ago). This would have been much too late for them to be present at the dinosaur nesting ground in "Dinosaur." Grasses were rare until the Oligocene Epoch, when the cooler and drier global climate allowed grasses to spread into the open spaces not occupied by temperate forests as the tropical forests retreated to the equatorial regions. Grasses really did not become common until the Late Miocene Epoch (22.5 to 5 million years ago); true grasslands (meaning savannas and prairies, rather than grassy patches in forests) would have appeared at this time.
The major radiation of angiosperms during the late Cretaceous Period and early Tertiary Period (65 to 5 million years ago), may have been in response to the Cretaceous-Tertiary Extinction Event. The K-T Extinction allowed mammals to diversify and to become the main large terrestrial vertebrate animals. The extinction of the dinosaurs, whether it was sudden or gradual, resulted in the loss of large herbivores that were not replaced by large mammals until the end of the Eocene Epoch (55 to 36 million years ago). The development of grasslands may have been a result of the presence of grazing mammals.
Feeding on grasses requires certain adaptations in herbivorous mammals. First of all, grasses are relatively poor in easily digestible nutrients when compared to meat or other plant products (such as fruits and roots). In order to gather enough calories to live, grazing animals must either eat huge quantities of grass and not digest them thoroughly, or to develop a better way of digesting the fibrous material. Most herbivorous mammals have developed methods for using microorganisms in their stomachs to digest the fiber more thoroughly. They have enlarged and expanded their stomachs, and are called ruminant animals (named after the rumen part of the expanded stomach). The rumen and the microcorganisms inside allow grasses to be processed more thoroughly.
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