These forests had a dramatic effect on Earth's atmospheric chemistry, resulting in a large increase in oxygen and a drastic reduction in carbon dioxide. The consequent reduction in greenhouse warming caused the climate to change to cooler, drier conditions in the Permian (286 to 248 million years ago), and fostered the rise of the seed plants known as
gymnosperms. The gymnosperms continued to dominate through the Mesozoic era (248 to 65 million years ago), providingsustenance for giant, herbivorous dinosaurs. Although flowering plants, known as angiosperms, were present by the Cretaceous period (144 to 65 million years ago) and were quite diverse late in this time frame, they shared dominance with gymnosperms until the famously destructive Cretaceous/Tertiary comet or asteroid impact about 65 million years ago. As a result of this event, many previously successful plant groups (as well as dinosaurs and other animals) became extinct. This created new opportunities for flowering plants, mammals, and birds, which consequently became very diverse.
Much of what we know about the origin and evolutionary diversification of plants comes from molecular systematics, the comparative study of DNA extracted from modern plants.
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