Botany is the study of plant life, so paleobotany is the study of ancient plants, often species of plants that are extinct.
The earliest "plants" were the blue-green algae (or "cyanobacteria"), which first show up in the fossil record 3,200 million years ago, during the Pre-Cambrian Era. With the fungi and bacteria, the algae provided oxygen and organic nutrients for early animal life, this went on for the first 2,500 million years of evolutionary time.
The first land plants appeared during the early Devonian Period (570 million years ago). The land plants underwent a time of rapid diversification throughout the Devonian Period, Carboniferous Period, and early Permian Period. At this time, the psilophytes, club mosses, calamites, ferns, and other gymnosperms appeared. The name "gymnosperm" means "naked seed," and refers to plants that form seeds from ovules that are not enclosed in an ovary, these include the conifers (the evergreens, such as pines and cedars), cycadeoids, cycads, and pteridosperms.
During this early phase of plant evolution, several groups of plants developed gigantic forms, such as giant ferns, which had a geologically short time spread. They went extinct by the middle of the Permian Period (about 260 million years ago), leaving only smaller forms that survive today.
The gymnosperms became the dominant plants in the late Permian Period (260 million years ago), and continued to flourish throughout the Mesozoic Era (230 to 65 million years ago). Their diversity was much higher at this time than in modern times, as many species of gymnosperms went extinct during the Cretaceous Period (140 to 65 million years ago).
During the Carboniferous Period (345 to 280 million years ago), gymnosperms formed large forests. At this time, the global climate was very warm, and many areas of the earth were covered by freshwater swamps and lagoons. Dead plants would fall into the warm water, and accumulated to form thick layers of peat. During the Carboniferous, changes in the sea level led to flooding by salt water, which killed off the vegetation growing on the peat, and deposited sand and mud on top of it. When the floods subsided, vegetation would grow again, and this process was repeated several times. The weight of the vegetation, mud, and sand eventually compressed the layers of peat, and it was turned into coal. A similar process helped to turn one-celled organisms into petroleum oil.
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