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Carboniferous period a geological period that began about 370 million years ago and ended 280 million years ago. It is often divided at about 325 million BP into Lower and Upper Carboniferous; the coal measures from which the period derives its name occurred mainly in the latter period. Club mosses, horsetails and ferns were the dominant plants during the period, and amphibians the commonest vertebrates, though this was also the time of the emergence of the reptiles. Britain crossed the equator.