Horsetails are one of the oldest groups of plants seen living today. Some of the earliest close relatives were alive 300 million years ago, forming dense thickets in swamp forests of giant clubmosses, the partially decayed remains of both forming the Carboniferous Coal Measures. As a major phylum of plants, the horsetails, Equisetopsida, are equivalent in status to the ferns or the flowering plants, but today the variation is small and only some 15 species are recognised, the majority in the Northern Hemisphere. Although crossing the equator in all three continents, they do not reach Australia or New Zealand.