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The Horsetail family (Equisetaceae) is a remarkable group of plants, now restricted to about 25 species, found in most parts of the world. They were formerly of greater importance: in Carboniferous times they were at their height and formed an important part of the vegetation. Many fossils of related species are known, with very diverse forms, and many grew into trees. These included Calamites and Sphenophylls (a completely extinct group) which were in many respects (i.e. cone and leaf structure) more complex and highly organized than their modest descendants.