Presently the horsetail lineage, whose diversity peaked in the Carboniferous, has only a single genus, Equisetum. A newly described fossil (American Journal of Botany) indicates that the genus appeared much earlier than previously thought. The anatomical detail of these fossils is quite amazing (stem cross section) and clearly shows that this late Jurassic (ca. 150 million years ago) horsetail has the characteristics of Equisetum. While that's a really long time for a genus to be around, the champion is still the clubmoss Selaginella, a genus that appeared in the Carboniferous which would make it about twice as old as Equisetum. And that's rock hard evidence.