Some ancient plants have close modern relatives. That's the case with calamites, an extinct, tree-sized plant that proliferated during the Carboniferous period some 250 to 360 million years ago. They looked like today's horsetails, but on steroids [source: Arens].
Calamite trunks consisted of hollow segments, giving them the outward appearance of bamboo. At the dividing ring between each segment, slender branches slanted upwards, branching a few more times before terminating in bunches of needlelike leaves. The whole plant stretched 33 to 66 feet (10 to 20 meters) high and was anchored by a massive rhizome, or underground stem, that allowed it to sprout clones of itself (it was the only plant of the period known to have that ability). Common on the sandy banks of rivers, calamites probably grew close together so they could support one another. Otherwise they tended to break fairly easily, which is actually one reason why so many fossils remain. Sediment would quickly fill the hollow, broken trunks, and as the outer tissues rotted away the inner cast remained [sources: Kenrick and Davies, University of Waterloo].