Horsetails are rare today, but flourished on Earth during the Carboniferous (about 325 million years ago), and remained important in many kinds of ecosystems into the early Mesozoic. Throughout their history, horsetails have preferred environments with disturbance, such as flooding and burial of vegetation in sediment. In contrast to the small horsetails of today, Carboniferous and Permian horsetails in the genus Calamites could be 10 meters tall (33 feet) with large leaves. Tree-like in shape, they formed the lower canopy (club mosses were even taller) of dense forests. Calamites, like modern horsetails, had segmented stems with circles (whorls) of branchess and leaves at the segment junctures (nodes). The stems of calamites had a hollow central area surrounded by a cylinder of woody tissue (picture a cardboard tube). Because these plants frequently lived in bars within streams, or on stream banks, they were often buried in floods. Their stems are often found still in place in ancient river deposits, as stem casts.