Paleozoic calamites, including a root (at right). Illustration from 1872. A calamite is any member of the lineage of giant horsetails, which belonged to the Sphenopsida, an important part of late Paleozoic vegetation. Calamites grew to be tree-sized plants with but with whorled branches seen in modern horsetails. A calamite root can be seen at right. The Paleozoic Era is the earliest of three geologic eras of the Phanerozoic Eon, spanning from roughly 541 to 252.17 million years ago.