Isoetopsida

Isoetopsida is one of the three classes of Lycopodiophyta. The only living members of this class are within the Selaginella or Isoëtes genera, with 4 other genera, Lepidodendron, Lepidophloios, Sigillaria, and Pleuromeia being extinct. Formerly, members of this class were placed within the Selaginellopsida or Lycopodiopsida classes. There are about 850 living species, living almost everywhere but Antarctica, and are rare in occurence. Some botanists have placed South American members of Isoëtes into a separate genus, Stylites.

The most famous members of this class are the scale trees (Lepidodendrales). These large trees lived during the Carboniferous period in marshlands. Isoëtes, or the quillworts, are considered to be the last remnants of these fossil trees, because they share unusual features, such as their development of wood and bark, a shoot system acting as roots, bipolar growth, and an upright stance.