a class of extinct gymnospermous plants that were predominantly treelike in form. The long stems, which were thin or massive, differed from the stems of true ferns in their development of a secondary xylem, the punctate porosity of their tracheids, and their characteristic reticulate system of mechanical fibers in the bark. The large pinnate leaves were visually indistinguishable from the leaves of true ferns; there were, however, anatomic differences in the structure of the epidermis, stomata, and leafstalks. The leaves bore seeds of varying structure (for example, in Lyginopteridaceae and Medullosaceae). The plants were wind-pollinated; the pollen chambers of the seed primordia often contained pollen. The microsporangia, which were marginal or apical, sometimes formed synangia. Caytoniales are usually assigned to Lyginop-teridopsida.
Most remains of Lyginopteridopsida have been found in Carboniferous deposits, but some have been discovered even in Jurassic deposits. In paleobotany this group of fossil plants is usually called Pteridospermae.