Pteridosperms

A group of extinct seed plants characterized by fernlike leaves that produced naked seeds. The discovery of the seed ferns was a major contribution to the study of plant evolution because it demonstrated the existence of a group of vascular plants that is today extinct. Although the seed ferns probably have their ultimate origin within the progymnosperm order Aneurophytales, the best evidence of the group comes from lower Carboniferous (Mississippian) and younger sediments. Some seed ferns are reconstructed as trees with stout stems, while perhaps the majority were vines or lianas that supported massive fernlike fronds. The seed ferns consist of six Paleozoic orders (Calamopityales, Buteoxylonales, Lyginopteridales, Medullosales, Callistophytales, Glossopteridales) and three orders (Peltaspermales, Corystospermales, Caytoniales) found in Mesozoic rocks. See also Caytoniales; Paleobotany.