an extinct division of plants with certain features in common with cycads (division Pinophyta) and grouped with them and the seed ferns (division Pteridospermophyta). Both the cycadeoids and the cycads dominated the vegetation in the Jurassic Period (about 200 to 145 million years ago)—called the “Age of Cycads”—and both are presumed to have originated from a seed-fern ancestor. The evolution of cycadeoids seems to have paralleled the rise and fall of the dinosaurs. Unlike the cycads, in which a few large strobili (conelike reproductive bodies) are borne at the stem tip, the cycadeoids produced many smaller strobili in rosettes among the leaf bases.

The Cycadeoidophyta contained two important families: Williamsoniaceae and Cycadeoidaceae (Bennettitaceae). Williamsonia, the best-known genus of its family, had a columnar trunk with frondlike leaves at branch tips; its fossil cones are not well defined. Williamsoniella, a related genus, was shrubby; fossil leaves placed in the genus Nilssoniopteris are believed to belong here. Cycadeoidea (Bennettites) had a barrel-shaped trunk, like a large pineapple, with a crown of leaves; its cones are reconstructed in detail from fossils. Monanthesia is the genus name reserved for fossil cycadeoid trunks with cones in the leaf axils.