Lepidodendronlĕpˌĭdōdĕnˈdrən and Sigillaria (sĭjĭlârˈēə) [key], two principal genera of an extinct group of primitive vascular trees. They dominated the forests of the early Carboniferous period until the ferns gained ascendancy. Related to the club mosses, they are sometimes called giant club mosses. The spore-bearing leaves formed cones. The tall, thick trunks, rarely branching, were crowned with a cluster of narrow leaves. The closely packed leaf scars left on the stems as the plants grew provide some of the most interesting and common fossils in shales and accompanying coal deposits. In Lepidodendron the leaf scars are diamond-shaped, and in Sigillaria they are arranged in vertical rows. The rhizomes, or root systems, of both genera, known as stigmaria, were thought to be distinct plants when their fossils were first discovered. Actually they served to support the trees and to produce new shoots. Lepidodendron and Sigillaria are classified in the division Lycopodiophyta, order Lepidodendrales.