Sigillaria, although less common than Lepidodendron, was widely distributed during the Pennsylvanian Period. It differed in growth habit from Lepidodendron in that it generally had fewer branches and not uncommonly was unbranched. Some species also possessed a thicker trunk, with hexagonal to elongate leaf cushions separated by vertical ribs. The trunk was crowned, in the manner of the modern palm tree, by a cluster of large, grasslike leaves.
The detached leaves of Sigillaria, extremely difficult to distinguish from Lepidophyllum (leaves of Lepidodendron), are referred to the genus Sigillariophyllum if preserved as compressions and to Sigillariopsis if preserved as petrifactions. Unbranched Sigillaria trunks have been found that are more than 100 feet long and six feet in diameter near the base, but the average height probably was closer to 50 feet.
Reconstruction of Calamites
(After Hirmer)
Not all Pennsylvanian trees were large, however. Small forms are known, including the important undergrowth genera Lycopodites and Selaginellites. In woody types the trunk consisted of an inner region of conducting and supporting tissues, surrounding concentric cortical layers, and an outer layer of corklike bark. Although the fossil impressions of the various bark layers have been given separate generic names, these are not commonly used.
SCOURING RUSHES
([Plate 2])
Although related to the small, inconspicuous horsetails of today, the ancient scouring rushes of the Pennsylvanian Period grew to the size of trees and were among the most widely distributed plant groups.
Some of these plants attained heights of 40 feet or more, but the average was closer to 20 feet. The trunks were jointed and bore a whorl of branches at the joints (nodes). Their small leaves also grew in whorls at nodes along the smaller branches. Internodal regions were ribbed in the same manner as present day horsetails. Fossils of the trunks are assigned to the genus Calamites and quite commonly are preserved in sandstone and shale.
The leaf whorls are placed in the genus Annularia. One form commonly found in Illinois has long, pointed, needlelike leaves and is given the name Asterophyllites. Calamostachys, shown on [plate 5], is one of the most common calamite cones.