Part 2

The fossilized plants of Pennsylvanian time belonged to only a few main categories: scale and seal trees, ancient scouring rushes (horsetails), herbaceous Sphenophyllum, ferns, seed ferns, and cordaitean trees.

SCALE AND SEAL TREES (Plate 1)

Scale and seal trees were abundant during the Pennsylvanian Period and were important contributors to coal beds. Although distantly related to the diminutive club-mosses and ground pines of the present, the trees grew on straight, slender trunks to heights of more than a hundred feet.

Scale trees were so called because their numerous, closely set, spirally arranged leaves left scarred “cushions” on the branches and trunk, making them appear scaly. Seal trees derived their name from the signetlike appearance of their leaf cushions. The two best known types belong to the genera Lepidodendron (scale tree) and Sigillaria (seal tree), and fossils of both are common in Illinois.

Lepidodendron had long, slender, somewhat tapering trunks. Some of the trees reached heights of more than 100 feet and measured more than two feet in basal diameter. The trunk ended in a spreading crown formed by repeated dichotomous branching. The leaves were awl-shaped or linear, ranging from one to 30 inches long.

The leaf cushions of Lepidodendron are diamond-shaped, longer than broad, and arranged in spiral rows around the trunk and branches. A different name, Lepidophyllum, is used for fossils of the long, bladelike leaf when it is found detached.

Spores were borne in long cylindrical cones at the tips of the branches. Those cones referred, or assigned, to the genus Lepidostrobus bore both small spores (microspores) and large spores (megaspores) in the same cones. Those in which only a large single spore, a somewhat seedlike structure, was developed in a spore sac (sporangium) are referred to the genus Lepidocarpon.

The rather commonly found genus Stigmaria comprises so-called “appendages” which, although stemlike in structure, apparently served as roots for the scale and seal trees. These appendages are identified by irregular spirals of circular scars (pits) that mark the attachment points of former rootlets.

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.

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.

SPHENOPHYLLUM (Plate 2)

The name Sphenophyllum refers to both stems and leaves of this extinct genus, which was related to the scouring rushes—note its resemblance to Annularia.

A small herbaceous plant, Sphenophyllum formed much of the swampy undergrowth of the Pennsylvanian Period and is abundant among Illinois fossils. It had a slender, ribbed stem bearing whorls of delicate, wedge-shaped leaves, generally less than three-fourths of an inch long, attached around the stem in multiples of three.

The cones of this group also are slender, delicate structures, bearing a number of sporangia, and are correctly called Bowmanites, although they also have been called Sphenophyllostachys. These fossil cones frequently are found in Illinois.

Sphenophyllum first appeared during the Devonian Period, some 300 million years ago, but did not become abundant until Pennsylvanian time. The genus continued through the Permian but died out in Triassic time.

FERNS (Plates 1 and 3)

True ferns, like those living in today’s woodlands, were common in the Pennsylvanian forests. Some species attained heights of 30 to 40 feet. Their fronds (compound leaves divided into segments or leaflets) commonly were five to six feet long.

True ferns do not produce cones or seeds, but spores, which develop in cases called sporangia. The sporangia frond showing are attached in clusters (sori) to the lower side or margins of the leaves. In modern ferns the sporangia may also occur on fertile spikes.

The shape and position of the sori are used to identify modern ferns, but because leaves that bear sori (“fertile” leaves) are rare among fossil specimens, the number, shape, and attachment of the leaflets and the pattern of the veins are more commonly used for identification.

Because fossils of complete fern plants have not yet been found, separate names have been adopted for detached leaves, stems, and other parts. For example, the fossil stems of some Pennsylvanian ferns found in Illinois have been referred to two genera, Megaphyton, whose leaf attachment scars are arranged in two vertical rows, one on either side of the stem, and Caulopteris, whose leaf scars are arranged in a steep spiral that becomes progressively flatter upward until near the top they appear to be whorled. When the stem is a petrifaction, with internal structures preserved, it is called Psaronius. The fronds are referred to a number of genera, but those most commonly found in Illinois are Pecopteris, Asterotheca, and Ptychocarpus.

Pecopteris Asterotheca Ptychocarpus

SEED FERNS (Plate 4)

Seed ferns resembled true ferns in general, but they produced seeds, borne on modified leaves. Where spore sacs and seeds are absent, the leaves of seed ferns are difficult to distinguish from those of spore ferns, although individual seed fern leaflets, called pinnae, are somewhat larger.