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.
Portion of fern frond showing sori on lower side of leaflets
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.