One of the most abundant kinds of fossils found in the i"roof shales" of the Herrin coal bed in Riola are the giant lycopsids, also known as giant "club mosses" or "scale trees". The lycopsids were the giants of the coal age forests, reaching heights of more than 100 feet (30 meters). They appear as reconstructions in many museum dioramas and reconstructions in biology and geology text books.
The only close modern relative of these trees is the diminutive plant Isoetes, the "quillwort" a plant only a botanist could love! Isoetes is common in wet habitats, around the margins of water bodies where it may sometimes be submerged. To the untrained eye, it will look like a small tuft of grass above ground. But below ground it has a peculiar root system that links it clearly with the extinct giant lycopsid trees.
Factoids about the giant lycopsids: Many species of these trees spent most of their lives growing as unbranched poles (up to 5 or 6 feet in diameter – nearly 2 m) covered with leaves. Branched crowns did not appear till late in life, when trees were tall. The stem branched many times to form the crown and with each branching the size of the stem diminished until growth ceased and the plant died. Botanists call this type of growth “determinate”, meaning that the plant had a fixed life span. Because the crown was not present for most of the life of the tree, it is unlikely that its main function was light capture (as in the crown of most flowering plant trees and conifers that we know today). Rather, its main role was reproductive.
The crown in the giant lycopsids seems to have served mainly as a launching pad for spores. In this kind of lycopsid tree, all the reproductive organs were borne in the branched crown. This means that the plant did not reproduce till the end of its life span and then only for a short time. With the reproductive organs high in the air, wide dispersal of spores was greatly enhanced.
Because of this kind of growth, most coal age forests should be thought of as a forest of poles of various heights. Those with crowns are entering or are well along in the final stages of their growth and are producing their spores. Instead of being dark at the forest floor, these forests may have allowed in considerable amount of light. In some cases, as at Riola, the giant lycopsid trees may have pushed through and towered over a much lower canopy of tree ferns and seed ferns, smaller trees and shrubs that were more like what we think of as trees today.
The main support tissue in the giant lycopsids was bark instead of wood. These trees had thick bark, sometimes more than a foot (~ 30 cm) thick. Packed one upon the other after death and collapse, the bark of these trees makes up most of the coal mined in the Eastern United States and Western Europe. A small cylinder of wood in the center of the trunk transported water throughout the trunk and to the leaves.
The giant lycopsids died out in the Late Permian, near the end of the Paleozoic Era, which ended about 250 million years ago. The last of them lived in what is now China, in swampy wetlands much like those of Illinois many years before.
All pictures are looking up at the roof of the mine. (Click on image for a larger view.)