ISGS - Fossil Mire Forest

One of the most abundant kinds of fossils found in the "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.

Description	Image (click for full size)
*Asolanus*. A small, rare lycopsid tree. Widespread in coal age wetlands but never very abundant. The biology of Asolanus is not well known because it is so uncommon
Diaphorodendron. Pictured here is a small branch from the crown of the tree. These appear to have been tall trees with highly branched crowns. The tiny diamond patterns seen on the stem are called "leaf cushions", the place at which leaves were attached to the stem when the plant was alive.
*Lepidodendron*. One of the best known of the lycopsid trees, Lepidodendron was one of the largest trees of the forest. The diamond patterns on its bark, like those of other lycopsid trees, were places where leaves where attached in life. Lepidodendron is more common in shales (originally muddy swamps) than in coal beds (originally peat-forming swamps or mires).
*Lepidophloios*. Although not as well known as Lepidodendron, the tree known as Lepidophloios has been known to scientists since the 1800s. It too has diamond shaped leaf cushions, but note that they are wider than high and overlap like shingles on a roof. Lepidophloios does not seem to have been as large as Lepidodendron or Diaphorodendron, but it has the same growth form – an long trunk with a crown present only at the end of the trees life, associated with reproduction. It was the most important lycopsid tree in most coal age peat-forming forests and was rather rare in muddy swamps (preserved as shales).
*Synchysidendron*. Small branch from the crown and a part of the main trunk of yet another kind of giant lycopsid tree, Synchysidendron. Like the other kinds of lycopsid trees in the Riola forest, this one appears to have been large, pole-like for most of its growth, with a crown only near the end of tree life. You can see that although the diamond-shaped leaf cushions are higher-than-wide, they are quite plump and almost round in shape, a form characteristic of the several species in this genus.
Leaves of giant lycopsid trees. These are typical leaves from the giant lycopsid trees. Such leaves are grass like, but could exceed 3 feet (1 m) in length. The densely covered the trunk and crown branches of the trees.
Cone of a giant lycopsid tree. The spore bearing cones of the giant lycopsids could reach more than a foot (30 cm) in length. They were borne in abundance in the crown branches and often fell off (abscised in botanical terms) from the tree after the spores were released. This cone bore small "male&" spores, very similar to pollen of seed plants.
Cone segment of the giant lycopsid tree. What are you looking at here? This is part of a "female" cone of one of the giant lycopsids, shown in a "cross section". The cone has been broken so that you are looking at it on-end. These kinds of cones bore "seed-like" structures – not really seeds, they functioned in a somewhat similar way, although fertilization probably took place in the water.
Trunk of giant lycopsid, probably Lepidodendron. Lycopsid trees could get huge. In this photo, you see Howard Falcon-Lang (University of Bristol) and John Nelson (Illinois State Geological Survey) standing below a tree trunk lying just above the contact of the coal bed with the roof shale (of course, the coal has been mined out to reveal the roof!). Their raised hands mark the edges of the trunk. This trunk was more than 6 feet wide (nearly 2 m) and more than 120 feet long (over 30 m) and we did not see the crown! Who knows how much bigger it was in life.
Fallen trunk section. A section of a large trunk has fallen from the roof and lies in the middle of the floor, to the right of the backpack. You can get a sense from this photograph what it is like to work in one of these mines. In the background, John Nelson and Howard Falcon-Lang are examining the roof for plant fossils. The sides of the "room" are the coal bed. Phil Ames walks along the left side of the passage.
Two views of a giant lycopsid tree stump buried while still upright. The trunk projects up into the roof shale. This stump would have been "rooted" in the very top of the coal bed (other such stumps were found along the "rib" where they could be seen to be in contact with the top of the coal). Notice the geological hammer for scale. The plate at the base of the stump is a roof bolt, put in to hold the stump in place, so that it will not fall out.