Pennsylvanian Age Mire Forest

John Nelson and Scott Elrick in the Bedrock Geology Section of the Illinois State Geological Survey, in cooperation with Bill DiMichele of the National Museum of Natural History Smithsonian Institution, Howard Falcon-Lang of the Univeristy of Bristol and Phil Ames of Peabody Energy published a paper in 2006, about the ecology of a fossil forest in the journal Geology.

ISGS geologists received more publicity from their fossil forest discovery near Danville, Illinois. Discover Magazine honored this discovery by including them in the “Top 100 Science Stories of 2007” in the January 2008 issue. A small summary article and interview with Scott Elrick are contained in the magazine article. The fossil forest was also featured in the July 2007 issue of Outdoor Illinois; an article entitled Underground Science (pdf).

Ecological gradients within a Pennsylvanian mire forest

Pennsylvanian coals represent remains of the earliest peat-forming rain forests, but there is no current consensus on forest ecology. Localized studies of fossil forests suggest intermixture of taxa (heterogeneity), while, in contrast, coal ball and palynological analyses imply the existence of pronounced ecological gradients. Here, we report the discovery of a spectacular fossil forest preserved over ~1000 ha on top of the Pennsylvanian (Desmoinesian) Herrin (No. 6) Coal of Illinois, United States. The forest was abruptly drowned when fault movement dropped a segment of coastal mire below sea level. In the largest study of its kind to date, forest composition is statistically analyzed within a well-constrained paleogeographic context. Findings resolve apparent conflicts in models of Pennsylvanian mire ecology by confirming the existence of forest heterogeneity at the local scale, while additionally demonstrating the emergence of ecological gradients at landscape scale

The location of this fossil forest is just to the south and west of Danville, Illinois, about 30 miles to the east of the ISGS in Champaign, Illinois (see picture to the right). This 300 million year old fossil forest was found directly on top of the Herrin coal seam in the Riola and Vermillion Grove coal mines, and represents the last stages of the peat mire forest responsible for forming the Herrin coal.

If you were walking down a hiking trail through a forest today, looking closely at the vegetation, you might notice that as the landscape changed, gradual changes in the types of plants and trees occured as well. For present day plant biologists examining the ecology of a modern day forest, this is a luxury that is easily taken for granted.

When fossil plant researchers (paleobotanists) want to look at the ecology of an ancient forest, it is rather difficult to simply go walking through an ancient forest! Instead, much study must go into looking at many plant fossils from multiple locations and times and settings, getting one puzzle piece of the ecology here... one puzzle piece of the ecology there. Through diligent study, a picture of ancient environments can be slowly be pieced together and good guesses about ancient ecologies can be made.

However, every once in a while a unique opportunity presents itself where a paleobotanist has an opportunity to do what his plant biologist colleagues do all the time. Walk through a forest... a fossil one! This paper represents the largest study of the plant ecology of a Pennsylvanian age forest to date and shows that subtle ecologic changes in the make-up of the forest were present 300 million years ago.

In the practice of geology in the field, scientists regularly look at vast spreads of time in small geographic slices. For example, standing at the rim of the Grand Canyon and peering across to the other side, your eye takes in millions of years of geologic time... but you are only able to see a thin 'slice' of each unit in profile. Do you want to know what a particular rock unit looks like 500 feet into the side of the canyon walls? The only way to find out is to drill a hole and take a core sample.

Geologic research in an underground mine such as these coal mines turns this notion on its ear!

In this study, we were able to look up at preserved trees and ferns in the mine ceiling, peering upwards at a single slice of time over a huge (relatively speaking) geographic area. This is a unique way to get a 'snapshot' in time look at the forest landscape of 300 million years ago. It's the 'worms eye' view of a fossil forest... allowing a veritable 'walk' through the forest and a chance to see first hand what plant species were present and how they were distributed across the landscape. In other words, a chance to see the ecology of a fossil forest!

If you would like to see a piece of this fossil forest in person, you are in luck! The coal mining exhibit at the Museum of Science and Industry in Chicago has a fossil covered slab of gray roof shale from the Riola mine on display. Now you can take a (mini) walk of your own.

How was this fossil forest discovered?

One of the responsibilities of the ISGS is to try to understand the geology of the state of Illinois. For the Coal Section at the ISGS, that means trying to visit the coal mines in the state on a regular basis. When the Riola mine opened in 1996, geologists from the ISGS visited and noted the presence of fossil plants in the roof of the mine. Plant fossils are not uncommon in Illinois coal mines, so while notes were made, nothing exceptional was thought of the discovery. As time went on, more coal was mined, more of the mine roof was uncovered and the plant fossils didn't stop! Fossils were numerous and showed excellent preservation.

Adding visits to the Vermillion Grove mine, Survey geologists soon realized that a very interesting story was waiting to be told about the fossil plants, and in 2004, contacted Bill DiMichele from the Smithsonian and Howard Falcon-Lang of the University of Bristol, both experts in paleobotany. With the assistance of Phil Ames of Peabody Energy, a large study was then undertaken to try to understand the mosaic of preserved plant fossils presented just over our heads in the gray shale of the mine roof.

Why was it preserved?

By examining the relative thicknesses of the rock units at the mine, such as the Herrin coal and overlying Energy shale, as well as noting the nature of the contact (either gradational or abrupt) between the afore mentioned rock units and adding in other observations from the mine, ISGS geologists John Nelson and Scott Elrick and Peabody geologist Phil Ames discovered that the ancient forest had grown in an estuary that was tectonically fault controlled. In other words, the fossil forest was growing in a low lying area that was 'low lying' because a fault was allowing part of the land to slowly sink.

More importantly however, was the realization from examining deposits above the fossils and the fossils themselves that this area had not only been sinking slowly, but the fault responsible for the slow tectonic sinking had also drowned the forest by way of an abrupt earthquake... quickly dropping the land surface below sea level. The extreme southern end of a nearby known fault called the Royal Center fault in Indiana was hypothesized to be the culprit.

An intriguing modern day analogue to this kind of phenomenon of earthquake created lowlands is Reelfoot lake in Tennessee.

Like many other fortunate fossil finds, especially ones that have such great preservation, this locality had the happy coincidence of being in the 'right place' at the 'right time'. A catastrophic event (earthquake) provided the opportunity for a large segment of forest to be preserved and reveal to us ecologic subtleties of Pennsylvanian age peat mires that we had guessed and inferred before, but can now see.

Fossil Forest Images

People and Places

The following images show the people involved and the place we were for this fossil forest discovery. (Click on image for a larger view.)

The landscape of East-Central Illinois on the surface above the coal mines where the fossil forest was found. You can see flat fields, corn and soybeans, a deciduous forest here and there... but no rainforest vegetation! We need to travel 250 feet underground to find our Pennsylvanian age rainforest vegetation.
Coal is found "underground". Here Howard Falcon-Lang (University of Bristol) and John Nelson (Illinois State Geological Survey) stand outside the mine headquarters before putting on their safety equipment and going through safety training, prior to entering the mine.
Howard's hard hat, cap lamp, reflective gear, SCSR (self contained self rescuer), battery pack and rock hammer, he is also wearing metatarsal guard boots and pants cuff straps. Don't forget your backpack, gloves, and lunch Howard!
From left to right - John Nelson from ISGS, Phil Ames from Peabody Energy, and Scott Elrick from ISGS discuss a series of geologic and geophysical logs.
John Nelson, in the mine and examining the coal "rib". In the "roof" you can see "roof bolts", which are used to stabilize the roof rock and keep it from collapsing into the area from which the coal has been removed. The roof rock and coal "rib" have been sprayed with limestone rock dust, used to help stop explosions from propagating in the mine. This is what causes many of the walls of the mine to appear white in color. It looks like John remembered his lunch.
Scott Elrick, in a mine in Illinois, examining a few thin stringers of coal above the main coal seam. Part of the 'rib' or wall of the passageway has come down, allowing us to see the black and shiny nature of the coal seam and the dull dark gray of the roof shale normally hidden by the limestone rock dust.Scott Elrick, in a mine in Illinois, examining a few thin stringers of coal above the main coal seam. Part of the 'rib' or wall of the passageway has come down, allowing us to see the black and shiny nature of the coal seam and the dull dark gray of the roof shale normally hidden by the limestone rock dust.
Bill DiMichele, caught in the act of wrapping a fossil sample in newspaper. By the end of the day, Bill will fill that backpack to the top! On Bill's left hip you can see the battery pack that powers his cap lamp.

Calamites

The calamites, or "giant horsetails" were a group of spore producing plants very closely related to modern "horsetails" (also known as "scouring rushes"), of the genus Equisetum. Much larger than modern horsetails, this ancient group also could produce wood. Trees could reach considerable heights, although those at Riola probably did not exceed 30 feet (10 m) in height, based on the sizes of the stems observed. The trunks had a hollow center that could be filled with mud after the tree died, leaving a so-called "pith cast", which is one of the most common remains of these plants. The calamites, like the modern horsetails, had "node-internode" architecture. All the leaves and branches were arranged in whorls borne at "nodes". These nodes can be seen in the fossils as lines that extend around the stem. All pictures are looking up at the roof of the mine. (Click on image for a larger view.)

Asterophyllities equisetiformis. Leafy branches of a calamite tree. Note how the leaves themselves (which are thin and elongate) are arranged in whorls on the stems.
Calamites pith cast. Note the nodes (lines around the stem) where leaves and leaf-bearing branches would have been attached. The picture on the right illustrates a long trunk segment that is thin throughout the preserved length.
Asterophyllities equisetiformis. Leafy branches of a calamite tree. Note how the leaves themselves (which are thin and elongate) are arranged in whorls on the stems.
Calamostachys, the spore-producing cone of a calamite tree. Note again, the whorled arrangement of the appendages.
Calamite cones mixed with tree-fern stems or frond axes. The triangular object in the lower right of the photograph is broken piece from a "female" cone of a giant.

Cordaites

The cordaites were a group of plants thought by paleobotanists to be closely related to the conifers, based mainly on the structure of their cone-like reproductive organs. Unlike conifers, which have relatively small to very small leaves, the cordaites had large, thin and grass-like to wide and strap-like leaves, depending on the species. Cordaite trees in the coastal wetlands appear to have had relatively small stature. Their trunks may have had prop roots, much like modern mangroves, although it has not been proven unequivocally that these ancient plants could grow in salt water. Leaves may have been borne on tufts at the end of branches. In contrast, cordaites of very large size have been found in drier environments, outside of the wet areas where coal beds formed. In these cases, the cordaites may have been the dominant large trees of the forests. All pictures are looking up at the roof of the mine. (Click on image for a larger view.)

Stem of Cordaites with an attached spray of long, relatively thin leaves at the end of the branch. (Just below the light colored piece of wood and roof bolt.)
Cordaites leaves, closer up. Again, these appear to be a tuft or spray at the end of a branch or trunk.

Ground Cover

Plants inferred from their growth forms to have been ground cover are not common at Riola. This suggests that the soil surface may have been inhospitable to the growth of small plants, perhaps due to flooding.

One plant in particular, Sphenophyllum, was widespread throughout the mine but rare. Sphenophyllum is a sphenopsid, the same higher-level group that includes the horsetails. Like that group of plants, it has "node-internode" construction and its leaves and branches are borne in whorls. In this instance, however, the leaves are wedge-shaped, a distinctive attribute of these plants.Some Sphenophyllum species have hooks or barbs on their leaves, suggesting that they too formed thickets or tangles, and perhaps may have climbed other trees for support.

Another potential ground cover plant, a possible small fern or seed plant, is Sphenopteris (53), which is rare in the Riola mine. Sphenopteris is characterized by small fronds that have small, variously lobed pinnules.

Sphenophyllum is a sphenopsid, the same higher-level group that includes the horsetails.
Sphenophyllum is a sphenopsid, the same higher-level group that includes the horsetails.
Sphenopteris, rare in the Riola mine. Sphenopteris is characterized by small fronds that have small, variously lobed pinnules.

Lycopsids

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.)

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.
Synchysidendron (close-up). 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.
Second view 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.
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. Peabody Energy safety official walks along the left side of the passage.
First view 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.

Seed Ferns

The seed ferns were a group of seed-producing plants with large fronds that, superficially, resembled the leaves of ferns (hence their name!). They have no close modern descendents.

The leaves of these plants were known prior to the discovery of their seed-bearing nature. In some species, the frond-like leaves could be more than 20 feet (7 m) in length. Trees have been found to be of two major growth forms: short, upright, free-standing forms, perhaps 15 feet (3 m) in height, and taller forms that were not self-supporting, but that formed tangles or thickets in which they leaned on one another and the large fronds were intertwined, lending support to the weak trunks.

Seed ferns were a diverse group of plants and so had many different kinds and sizes of reproductive organs. Their seeds ranged from small, perhaps the size of a modern "field pea", to quite large, roughly the size of, or even a little larger than, an avocado seed.

All pictures are looking up at the roof of the mine. (Click on image for larger view.)

Laveineopteris rarinervis. Uncommon
Neuropteris ovata.Neuropteris ovata. Common in parts of the mine and often forming dense mats of leaves.
Neuropteris ovata (close-up). Common in parts of the mine and often forming dense mats of leaves.
Plant fossils above the Herrin coal from an underground coal mine Possibly Odontopteris.Plant fossils above the Herrin coal from an underground coal mine Possibly Odontopteris. This form was rare and only found in fragmentary preservation.

Tree Ferns

The second most important group of plants in the Riola forest were the tree ferns. These tree ferns are not closely related to those of the modern day. They are, instead, relatives of a small group of ferns known as the Marattiales. Members of this group of ferns are still present today, mainly in moist tropical environments; they can be found in many greenhouse botanical collections in temperate countries. Marattialean ferns are no longer trees, although they may have large leaves (fronds).

The tree ferns are spore producting plants. During the coal age, these plants were perhaps 30 feet (10 m) or so in maximum height with a crown of large, feathery fronds, each frond reaching many feet in length. Tree ferns were able to grow tall because their stems were surrounded by a thick layer, or "mantle" or roots, which supported the spongy, soft tissues of the trunk. Surprisingly, the roots of tree ferns were mostly air spaces, but with a tough outer layer. Individually not very strong, when all grown together, the root mantle, though light and cheaply constructed, gave the tree-fern trunk great strength.

All pictures are looking up at the roof of the mine. (Click on image for larger view.)

Parts of the leaves (fronds) of tree ferns. These fossils are classified as Pecopteris. The frond of these plants was quite large, man feet (several meters) in length and was highly "divided". The small leaf-like structures seen on these fronds are the "pinnules", many of which were presenton a single leaf.
Pecopteris. A closer view of the pinnules of a tree-fern leaf.Pecopteris. A closer view of the pinnules of a tree-fern leaf.
A tangle of tree fern leaves and stems in the roof shale. The delicacy of these leaves and their excellent preservation indicates that they were not transported far, if at all, from the point at which they fell onto the floor of the swamp (perhaps into standing water) and were buried in mud.
Tree fern trunk. On this section of a tree fern trunk you can see the irregularly lined surface. These lines represent the roots that ran down and out from the trunk, both supporting the tree and transporting water to the upper parts of the stem and to the crown of large, frond-like leaves. In the upper right part of the trunk is a small oval mark. This is the scar from the point where a large leaf was attached. After death, the leaf would fall off, leaving a scar, as shown. (which would then get grown over by roots, as can be seen in this stem.)