Carboniferous Forests

Ralph E. Taggart, Professor

Department of Plant Biology

Department of Geological Sciences

Michigan State University


By the Carboniferous Period, the evolution of terrestrial plants and animals had advanced to the point where true forests were developed in lowland, coastal sites. Most of these forests would be characterized as swamps and they were dominated by plants and animals that were quite different, in many ways, from such sites today. Shown above is a classic diorama of a reconstructed Carboniferous swamp forest executed and displayed at the Field Museum of Natural History in Chicago. The primary plant and animal groups that constituted such forests will be discussed below.

Climate

Although study of fossils from this period provides the basis for understanding the individual organisms, some major aspects of the communities are not well understood. One of these aspects is climate. The thick vegetation suggests a tropical biome, but this is by no means certain. Plants with secondary growth generally lack well-developed growth rings, which is characteristic of modern rain-forest habitats. However, as long as water was abundant throughout the year and temperatures were not highly variable, almost any climatic regime from tropical to temperate would produce the same result. We can assume the climate was tropical to sub-tropical, but that may not be strictly true.

Lycopods

Modern lycopods, such as the Lycopodium shown above, a plants generally only a few inches tall (maximum of 1 meter), are characterized by small, pointed leaves that cover the dichotomously-branched stem and branches in a dense spiral. Spore-bearing cones, located at the ends of the main branches, form the basis for a fern-like free-sporing life cycle.

Although the Carboniferous swamp forests had small, herbaceous lycopods like those of today, the most impressive plants of these communities were several types of tree-sized (arborescent) lycopods like this Lepidodendron. The large tree-like trunks in the Field Museum reconstruction are various kinds of these spectacular plants. Quite unlike trees of today, these plants were either sparsely dichotomously branched, as in this specimen, or completely un-branched, reaching a height of 30-40 meters. The leaves were long and grass-like and may have covered the entire plant, not just the terminal portions of the branches as shown here. Spore-bearing cones were produced near the apex of the branches. What appear to be roots at the base of the plant are also branched stems, which produced highly modified ribbon-like leaves that functioned as rootlets. The trunks contained very little secondary wood and the plant was supported by a thick, bark-like periderm that enclosed soft, pith-like tissue. Without abundant water to keep the internal cells fully expanded, the plants would collapse under their own weight.

Horsetails

Horsetails were another important component of the Carboniferous plant communities. Modern horsetails, such as this Equisetum (the only living genus in the group), reach a height of just about 1-2 meters, but are easily recognized by the division of the stems and branches into definite nodes, with both leaves and branches occurring in whorls at the nodes. Spores are produced in terminal cones, which feature the same nodes/whorls of the other parts of the plant.

Carboniferous horsetails were of two general types. The larger forms, like this Calamites (A above), reached a height of 10-15 meters and probably grew in densely packed clumps like modern bamboo. A Calamites can be seen in the right foreground of the Field Museum image. Typical plants of this type were very much like gigantic versions of modern horsetails.

The second major type of Carboniferous horsetail was a bushy or vine-like plant known as Spenophyllum.

Ferns

Modern ferns exhibit a wide range of growth forms from small, understory plants to these large, tree-ferns from New Zealand (Cyathea).

These large tree ferns, known as Psaronius, were the secondary dominants of the Carboniferous forests, second only to the large arborescent Lycopods. Growing to a height of 20 meters, with large, compound fronds, the plants would have been even more spectacular than the living Cyatheas illustrated above. The forests also contained a wide range of smaller ferns, including epiphytes found attached to the Psaronius trunks.

Seed Ferns

In addition to true ferns, the Carboniferous forest also provided habitat for a group of early seed plants known as seed ferns or pteridosperms, such as this Medullosa. Seed ferns had fern-like leaves but a seed-type life cycle, often with walnut-sized seeds produced directly on the foliage. Plants of this type ranged from shrub-like to very small trees and would have made up much of the under-story vegetation. If you look carefully and the Field Museum reconstruction, you can see pteridosperm foliage with large seeds on the left side of the foreground. Seed ferns, as a group, almost disappeared with the Permo-Triassic mass extinction and did become extinct by the end of the Mesozoic Era

Early Conifers

The first conifers appear in the late Carboniferous but from most unlikely-looking ancestors. The plants that gave rise to the conifers were Cordaites, a gymnosperm with long, strap-like leaves. These plants ranged from large shrubs to very small trees and many probably grew like modern mangroves, living on mud flats in brackish water. Water acquisition is difficult in such habitats and may suggest the source of the xeric adaptations found in most conifers.

The first true conifers had branches and needles that strongly resemble this modern Araucaria. Their cones however and much more loosely organized than any modern conifer and reflect their derivation from the cone-like branch systems of Cordaites. While they were the most advanced plants on the Carboniferous landscape, adaptive radiation of the conifers was delayed until the Permo-Triassic mass extinction opened up niches.

Insects

The Carboniferous insect fauna was moderately diverse and only two examples are shown below:

This dragonfly (Cowen, 1990) had a wingspan of two feet!

This is an example of a small Carboniferous cockroach - it only measured about 2 inches in length! Larger species were up to 1 foot long.

Amphibians

The largest land animals of the Carboniferous were amphibians, such as this Eryops (American Museum of Natural History). About the size of a large dog, this particular animal, like virtually all amphibians of the period, probably fed on fish.

A remarkable example of convergence, this animal would have looked like a small snake, but was actually an aistopod amphibian! About 75 cm long, the animal may have had a niche similar to modern water snakes. (Cowen, 1990)

Reptiles

This Petrolacosaurus (Science magazine), found in late Carboniferous deposits from Kansas, is one of the earliest known reptiles. Significant adaptive radiation of reptiles, like the conifers, was delayed until the aftermath of the Permo-Triassic extinction.

The End of an Era

Most of the plants and animals of the Carboniferous swamp forests were dependent on abundant supplies of water. The loss of coastal habitats and the drier and more extreme climatic conditions that developed during the Permian, with the gradual accretion of Pangea, caused the extinction of most of the species characteristic of these first forest communities. Conifers and other advanced gymnosperms and the reptiles were pre-adapted to drier conditions and would dominate the terrestrial communities of the Mesozoic.


Ralph E. Taggart (taggart@msu.edu)