Great Coal-Age Swamps
Carboniferous reconstruction with horsetails (front-right), ferns and tree ferns (front-left), scale trees (tall trees in back), and seed ferns (in front of scale ferns)
What was the Earth like during this time?
The Late Devonian extinction was a period of global cooling, but the Earth recovered and began to get warm and humid
Large swaths of the Earth were now tropical with expansive swamps.
Remember, swamps are areas of saturated soil or shallow water that support woody plants, including trees and shrubs.
Swamps are usually flooded with freshwater, but may also line saltwater or brackish water.
Cypress swamps in southern United States are an example of freshwater swamps; Mangroves swamps, such as the Everglades in Florida, are an example of saltwater swamps.
Which plants dominate?
During this time, known as the Carboniferous Period, vast numbers club-mosses, ferns, horsetails, and seed plants populate the Earth. In the modern world, most club-mosses, horsetails, and ferns are short in stature, but during this time all of these groups had tree-sized members.
Club-mosses evolved into enormous plants, called scale trees. The bark of these trees gave them a scaly-like appearance, thus the name (see image below).
Scale trees, like Lepidodendron or Sigillaria, were able to attain heights of 130 feet, and diameters of 7 ft.
These plants had strange, root-like structures which were actually stems. On the bottom, the structures had leaves that acted like roots. On the top, these root-like stems had leaves that were green and photosynthetic. These plants had roots that could do photosynthesis!
Research seems to indicate that these plants produced a trunk and branches for the primarily function of reproduction. A very massive and tall "fruiting" structure.
Above: Reconstructions of tree-sized clubmosses called scale trees
What plants are in the understory?
Ancient tree ferns, that had long trunks and enormous fronds, grew in the under-story
Also living in the under-story were plants that looked like ferns, but reproduced by seeds. These early seed plants, called "seed ferns" or pteridosperms, were the great grandparents to modern-day seed plants.
Why is this time called the "Coal Age"?
Coal is the compressed carbon remains of plants that lived millions of years ago. Through photosynthesis, these plants took in carbon dioxide to make food. Carbon plays a major part in making glucose and other structural parts of the plant.
All of these plants thrived in a hot and humid environment, but some environmental factors prevented the full decay of plant debris. Normally plant material like leaves and wood decay into compost or soil, but something about the environment during this time prevented this decay.
When the plant died, the leaves and stems were buried and over hundreds of years changed into peat. Over longer periods of time, this peat turns into brown coal (lignite), and then it changes into black coal, which is called anthracite.
Some researchers claim that fungi had not yet evolved the ability to decompose wood. The evolution of wood was relatively new on the Earth, and a chemical called lignin (which makes wood cells so tough), was indigestible by any known fungi or animals. Therefore plant material started to accumulate, instead of be decomposed.
Others claim that there were wood-degrading fungi around during the Carboniferous, but the tropical conditions and extensive amount of plant material being dropped, was the perfect recipe for creating enormous amounts of coal over millions of years (Nelsen et al. 2015).
We burn coal for energy, and we are releasing that ancient carbon back into the atmosphere. This addition of carbon, from millions of years ago, increases CO2 level, thus causing our modern-day global warming.
Above: Diagram of how peat turns into black coal
What animals lived during this time?
As these massive swamp plants flourished, the oxygen levels continued to increase. In fact, this period in Earth's history has the highest known oxygen levels ever recorded (32% during Carboniferous, versus 21% today)
High oxygen levels meant that invertebrates could get bigger. One of the major factors that limits the size of these creatures is the amount of oxygen they can passively absorb from the air.
Some species grew to sizes not seen before or after. The millipede, Arthropleura, could grow to over seven feet long, and Meganeura, a distant relative of dragonflies that had a wingspan of over two feet wide! This was a truly terrifying time in earth's history for those that hate creepy-crawlers.
During this time, amphibians were clearly on the land, and some were the size of small dogs. Early reptiles were also present and also small.
Above: Reconstruction of Meganeura
Above: Reconstruction of Arthropleura
How long did the coal forests last?
The cause of the high oxygen levels was productive photosynthesis by the world-wide swamps and forests. This also meant that carbon dioxide levels would continue to drop through the Carboniferous. This created global cooling, once again.
Progressively the Earth becomes colder and drier with the beginning of an ice age during the Late Carboniferous.
Around 315 million years ago, life on Earth experiences another great extinction event with a collapse of these tropical swamps.