A quick look at the term “Carboniferous” in the dictionary will give you a hint why this period is named as such. Merriam-Webster defines the word as:
producing or containing carbon or coal
True to its moniker, the Carboniferous period refers to the span of 60 years (approximately 358.9- 298.9 million years ago) characterized by its coal-bearing strata formed from prehistoric vegetation.
That means we all have the Carboniferous period to thank for the vast reserves of coal and natural gas we’re enjoying today. But how did that come about? Now let’s dig deeper, and stay with me because it gets more interesting.
How the plants put the “carbon” in Carboniferous
The Carboniferous period witnessed a variety of plant species – ranging from small, shrubby growths to trees exceeding heights of 100 feet. But, the most important plants of the Carboniferous period were the ones inhabiting the swamp forests around the equator.
They are composed of bark-bearing trees including giant club mosses, tree ferns, great horsetails, and towering trees with strap-shaped leaves. When these vegetations died, they fell into waters that did not have bacteria to help them decompose.
Because of this, layers and layers of fossil plant materials formed peat beds. In turn, the deposits of plant debris turned into coal and the explosion of coal bed formations during this period became the reason for the name “Carboniferous“.
How coal forms
Let’s take a minute to discuss how coal formed during the Carboniferous period.
Because the conditions were right during this particular epoch, accumulating dead plants formed peat beds. Buried but not decayed, the peat beds were subjected to heat and pressure as additional sediment layers continued to accumulate and add weight.
During the deep burial, the peat undergoes coalification. It means that 98% of the water was squeezed out as well as some of the volatile hydrocarbons. So the older and more deeply-buried a coal seam is, the less water and volatile matter it contains.
According to the coal rank, the higher the ratio of fixed carbon to volatile matter is, the higher the rank will be. We have peat as the lowest rank of coal. Next comes lignite, then sub-bituminous, bituminous, and in tectonically active regions– anthracite.
When it comes to coal beds of the Carboniferous Period, they are almost all ranked bituminous, or higher, because of their great age and the very deep burial.
What was the climate like in the Carboniferous period?
According to a publication from the University of Berkeley, the beginning of the Carboniferous period had a more uniform, tropical, and humid climate than exists today. There’s no changing of seasons based on “comparisons between fossil and modern-day” plant morphology.
The same article claims that the Carboniferous plants resemble those that live in tropical temperate areas today due to the lack of growth rings, suggesting a uniform climate.
During this period, the temperature was warm and the humidity is high. With this climate, new plants emerged in the swampy conditions of this period. The growth of swamp forests removed huge amounts of carbon dioxide from the atmosphere, which resulted in oxygen surplus.
In fact, the atmospheric oxygen levels peaked around 35 percent, compared with 21 percent today according to National Geographic.
Why is the Carboniferous period divided?
According to Brittanica, the Carboniferous Period is formally divided into two – the Mississippian (358.9 to 323.2 million years ago) and the Pennsylvanian (323.2 to 298.9 million years ago) epochs.
This division, according to this article, was established to distinguish the coal-bearing layers as well as the differing stratigraphy on the different continents.
The Mississippian subperiod is characterized by deposition of mostly thick marine limestones because the Mississippian environment of North America was heavily marine. In turn, Mississippian rocks are composed of the remains of crinoids, lime-encrusted green algae, or calcium carbonate shaped by waves.
On the other hand, the Pennsylvanian Period contains mostly sediments and coal seams created by the transgression and regression of the seas caused by glaciation. When the seas covered the swamp forests, the plant deposits did not decay. Instead, the pressure and heat for over millions of years transform the plant debris into coal.
What animals were in the Carboniferous period?
The Carboniferous period witnessed diversity when it comes to animal life. Some of them are:
Although insects have already occupied the terrestrial environment since the Devonian period, they continued to diversify during the Carboniferous age. They developed wings during the Mississippian and one theory, according to NatGeo, suggests that:
…wings may have developed from structures used to regulate temperature (such as by catching the sun’s rays to heat up), or perhaps they evolved from brightly colored signals used to attract mates and ward off rivals. Another idea is that insect wings developed from appendages used to glide between trees in the Carboniferous forests.
By the Pennsylvanian epoch, dragonflies and mayflies flourished and radiated, as well as the other giant creepy-crawlies that now emerged.
The same article suggests that the oxygen level is the reason why creatures grew at such gigantic measurements since the size reached by insects is thought to be limited by the amount of air they are able to breathe.
Just think of six-feet centipedes, three-feet cockroaches, ten-feet millipedes, mammoth scorpions, and dragonflies the size of a seagull.
2. Marine Life
According to New World Encyclopedia, the arthodires (an armored, jawed fish) became extinct immediately during the Mississippian period.
Sharks such as the Cladoselache and Orthacanthus inhabited the waters as well as the Palaeonisciformes, Sarcopterygian, Rhizodonts, Ctenodus, Uronemus, Acanthodes, Cheirodus, and Gyracanthus.
But apart from sharks, corals, crinoids, and arthropods, the Carboniferous isn’t especially well-known for its marine life.
The Carboniferous period is sometimes referred to as the “Age of Amphibians” because it was the time of peak amphibian development.
During the late Devonian period, the tetrapods only recently evolved from lobe-finned fish and were well on their way toward becoming true amphibians. But by the late Carboniferous, amphibians like the Amphibamus and Phlegethontia were now laying their eggs in water and keeping their skin moist – a characteristic of a modern amphibian.
It’s believed that the reptiles evolved through the increasingly cold, dry climate of the late Carboniferous period. During this time, a major biological evolutionary innovation took place – the development of the amniote egg.
The amniote egg is better able to withstand dry conditions, and thus don’t need to be laid in water or moist ground. In turn, it allowed for the further exploitation of the land by certain tetrapods and became the ancestors of birds, mammals, and reptiles.
By the end of the Carboniferous, reptiles had migrated well toward the interior of Pangea and they went on to spawn the archosaurs, pelycosaurs, and therapsids for the next period, the Permian age. And a hundred million years later, the archosaurs then bred the first dinosaurs.
What plants lived during the Carboniferous Period?
Early Carboniferous land plants were very similar to those of the preceding Latest Devonian. However, new groups of flora also appeared at this time.
In fact, according to the Columbia Electronic Encyclopedia, the plant life of the Carboniferous period was extensive and luxuriant, especially during the Pennsylvanian epoch.
The terrestrial environments were dominated by vascular land plants such as the lycopods, sphenopsids, Cordaites, seed ferns, and true ferns. But the most common was the Calamitesis which flourished in drier, more upland environments.
Distinctive features of the Carboniferous Period
1. Orogenic activity (mountain building)
According to Geocraft:
During the Upper Carboniferous Period (a.k.a. Pennsylvanian Period: 286 – 320 mya) nearly all the continents were joined as one giant landmass called Pangea (meaning “all lands”). While massive glaciers existed at the south pole, tropical swampland forests along the equator produced vast peat beds which after deep burial and subsequent heat and pressure were transformed into the Great Bituminous Coalfields of the eastern U.S and western Europe.
With this, the Carboniferous period was a time of active mountain-building. During this time, the southern continents (which remained tied together in the supercontinent Gondwana) collided with North America-Europe along the present line of eastern North America.
This collision gave birth to the Appalachian Mountains and Allegheny Mountains as well as welded the Eurasian plate to Europe along the line of the Ural mountains.
2. The emergence of new species
During the Carboniferous period, more and more species of tetrapods evolved. This age saw some of the early amphibians that began their lives in the water and later moved onto land since the land areas grew.
The first reptiles also appeared in the late portion of the Pennsylvanian epoch. Some reptiles developed leathery skin as they moved to the parts of land that were very dry. Additionally, these same reptiles lay amniotic eggs, as mentioned above.
3. Coal formation
Perhaps the most well-known attribute of the Carboniferous period, coal formation was possible because of the continuous peat accumulation from this era. According to Brittanica, more coal was formed during Pennsylvanian epoch of the Carboniferous period than at any other time in the entire geologic record.
Coals of the said subperiod contain abundant remains of unusual vascular plants, such as the sphenopsids, lycopods (or lycopsids), and seed ferns. On the other hand, seed ferns of the cold Southern Hemisphere became a source of coal deposits as well.
Thanks to the Carboniferous period, we have great coal basins which include the basins of central and eastern North America, England, western and eastern Europe, Russia, Ukraine, China, and Australia.
The end of the Carboniferous Period
According to an article from Denison University, the formation of Pangea signaled the end of the Carboniferous Period. It was marked by a dry climate, disappearance of the coastal coal swamps and changes in plants and animals.
It was one of Earth’s great mass extinction events which affected not only organisms in the ocean but also those on land, thus giving way to the final period of the Paleozoic Era – the Permian Period.
A few thoughts to ponder…
The geological information locked up in the peats of the Carboniferous period is like a book waiting to be opened.
As of now, what we know is limited but with our future scientists, we will surely learn to decrypt our planet’s history and unravel the mysteries written in stone, more than 300 million years ago.