⭐ Carboniferous Period

We are talking about the fifth period of the Paleozoic Era that happened after the Devonian and before the Permian. It extended from 359 to 299 million years BC. It was highlighted by large swampy forests that ended up forming the coal deposits that we exploit today.

It was named in honor of the large coal deposits that abound in England. They are also prevalent in Europe, Asia, North America, and the Midwest. Although it is the way in which this period of time is universally recognized, in the United States they added two subdivisions.

This is from the Mississippian and Pennsylvanian periods. It is a partition with which the carboniferous layers are differentiated from the limestone layers, along with the different stratigraphies on each continent.

Characteristics of the Carboniferous

In addition to it being the era of coal development, there were notable biological, climatic, and geological events of this time. One of these was the emergence of amniote eggs in birds. The ancestors of birds were able to lay eggs in the ground without having to fear that they would dry out.

In the field of geology, in the late Carboniferous the continents of Laurasia and Gondwana collided, which developed important mountain ranges such as the Appalachian Mountains and the mountains in the United Kingdom. On the other hand, the collision of Siberia and Eastern Europe created the Ural Mountains of Russia.

Temperatures were mild during this period, as can be seen in the decrease in lycopods and large insects. On the other hand, this promoted the increase of tree ferns.

Mississippian and Pennsylvanian

There are definite characteristics that differentiate the Mississippian and Pennsylvanian sub-era. The Mississippi environment of North America had very marine characteristics. That is why its rocks are limestone and abundant in remains of crinoids or green algae embedded in lime or calcium carbonate.

The environment of the Pennsylvanian changed from terrestrial and marine, due to the regression of the seas during the glaciation.

In this way, a great variety of plant material provided by the forests was formed. This did not decompose when it was covered by the sea. Then the pressure and heat transformed it into charcoal.

Life during the Carboniferous

At the beginning of the Carboniferous, a uniform, tropical and humid climate abounded. The seasons were not well defined, something that can be observed in fossil and current plant morphology.

The vegetation at that time resembled that of the tropical areas we know today. For example, they lack growth rings, which is a sign of a uniform climate. This may have been due to the large expanse of ocean that covered almost the entire globe. The exception to this was Pangea, which existed between the Late Paleozoic and the Triassic.

The marine waters were warm and shallow and frequently flooded the continents. There were life forms that served as filter feeders such as bryozoans, in particular the phenestelids, which were abundant. The seabed belonged to the brachiopods.

On the other hand, trilobites became increasingly scarce, as opposed to the foraminifera that were thriving successfully. Armored fish became extinct and were replaced by others with modern characteristics.

Two large ice sheets at the south pole locked up large amounts of ice water. With so much water taken out of the water cycle, sea levels began to drop. This increased the development of terrestrial habitats. Such sea level fluctuations can be observed in streak patterns in the rocks of alternating layers of shale and coal.

Plant Life

Many spectacular plants that developed in the Carboniferous became extinct at the end of the Paleozoic Era.

Some notable ones were the Neuropteris whose shape of leaves were associated with ferns with seeds that resembled cycads. In the same way there was the Lepidodendron stenbergii which was a tree with large scales. These in particular became extinct at the end of the Middle Pennsylvanian.

The uplift of the continents caused a transition to a more terrestrial environment during the Pennsylvanian. Seedless plants, such as lycopsids, flourished in the swamp forests and were the main carbon source for the charcoal formed in that era.

Lycopods suffered a major extinction event due to the drought that resulted from the glaciation during the Pennsylvanian. Ferns and sphenopsids prospered and relatives of conifers appeared.

animal life

The uplift that occurred near the Middle Mississippian caused extensive erosion and an increase in floodplains and deltas. Around these deltas it housed fewer corals, crinoids, blastoids, cryozoans and bryozoans that were abundant in the Carboniferous.

Instead, freshwater clams appeared and gastropods, bony fish and sharks diversified. Furthermore, as the continents moved closer together to form the continent of Pangea, the coastlines shrank. This harmed the diversity of marine life where continental waters were shallow.

Many groups that appeared in the Carboniferous would give rise to groups that would dominate in the Permian and Mesozoic. An example of this was the Amphibiamus lyelli which was a primitive temnospondyl. The first amphibian-like tetrapods grew to the size of crocodiles in the Permian and Triassic.

The first land snails also appeared, at the same time as winged insects such as dragonflies and mayflies. Added to this is the fact that millipedes, scorpions and spiders acquired importance for the ecosystem.

Aridity and the increase in terrestrial habitat increased the importance of the amniotic egg in reproduction. The example of this is the amniote fossil of Hylonomus which was a species of lizard, with a light structure, strong jaws and thin limbs.

Basal tetrapods diversified as well. Predators with long snouts, short, extended limbs, and flattened heads appeared, such as temnospondyls, including the Amphibiamus.

Anthracosaurs, basal tetrapods and amniotes with deep skulls appeared in the Carboniferous. They were followed by the diapsids, which divided into marine reptiles, lizards, snakes, archosaurs and crocodiles. Synapsids first appeared, presumably alongside anapsids.

Carboniferous stratigraphy

The appearance or extinction of certain specimens of fauna has served to recognize the limits between periods or even eras. An example of this is the beginning of the Carboniferous, which is separated from the Devonian through the appearance of the conodont. Siphonodella sulcataalso known as Siphondela duplicata.

Conodonts are fossils whose appearance is similar to the teeth or even the jaws of primitive fish. These looked like hagfish or eel.

On the other hand, the limit between the Carboniferous and the Permian is given by the appearance of the fusulinid foraminifera Shaeroschwagerina fusiformis in Europe and Pseudoschwagerina beeder in North America.

Fusilinids were giants among protists and could reach a centimeter in length. They were so abundant that they formed important deposits which are called rice rocks. They were named so because these beings resembled grains of rice.

The Mississippian

The Mississippian subsystem is separated from the Pennsylvanian with the appearance of the conodont Declinghathodus nuduliferus which belonged to the ammonoid genus Homoceras and foraminifera Millerella pressa and Millerella marblensis. Although these markers apply only to marine deposits.

Another factor that is considered to indicate this difference is the break in the flora due to the transitional changes from a marine environment to a more terrestrial one.

The Mississippian stratigraphy is characterized by shallow water limestones, some of which are partly composed of organisms. Mostly these were the rest of the crinoids that thrived in shallow seas.

Other limestones that predominated then were salt shales, which included carbonated mud from green algae. Oolitic limestones that had calcium carbonate in concentric spheres, which were produced by wave energy, also originated there.

Also found in the strata of the Mississippi, although they are not as common, are sandstones, which are sedimentary rocks composed of quartz sand and cemented with silica or calcium carbonate. In the same way, it is possible to find siltstones, which are rocks made up of hardened silt.

Charcoal

Coal beds that can contain 11 to 12 meters thick are characteristic of the late Carboniferous. They originated from seedless vascular plants that existed in the tropical swamp forests of Europe and North America.

The dead plants did not decompose completely and were transformed into peat in the swampy forests. At the time when the sea covered the swamps, marine sediments covered the peat.

Eventually, the heat and pressure turned them into the coal we know today. Some plant waste that was transformed into fossils, but not into coal, is usually found within the coal layers.

Evidence and fossils

Multiple transgressions and regressions of the Pennsylvanian seas can be observed in the rocks. They can even be counted, because they leave a revealing sequence of layers.

Some of these are indicative of sea level rise. Such is the case of sandstone that is mostly abundant on the beach, silty slate or siltstone common in the tide, limestone that is abundant in lagoons, subclay and charcoal that is common in terrestrial swamp forests.

As the sea recedes it is possible to observe a certain level of shale that is common in the tide near the coast, limestone from a shallow marine environment and black shale that is not common because it would come from a deep sea.

Index fossils are remains of plants and animals that characterize a well-defined period of time. They occur over a wide geographic range. Those of marine life characterized the Mississippian. This is because shallow continental seas covered what is now the United States.

These fossils include solitary corals, Syringoporas, and tubular colonial corals. Other fossil colonial corals included were the Stelechophyllum and Siphonodendrum. Due to the worldwide distribution of conodonts, they are used internationally to date Mississippian rocks.

The Pennsylvanian

The fossils used to identify the Pennsylvanian are fusulinid foraminifera, pollen and spores from the charcoal forests that predominated in that…