Carboniferous

The Carboniferous is a geologic period and system of the Paleozoic era that lasted 60 million years, from the end of the Devonian Period 358.86 million years ago to the beginning of the Permian Period, 298.9 million years ago. It is the fifth period of the Phanerozoic eon. In North America, it is often split into two parts: the earlier Mississippian and the later Pennsylvanian.

The name Carboniferous means "coal-bearing," referring to the many coal beds formed worldwide during this time. It was named by geologists William Conybeare and William Phillips in 1822 after studying British rock layers.

This period was a time when life on land became well established. Four-limbed vertebrates, including early amphibians and the first reptiles and mammal ancestors, appeared and thrived. Insects, spiders, and other land animals also evolved rapidly. Huge forests and swamps covered the land, and these eventually turned into the coal we find today. Towards the end of the Carboniferous, the continents came together to form Pangaea.

Etymology and history

The name "Carboniferous" started being used in the late 1700s. It was first used by an Irish geologist named Richard Kirwan in 1799. Later, in 1811, another scientist named John Farey Sr. used it in a book about coal rocks. By 1822, two scientists, William Conybeare and William Phillips, put these rock layers into a formal group called the Carboniferous System.

Geologists in different places grouped these layers in different ways. In North America, they split the Carboniferous into two parts: the Mississippian and the Pennsylvanian. In Europe, they divided it into the lower Dinantian and the upper Silesian. Over time, international teams agreed on stages like Tournaisian, Viséan, and others to help organize these layers worldwide.

Geology

Stages can be defined globally or locally. For global comparison, special points mark the start of each stage.

The Mississippian name comes from rocks found near the Mississippi River. During this time, seas connected different parts of the world.

The Pennsylvanian name comes from coal-rich rocks in Pennsylvania. As continents moved and glaciers grew, climates changed a lot.

Cyclothems are layers of rock formed when sea levels rose and fell. These changes happened because of ice ages. When ice grew, sea levels fell. When ice melted, seas rose again.

Coal formed from plants that grew in wet, swampy areas. These swamps were buried under sand and mud as seas rose, turning the plant material into coal over millions of years. Most of the world’s coal comes from this time.

Palaeogeography

During the Carboniferous, the continents moved as pieces of Earth's crust shifted. Pangea, a huge supercontinent, began to form when several continents came together. The continents arranged themselves around a large ocean called the Paleo-Tethys, with another huge ocean, the Panthalassic Ocean, beyond.

Important mountain ranges formed as continents collided. The Central Pangean Mountains grew when landmasses pressed together. These mountains stretch from the Gulf of Mexico to Turkey today. Another mountain range, the Urals, formed where Siberia met other landmasses. These events changed the shape of Earth and created new landscapes.

Climate

The Carboniferous period had a climate shaped by a long ice age called the Late Paleozoic Ice Age. This was the biggest and longest ice age of its time. During this time, temperatures dropped because the amount of a gas in the air called CO₂ went down. This happened because more plants were buried and there was less oxygen in the oceans.

The coldest part of this ice age started in the late Visean stage. The climate got colder, and CO₂ levels dropped even more. This made sea levels fall and created gaps in the rock record. There were many short, cold times when ice grew in many places in the southern part of the world.

Warmer times happened too, especially in the Bashkirian, late Moscovian, and latest Kasimovian to mid-Gzhelian stages. We know this because we find rocks from warmer times and see that sea levels rose.

In the early Kasimovian, there was a short time when the gas levels in the air dropped very low. Then they quickly went up again, making the climate warmer. This might have been because of lots of volcanic activity or because less plant material was being buried.

The ice age was at its peak at the end of the Carboniferous and beginning of the Permian. There were big sheets of ice across many southern lands. As mountains rose and wore down, CO₂ levels dropped even more and stayed low for about 10 million years.

Temperatures

The temperature changes in the Carboniferous matched the ice age stages. During the coldest time, the world’s average temperature was about 13°C (55°F). In the tropics, it was around 24°C (75°F), and in the poles, it was about -23°C (-10°F). During a warmer time early in the Carboniferous, the world’s average temperature was about 22°C (72°F), with tropics at 30°C (86°F) and poles at 1.5°C (35°F). Overall, during the ice age, the world’s average temperature was about 17°C (62°F), with tropics at 26°C and poles at -9°C (16°F).

Atmospheric oxygen levels

Scientists use different ways to figure out how much oxygen was in the air long ago. These include looking at charcoal from wildfires, tiny pieces of salt, how much carbon was buried, and other clues from rocks. These methods show that oxygen levels went up during the Carboniferous, from about 15-20% at the start to as high as 25-30% during the time. But it wasn’t a steady rise—there were ups and downs.

Effects of climate on sedimentation

The changing climate changed how sediments were laid down. In warmer times, certain types of rocks formed deep in the oceans around some continents. As the climate cooled, these rocks formed only in very shallow water. Later, as ice sheets grew and shrank, layers of different rocks were deposited on land and shelves.

When glaciers melted each season, very cold water formed around the edges of a big landmass. We know this because we find special minerals in the rocks there. The grinding of glaciers also created lots of fine dirt that was blown by the wind to form big deposits in some places.

Effects of climate on biodiversity

The cold period was thought to be hard on sea life, causing many species to disappear. But newer studies show that the quick changes in climate and environment might have helped new sea species to appear fast.

On land, the changing climate caused forests to change between wet and dry areas. This led to new types of four-legged animals appearing. During a very cold time, some wet forest plants disappeared, and the forests changed. This happened because the gas levels in the air dropped too low. This change wasn’t the end of forests—it was more like one kind of forest replacing another.

At the end of the Carboniferous, as CO₂ levels dropped and it became drier, forests changed forever to more dry woodland plants. Land animals developed new ways to live on land, and many new types of animals with eggs that could be kept safe from drying out appeared.

Geochemistry

As the continents came together to form Pangea, big mountains grew. This brought more rain and wore away rocks. This added minerals to the ocean floor and helped huge rainforests grow. These changes took carbon from the air, cooling the Earth and changing the ocean’s chemistry.

During this time, the balance of minerals in seawater changed. One big change was in the amount of magnesium and calcium. As the oceans changed, some sea creatures found it easier to build their shells and reefs. Another change was in strontium, a mineral that comes from weathering rocks on land and deep ocean processes. Strontium levels changed over millions of years as mountains grew and weather patterns shifted.

We can also study oxygen and carbon in ancient seawater using fossils. These records show that Earth’s climate cooled in stages during this time, with big changes happening over millions of years. Some of these changes happened when many plants were buried, taking carbon from the air and cooling the planet. Other times, drier climates and fewer rainforests let carbon build up again in the air.

Life

Plants

Early Carboniferous land plants looked similar to those from the Late Devonian, but new groups also appeared. The main early Carboniferous plants included horse-tails, scrambling plants, club mosses, scale trees, ferns, seed ferns, and Cordaitales. These plants stayed important, but during the late Carboniferous, new groups like cycads, another group of seed ferns, and Voltziales appeared.

The Carboniferous lycophytes, such as scale trees, were huge with trunks up to 30 meters high. Ferns from this time look almost identical to living ferns today. Giant forms of Equisetales, like Calamites, had trunks up to 20 meters tall. Cordaites were tall plants with strap-like leaves, related to cycads and conifers.

Marine invertebrates

The oceans had many groups of marine invertebrates, including Foraminifera, corals, Bryozoa, Ostracoda, brachiopods, ammonoids, and echinoderms. Brachiopods and fusilinid foraminiferans became very diverse. Genera like Fusulina were abundant in regions now known as Russia, China, Japan, and North America. Sponges, reef-building corals, and other invertebrates also thrived.

Freshwater and lagoonal invertebrates

Freshwater environments had various bivalve molluscs and crustaceans. Eurypterids, often called sea scorpions, were diverse and included large forms like Hibbertopterus. Some of these creatures could live both in water and on land.

Terrestrial invertebrates

Fossil remains of insects, myriapods, and arachnids from the Carboniferous show they were well-developed and numerous. Some grew to large sizes, like the millipede-like Arthropleura, which was up to 2.6 meters long. Winged insects appeared in the mid-Mississippian, including large predatory griffinflies like Meganeura, with a wingspan of about 75 cm.

Fish

Many fish lived in Carboniferous seas, especially sharks and their relatives. Some had special teeth for grinding shells. Other fish had piercing teeth or unique shapes. Freshwater fish were also abundant, including genera like Ctenodus and Gyracanthus. The decline of placoderms allowed many new fish groups to evolve and thrive.

Tetrapods

Carboniferous amphibians were diverse and common, some growing as long as 6 meters. Amniotes, including the earliest reptiles and synapsids, prospered due to adaptations like the amniote egg. These adaptations allowed them to live in drier environments. Reptiles and synapsids diversified quickly, setting the stage for their future dominance.

Fungi

As plants and animals grew larger and more abundant, land fungi also diversified. All modern classes of fungi were present by the late Carboniferous. Marine fungi continued to occupy the oceans.

Extinction events

Romer's gap

Main article: Romer's gap

The first 15 million years of the Carboniferous had very few fossils from land animals. Scientists think this might be because of a drop in oxygen in the air. During this time, some older fish-like animals disappeared, and new types of amphibians began to appear.

Carboniferous rainforest collapse

Near the end of the Carboniferous, a big change happened. The huge tropical rainforests suddenly shrank because the climate turned from warm and wet to cool and dry. This change was likely caused by large glaciers forming and sea levels falling. The new, drier conditions were hard for rainforests and the animals living there. The forests became smaller and more isolated. Amphibians lost many kinds of species, while reptiles began to thrive.