CARBONIFEROUS

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Deadline - 9th October 2025

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CARBONIFEROUS

358.9 - 298.9 mya

The Carboniferous is a period from the Paleozoic era that spans roughly 60 million years, from the end of the Devonian, around 358.9 mya, and up until the start of the Permian period, 298.9 mya. It was a time characterized by immense expansion of terrestrial life, with land arthropods such as arachnids (for example, trigonotarbids and the large scorpion Pulmonoscorpius), myriapods (such as the giant Arthropleura) and insects, especially flying insects, going through major evolutionary radiations, especially more in the late part of the period. The period suffered a minor extinction event at the end of the period that affected both terrestrial and aquatic ecosystems, the so called Carboniferous rainforest collapse, some 305 million years ago, caused by climate change.

Four stratigraphic units were initially consolidated in the early 20th century to create a formalised Carboniferous unit by William Conybeare and William Phillips in 1822 and then into the Carboniferous System by Phillips in 1835, consisted by the Old Red Sandstone (now considered to be from the Devonian), Carboniferous Limestone, Millstone Grit and the Coal Measures. The similarity in successions between the British Isles and Western Europe led to the development of a common European timescale with the Carboniferous System divided into the lower Dinantian (358.9 - 326.4 mya), dominated by carbonate deposition and the upper Silesian (326.4 - 298.9 mya) with mainly siliciclastic deposition. The Dinantian was divided in two stages, which translate to the ICS' (International Commission of Stratigraphy) Tournaisian (358.9 - 346.7 mya) and the Viséan (346.7 - 330.9 mya), noting that with the Dinantian, the Viséan was extended a bit further initially, lasting until 326.4 mya. In North America, geologists recognized a similar stratigraphy for the Carboniferous but divided it into two systems rather than one. These would be the lower carbonate-rich sequence of the Mississippian (358.9 - 323.2) and the upper siliciclastic and coal-rich sequence of the Pennsylvanian (323.2 - 298.9 mya). In Russia, in the 1840s British and Russian geologists divided the Carboniferous into the Lower, Middle and Upper series based on Russian sequences. In 1975, the ICS formally ratified the Carboniferous System, with the Mississippian and Pennsylvanian subsystems from the North American timescale, the Tournaisian and Visean stages from the Western European and the Serpukhovian (330.9 - 323.2 mya), Bashkirian (323.2 - 315.2 mya), Moscovian (315.2 - 307 mya), Kasimovian (307 - 303.7 mya) and Gzhelian (303.7 - 298.9 mya) from the Russian.

Stages can be defined globally or regionally, and for global stratigraphic correlation, the ICS ratify global stages based on a Global Boundary Stratotype Section and Point (GSSP) from a single formation (a stratotype) identifying the lower boundary of the stage; only the boundaries of the Carboniferous System and three of the stage bases are defined by global stratotype sections and points because of the complexity of the geology. The ICS formalizes the Carboniferous, by starting with the Mississipian, which was proposed by Alexander Winchell in 1870 named after the extensive exposure of lower Carboniferous limestone in the upper Mississippi River valley. It begins with the Tournaisian stage, which was named after the Belgian city of Tournai. It is defined by the first appearance of the conodont Siphonodella sulcata within the evolutionary lineage from Siphonodella praesulcata to Siphonodella sulcata. However, in 2006 further study revealed the presence of Siphonodella sulcata below the boundary, and the presence of Siphonodella praesulcata and Siphonodella sulcata together above a local unconformity. The following Viséan stage has a GSSP for its base at the Bed 83 of the sequence of dark grey limestones and shales at the Pengchong section, Guangxi, southern China. It is defined by the first appearance of the fusulinid Eoparastaffella simplex in the evolutionary lineage Eoparastaffella ovalis – Eoparastaffella simplex and was ratified in 2009. To finalize the Mississipian, we have the Serpukhovian stage, which was proposed in 1890 by russian stratigrapher Sergei Nikitin. The Visean-Serpukhovian boundary coincides with a major period of glaciation. The resulting sea level fall and climatic changes led to the loss of connections between marine basins and endemism of marine fauna across the Russian margin. Work is underway in the Urals and Nashui, Guizhou Province, southwestern China for a suitable site for the GSSP of this stage, with the proposed definition for the base of the Serpukhovian as the first appearance of the conodont Lochriea ziegleri. After the Mississipian, we have the Pennsylvanian, which had more extensive volcanic events associated with the formation of the supercontinent Pangaea, allowing for more precise radiometric dating, compared to the earlier Mississipian. The Pennsylvanian begins with the Bashkirian stage, and is later followed by the Moscovian stage, this later one being named after shallow marine limestones and colourful clays found around Moscow, Russia. The next stage is the Kasimovian, whose boundary covers a period of globally low sea levels, resulting in disconformities that cause difficulties in finding suitable marine fauna that can be used to correlate boundaries worldwide. The Kasimovian, therefore, currently lacks a defined GSSP (with potential sites in the southern Urals, southwest USA and Nashui, Guizhou Province, southwestern China being considered), a characteristic it shares with the last stage of the Pennsylvanian, the Gzhelian. The Carboniferous is characterized by the presence of cyclotherms, a succession of non-marine and marine sedimentary rocks, deposited during a single sedimentary cycle, with an erosional surface at its base, having been deposited along continental shelves where the very gentle gradient of the shelves meant even small changes in sea level led to large advances or retreats of the sea. The main period of cyclothem deposition occurred during the Late Paleozoic Ice Age from the late Mississippian to early Permian, when the waxing and waning of ice sheets led to rapid changes in eustatic sea level. As sea levels began to rise, the rivers flowed through increasingly water-logged landscapes of swamps and lakes. As fully marine conditions were established, limestones succeeded these marginal marine deposits. The Carboniferous period is named after the period's infamously large amounts of coal deposits preserved from this time, with the majority of global coal coming from the late parts of this period, as well as from the early Permian. During glacial periods, low sea levels exposed large areas of the continental shelves, forming lowland coastal wetlands. These wetlands were then buried by sediment as sea levels rose during interglacials. There is ongoing debate as to why this peak in the formation of Earth's coal deposits occurred during the Carboniferous, with the dominating theory being that there weren't many organisms that could effectively decompose lignin from woody plants, leading them to accumulate their organic matter over long periods of time.

The continental arrangement of the Carboniferous was composed of Gondwana in the south pole, Laurussia to the northwest, Siberia and Amuria to the north, and to the east there's Kazakhstania, North China and South China forming the northern margin of the Paleo-Tethys, with Annamia laying to the south. The Central Pangaean Mountains were formed during the Variscan-Alleghanian-Ouachita orogeny. Today their remains stretch over 10,000 km from the Gulf of Mexico in the west to Turkey in the east. With the collision between Gondwana and Laurussia, the resulting Variscan orogeny involved a complex series of oblique collisions with associated metamorphism, igneous activity, and large-scale deformation between these terranes and Laurussia, which continued into the Carboniferous. During this period we also observe the Uralian orogeny, which began in the Late Devonian and continued, with some hiatuses, into the Jurassic. Accretion of an island arc around the european and central asian terranes was complete by the Tournaisian, but subduction of the Ural Ocean between Kazakhstania and Laurussia continued until the Bashkirian when the ocean finally closed and continental collision began. During the Carboniferous, Laurussia had extensive coal deposits developed within the cyclothem sequences that dominated the Pennsylvanian sedimentary basins associated with the growing orogenic belt that formed the Central Pangaean mountains. Gondwana, on the other hand, is dominated by large glacial deposits, due to its south polar geography. Along the southeastern and southern margin of Gondwana (eastern Australia and Antarctica), northward subduction of Panthalassa was ongoing. Retreated sea levels in the siberian region were present in the Pennsylvanian and, as the continent drifted north into more temperate zones, extensive coal deposits formed in the Kuznetsk Basin. The Devonian to early Carboniferous Siberian and South Chinese Altai accretionary complexes developed above an east-dipping subduction zone, whilst further south, the Zharma-Saur arc formed along the northeastern margin of Kazakhstania. Continuing strike-slip motion between Laurussia and Siberia led the formerly elongate Kazakhstania microcontinent to bend into an orocline. During the Carboniferous, the Tarim craton lay along the northwestern edge of North China. No sediments are preserved from the early Carboniferous in North China. Cyclothem sediments with coal and evaporites were deposited across the passive margins that surrounded both South China and Annamia (corresponding to southeast Asia).

The Carboniferous is characterized by the formation of the Late Paleozoic Ice Age (LPIA), whose main phase started in the late Viséan, some 335 mya, as the climate cooled and atmospheric CO2 levels dropped, and ended in the early Permian, 290 mya. This main phase consisted of a series of discrete several million-year-long glacial periods during which ice expanded out from up to 30 ice centres that stretched across mid- to high latitudes of Gondwana in eastern Australia, northwestern Argentina, southern Brazil, and central and Southern Africa. Warmer periods with reduced ice volume within the Bashkirian, the late Moscovian and the latest Kasimovian to mid-Gzhelian are inferred from the disappearance of glacial sediments, the appearance of deglaciation deposits and rises in sea levels. The LPIA peaked across the Carboniferous-Permian boundary. Overall, for the Ice Age the GAT (global average temperature) was around 17 °C, with tropical temperatures around 26 °C and polar temperatures around -9 °C. The changing climate was reflected in regional-scale changes in sedimentation patterns. Seasonal melting of glaciers resulted in near freezing waters around the margins of Gondwana, with this being evidenced by the occurrence of glendonite (a pseudomorph of ikaite; a form of calcite deposited in glacial waters) in fine-grained, shallow marine sediments. The later part of the period is characterized by what is referred to as the Carboniferous rainforest collapse, but this was simply a complex replacement of one type of rainforest by another, not a complete disappearance of rainforest vegetation, which nevertheless caused great eccological shifts on land.

As the continents assembled to form Pangaea, the growth of the Central Pangaean mountains led to increased weathering and carbonate sedimentation on the ocean floor, whilst the distribution of continents across the paleo-tropics meant vast areas of land were available for the spread of tropical rainforests; together these two factors significantly increased CO2 drawdown from the atmosphere, lowering global temperatures, increasing ocean pH and triggering the Late Paleozoic Ice Age. The concentration of calcium in seawater is largely controlled by ocean pH, and as the Mg2+/Ca2+ ratio in seawater increased the calcium concentration was reduced. The strontium isotopic composition (87Sr/86Sr) of seawater represents a mix of strontium derived from continental weathering which is rich in 87Sr and from mantle sources e.g. mid-ocean ridges, which are relatively depleted in 87Sr, with these values varying across the period. Oxygen and carbon isotope ratios in seawater also varied regionally, but some large scale trends can still be determined. δ13C rose rapidly from around 0 to 1‰ (parts per thousand) to about 5 to 7‰ in the early Mississippian and remained high for the duration of the Late Paleozoic Ice Age (around 3–6‰) into the early Permian. The mid-Tournaisian positive δ13C and δ18O excursions lasted between 6 and 10 million years and were also accompanied by about 6‰ positive excursion in organic matter δ15N values, a negative excursion in carbonate δ238U and a positive excursion in carbonate-associated sulphate δ34S. During the early Kasimovian there was a short (<1myr), intense glacial period, which came to a sudden end as atmospheric CO2 concentrations rapidly rose.

During the early Carboniferous, the dominating forms of plants were Equisetales (members of the horsetail lineage), Sphenophyllales (scrambling plants), Lycopodiales (club mosses), Lepidodendrales (scale trees), typical ferns, Medullosales (informally designated as "seed ferns" a paraphyletic assemblage of early seed-bearing plants) and the probable stem-gymnosperm Cordaitales clade. Lepidodendrales included genera such as Lepidodendron (with its cone called lepidostrobus), Anabathra, Lepidophloios and Sigillaria. The cladoxylopsids were large trees, that may be ancestral to ferns, first arising in the Devonian, and diversifying much more in the Carboniferous. Many Carboniferous ferns had fronds surprisingly identical to our present's forms. The Equisetales clade was represented, for instance, by the giant Calamites that could reach a trunk diameter of 30 - 60 centimeters and a height of up to 20 meters. The late Carboniferous genus Cordaites was a tall plant ranging from 6 to over 30 meters in height, with strap-like leaves, being distantly related to modern cycads, ginkgos and conifers, having catkin-like reproductive organs named cardiocarpus, that bore ovules/seeds. During the Viséan, a surge of diversity of brachiopods and fusilinid foraminiferans began, and extended through the end of the Carboniferous, however nektonic conodont and cephalopod diversity declined. Some genera that lived in the Carboniferous are still alive in our timeline's present. The microscopic shells of radiolarians are found in cherts of this age in the Culm of Devon and Cornwall, and in Russia, Germany and elsewhere. Sponges are known from spicules and anchor ropes, and include various forms such as the calcisponges Cotyliscus and Girtycoelia, the demosponge Chaetetes, and the genus of unusual colonial glass sponges Titusvillia. Brachiopods were also abundant, including the productidans, some of which reached very large for brachiopod size and had very thick shells (for example, the 30 centimeter wide Gigantoproductus), while others like Chonetes were more conservative in form. Among mollusks, bivalves continued to become more abundant and eccologically relevant. Trilobites, on the other hand, were steadily declining, with proetidans being the only ones to be found in the fossil record. Echinoids such as Archaeocidaris and Palaeechinus were also present in the Carboniferous seas. In freshwater regions, eurypterids were present, many of them being amphibious. On land, some terrestrial arthropods grew massively, such as Arthropleura, a 2.6 meter long member of the myriapods and the largest terrestrial invertebrate preserved in the fossil record of our timeline's past. Incredibly well preserved insect fossils have been found from coal fields, such as Archaeoptilus, from the Derbyshire coalfield, with a large 4.3 centimeter wing preserved and some specimens of Brodia that still exhibit traces of brilliant wing colors. In fossilized tree trunks, land snails, such as Archaeozonites and Dendropupa, have been found, showing that shelled gastropods have started to experiment on life inland. Carboniferous fish were very diverse, sporting varied dentitions, and therefore ecologies, ranging from the piercing teeth of symmoridans to the cycloid cutting teeth of petalodonts. Most cartilaginous fish of the period were marine, but some, like xenacanths and the strange paddlefish-like Bandringa, inhabited the freshwater courses and coal swamps. Most species of Carboniferous marine fish are described by just teeth, fin spines or dermal ossicles, with only smaller freshwater fish being preserved whole, limiting our understanding of their broader physical appearance. Some well preserved fish show that some had extraordinary features, such as with Stethacanthus, which had an unsual fin probably used for mating rituals. Amphibians were abundant in the Carboniferous, with some large predatory aquatic forms (informally named "labirynthodonts) reaching sizes of over 2 meters long. Smaller tetrapods included the lepospondyls, some being as long as a human hand. These tetrapodomorphs ranged from aquatic (such as Loxomma, Eogyrinus, Anthracosaurus and Proterogyrinus), semi-aquatic (such as Ophiderpeton, Amphibamus and Hyloplesion) and terrestrial (such as Dendrerpeton and Tuditanus). Cooler and drier conditions began to develop in the later parts of the period, leading to the evolution of the amniotes, which had specializations for such conditions, such as the development of the amniotic egg, that keeps the developing embryo protected from the dry exterior, allowing the eggs to be laid on land.

The first 15 million years or so of the Carboniferous period are characterized by what is called the Romer's gap, where very few terrestrial fossils are preserved, perhaps caused by a combination of an eccological collapse and a lack of proper sedimentological conditions for fossil preservation. The most well documented extinction event from the period is the Carboniferous rainforest collapse, caused by intense glaciation and a drop in sea levels.

main source: Wikipedia

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