The Carboniferous Period

Article by: J. D. Dixon
Edited by: Harry T. Jones

The Carboniferous is the period of time spanning between 358.9 ± 0.4 million years ago and 298.9 ± 0.15 million years ago. It is the fifth period of the Palaeozoic Era, and was described by William Daniel Conybeare and William Phillips. During the Carboniferous, the two continental masses Gondwana and Larussia collided, closing the Rheic Ocean and forming the supercontinent Pangaea, which went on to cover the Earth for the next 150 million years. Mountain building events, such as the Variscan and Ural orogenies, started or continued to occur during this time.

Carboniferous means ‘carbon-bearing’, a name alluding to the plentiful coal deposits from this period. Coal is a type of sedimentary rock, and has been used as an important global resource since the Industrial Revolution. It is known to form when plant material in swamp-like conditions does not decay, but is instead compacted by sedimentation over extensive periods of time. Therefore, the presence of this rock shows that the terrestrial environment of the Carboniferous must have been ruled by plant life, with a majority of the planet covered in dense swamps. Fossils from this time also show that this was the case. Some of these plants are even known to exhibit features similar to their Devonian predecessors. Giant lycopod plants, such as Lepidodendron and Lepidophloios, dominated vast expanses of land. Other easily distinguishable structures of this time are plants of the genus Calamites, a common fossil found across many UK localities. These Carboniferous plants were much larger and much more widespread than those in the Devonian, and so photosynthesis soared. Oxygen was pumped into the atmosphere in large concentrations, while carbon dioxide started to be sequestered into the biosphere. In the Early Carboniferous, global atmospheric carbon dioxide levels were around 1,500 parts per million, astronomical in comparison to modern values. However, carbon dioxide levels had decreased to around 350 parts per million by the Middle Carboniferous.

A depiction of the Braidwood Biota (originating from the Mazon Creek fossil beds) during the Carboniferous. It shows plants like Sigillaria, Lepidodendron, and Calamites. A relative of the giant arthropod Arthropleura, which is known from indirect evidence at this locality, crawls across the forest floor. Artwork by Franz Anthony. Available at 252 MYA.

Fossils from the later Carboniferous reveal how life differed from that seen in prior periods, however, there is a massive issue with obtaining earlier Carboniferous fossils. The period starts with Romer’s Gap, a 20 million year break in the fossil record, for which there is little fossil material available. A vast diversity of organisms is seen in the rocks after this break, so the groups during this time must have undergone significant radiations. Sadly, we might never know what these forms of life may have looked like, but substantial insights have been gained from other Carboniferous fossils.

In the seas lived invertebrates such as bivalves, brachiopods, cephalopods, corals, crinoids, and some trilobites. Advanced jawed fish (e.g. chondrichthyans and acanthodians) and ray finned fish were common, and sharks diversified. Arthropods continued to roam the Earth, but they got much bigger. Giant invertebrates such as Meganeura and Arthropleura developed, while the terrestrial vertebrates that began their journey landward in the Devonian continued to adapt. Vertebrate genera such as Proterogyrinus, Amphibamus, and Hyloplesion remained aquatic or semi-aquatic. However, genera such as Dendrerpeton and Pholiderpeton are thought to have been fully terrestrial, and probably ate fish or insects.

The Carboniferous amniote Dendromaia unamakiensis from Nova Scotia. Its remains may actually represent the oldest account of parental care in the fossil record. Artwork by Henry Sharpe.

The development of more advanced eggs was also critical to continued terrestrialisation. Amphibious reproduction is the method thought to have been used by many Devonian vertebrates, and is still used today by animals such as frogs and toads. This method requires proximity to water, as it keeps the eggs from drying out. However, this proximity isn’t possible for more inland animals. Therefore, the amniotes (a group of terrestrial vertebrates emerging in the Carboniferous) developed an egg structure equipped with more membranes and a protective shell, which could keep an embryo safe while on land. They also developed new ways of reproducing. Their amphibious predecessors needed to release gametes into the water in order to fertilise eggs, again meaning water pools had to be close, but the amniotes developed internal sexual intercourse, meaning they could procreate wherever they lived.

The Carboniferous ended with the Carboniferous Rainforest Collapse (CRC), a dramatic decline of all rainforest ecosystems. This is thought to have been caused by many regions becoming drier, yet the precise cause of this aridification is unclear. It may have been driven by a short-lived glacial episode, or moderate global warming. The CRC was a stepwise decline. First there was a gradual rise of opportunistic ferns, followed by an abrupt extinction of the dominant lycopsids and a new dominance of treeferns. Finally, the rainforests either disappeared or became isolated. The CRC is associated with the development of new feeding strategies in vertebrates as a result of habitat breakup and the subsequent restriction of resources. The CRC was followed by a lack of dispersal barriers succeeding the decline of plant life, and the further diversification of amniotes seen in the Permian.

Image References
[1] A depiction of the Braidwood Biota (originating from the Mazon Creek fossil beds) during the Carboniferous. It shows plants like Sigillaria, Lepidodendron, and Calamites. A relative of the giant arthropod Arthropleura, which is known from indirect evidence at this locality, crawls across the forest floor. Artwork by Franz Anthony. Available at 252 MYA.
[2] The Carboniferous amniote Dendromaia unamakiensis from Nova Scotia. Its remains may actually represent the oldest account of parental care in the fossil record. Artwork by Henry Sharpe.

Information References and Further Sources
[1] Blakey, R. C. and Wong, T. E. (2003). ‘Carboniferous-Permian paleogeography of the assembly of Pangaea’, Proceedings of the XVth International Congress on Carboniferous and Permian Stratigraphy, 10, pp. 443-456). Accessed 5th February 2020. Click Here.
[2] Bolt, J. R. (1979). ‘Amphibamus grandiceps AS A JUVENILE DISSOROPHID: EVIDENCE AND IMPLICATIONS’, in Mazon Creek Fossils. Academic Press. pp. 529-563. Accessed 5th February 2020. Click Here.
[3] Chart drafted by K. M. Cohen, D. A. T. Harper, P. L. Gibbard, and J.-X. Fan (c) International Commission on Stratigraphy, August 2018. To cite: Cohen, K. M., Finney, S. C., Gibbard, P. L. & Fan, J.-X. (2013; updated). The ICS International Chronostratigraphic Chart. Episodes 36: 199-204. Accessed 28th January 2020.
[4] Clack, J. A. (2008). ‘The stapes of the Coal Measures embolomere Pholiderpeton scutigerum Huxley (Amphibia: Anthracosauria) and otic evolution in early tetrapods’, Zoological Journal of the Linnean Society, 79 (2), pp. 121-148. Accessed 5th February. Click Here.
[5] Conybeare, W. D., and Phillips, W. (1822). Outlines of the Geology of England and Wales: With an Introductory Compendium of the General Principles of that Science, and Comparative Views of the Structure of Foreign Countries. W. Phillips. Accessed 5th February 2020. Click Here.
[6] Dorling Kindersley. (2009). ‘Carboniferous’ in Prehistoric. Great Britain: Dorling Kindersley Limited. pp. 141-169.
[7] Dunne, E. M., Close, R. A., Button, D. J., Brocklehurst, N., Cashmore, D. D., Lloyd, G. T., and Butler, R. J. (2018). ‘Diversity change during the rise of tetrapods and the impact of the ‘Carboniferous rainforest collapse’’, Proceedings of the Royal Society B: Biological Sciences, 285 (1872), pp. 20172730. Accessed 28th January 2020. Click Here.
[8] Godfrey, S. J., Fiorillo, A. R., and Carroll, R. L. (1987). ‘A newly discovered skull of the temnospondyl amphibian Dendrerpeton acadianum Owen’, Canadian Journal of Earth Sciences, 24 (4), pp. 796-805. Accessed 5th February 2020. Click Here.
[9] Hedge, J., Shillito, A., Davies, N., Butler, R., and Sansom, I. (2019). ‘Invertebrate trace fossils from the Alveley Member, Salop Formation (Pennsylvanian, Carboniferous), Shropshire, UK’, Proceedings of the Geologists’ Association, 130 (1), pp. 103-111. Accessed 28th January 2020. Click Here.
[10] Holmes, R. (1984). ‘The Carboniferous amphibian Proterogyrinus Scheelei Romer, and the early evolution of tetrapods’, Philosophical Transactions of the Royal Society of London. B: Biological Sciences, 306 (1130), pp. 431-524. Accessed 5th February 2020. Click Here.
[11] Lund, R., and Poplin, C. (1999). ‘Fish diversity of the Bear Gulch Limestone, Namurian, Lower Carboniferous of Montana, USA’, Geobios, 32 (2), pp. 285-295. Accessed 5th February 2020. Click Here.
[12] Olori, J. C. (2013). ‘Morphometric analysis of skeletal growth in the lepospondyls Microbrachis pelikani and Hyloplesion longicostatum (Tetrapoda, Lepospondyli)’, Journal of Vertebrate Paleontology, 33 (6), pp. 1300-1320. Accessed 5th February 2020. Click Here.
[13] Roberts, J., Hunt, J. W., and Thompson, D. M. (1976). ‘Late Carboniferous marine invertebrate zones of eastern Australia’, Alcheringa: An Australasian Journal of Palaeontology, 1 (2), pp. 197-225. Accessed 5th February 2020. Click Here.
[14] Sahney, S., Benton, M. J., and Falcon-Lang, H. J. (2010). ‘Rainforest collapse triggered Carboniferous tetrapod diversification in Euramerica’, Geology, 38 (12), pp. 1079-1082. Accessed 5th February 2020. Click Here.
[15] Ward, P., Labandeira, C., Laurin, M., and Berner, R. A. (2006). ‘Confirmation of Romer’s Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization’, Proceedings of the National Academy of Sciences of the United States of America, 103 (45), pp. 16818-16822. Accessed 28th January 2020. Click Here.