Coal forest

Etching depicting some of the most significant plants of the Carboniferous.

Coal forests were the vast swathes of wetlands that extended over much of the tropical land areas during late Carboniferous (Pennsylvanian) and Permian times.^[1][2] These forests got their name because they accumulated enormous deposits of peat which later changed into coal. As much of the carbon in this enormous amount of peat came from photosynthesis splitting existing carbon dioxide, it is thought that the accompanying split-off oxygen went into the atmosphere, greatly increasing its ppO₂: one estimate says to about 35%, making it easier for animals to breathe air as a source of oxygen, as seen in the size of Meganeura compared to modern dragonflies.

Coal Forests covered tropical Euramerica (Europe, eastern North America, northwesternmost Africa) and Cathaysia (mainly China). Climate change devastated these tropical rainforests during the Carboniferous Period. The Carboniferous Rainforest Collapse was caused by a cooler, drier climate that initially fragmented, then collapsed the rainforest ecosystem.^[2] During most of the rest of Carboniferous times, the Coal Forests were mainly restricted to refugia in North America (such as the Appalachian and Illinois coal basins) and central Europe. At the very end of the Carboniferous period, however, the Coal Forests underwent a resurgence, expanding mainly in eastern Asia, notably China; they never recovered fully in Euramerica. The Chinese Coal Forests continued to flourish well into Permian times. This resurgence of the Coal Forests in very late Carboniferous times seems to have coincided with a lowering of global temperatures and a return of extensive polar ice in southern Gondwana, perhaps due to lessening of greenhouse effect due to massive coal deposition abstracting much carbon dioxide from the atmosphere.

Environment

Coal forest of tree ferns and lycopod trees

The coal forests seem to have been areas of flat low-lying swampy land with rivers flowing through from higher drier land.^[3] The rivers, when they flooded, dropped most of their silt near the river, gradually building up natural levees. Sometimes areas subsided and became lakes; sometimes lakes silted up and became land. Sometimes a high flood would break a levee and bury an area in new silt. Sometimes a river would change its course. Sometimes the forest dried enough to be set on fire by lightning; the fusain component of coal is derived from charcoal left by forest fires.

Plant Life

There seems to have been a rich and varied flora, with sets of species for each type of growing condition. The most varied flora seems to have been levee vegetation, with many species of trees, bushes, creepers, etc. Calamites thickets seem to have favored the edges of lakes and waterways. There seem to have been genera of lycopsids specialized for various roles: Paralycopodites as a pioneer on lakes newly silted shallow enough for land vegetation to start; Diaphorodendron later when the ground had become peaty. Other species specialized in re-settling land which had been briefly deforested by flooding: Synchysidendron and Lepidodendron in mineral-soil areas and Lepidophloios in peat areas. Cordaites may have favored drier areas of the swamp. One author thinks that Sigillaria favored the intermediate areas between levee habitat and swamp habitat. In the later part of this period tree ferns tended to take over from lycopsid trees.

Animal life

Animal life was invertebrates (particularly insects), fish, labyrinthodont amphibians, and early reptiles. One evidence for plant-eating is tree lycopsid tracheids found in an Arthropleura's gut. Amphibians were widespread but once the coal forests fragmented, the new environment was better suited to reptiles, which became more diverse and even varied their diet in the rapidly changing environment.^[2]

Species list

Some of the characteristic plants of the Coal Forests were:

• Sigillaria
• Lepidodendron
• Calamites
• pteridosperms

Genera recorded in Great Britain include^[3]:

• Pteridosperm leaves: Alethopteris, Callipteridium, Cyclopteris (leaf bases), ?Desmopteris, Dicksonites, Eusphenopteris, Fortopteris, Hymenophyllites, Karinopteris, Laveinopteris, Linopteris, Lonchopteris, Lyginopteris, Macroneuropteris, Margaritopteris, Mariopteris, Neuralethopteris, Neuropteris, Odontopteris, Palmatopteris, Paropteris, Reticulopteris
• Pteridosperm spore organs: Aulacotheca (male), Boulaya, Potoniea (male), Whittleseya (male)
• Pteridosperm seeds: Gnetopsis, Hexagonocarpus, Holcospermum, Lagenospermum, ?Polypterocarpus, Rhabdocarpus, Trigonocarpus
• Fern fronds: Aphlebia, Bertrandia, Corynepteris, Crossotheca, Cyathocarpus, Lobatopheris, Oligocarpia, Pecopteris, Polymorphopteris, Renaultia, Sphyropteris, Sturia, Zeilleria
• Tree-fern leaves: Caulopteris
• Tree-fern stems: Artisophyton, Megaphyton
• Lycopsid tree stems and leafy shoots: Cyperites, Lepidodendron, Ulodendron
• Lycopsid tree stems: Asolanus, Bothrodendron, Cyclostigma, Lepidophloios, Sigillaria, Sublepidophloios, Syringodendron (de-barked)
• Lycopsid reproductive parts: Flemingites, Lepidodostrobus, Lepidodostrobophyllum (sporophylls), Sigillariostrobus
• Lycopsid (herbaceous) stems: Lycopodites, Selaginellites
• Sphenopsid leaves: Annularia, Asterophyllites
• Sphenopsid stems: Calamites
• Sphenopsid reproductive parts: Bowmanites, Calamostachys, Macrostachya, Palaeostachya
• Cordaite leaves: Cordaites
• Cordaite stem pith case: Artisia (pith cast)
• Cordaite seeds: Cordaicarpus
• Cordaite cones and seeds: Cordaitanthus
• May be progymnosperm: leaves: Noeggerathia
• Conifer leaves: Walchia
• Seeds: Carpolithus, Cornucarpus, Samaropsis

Palaeontologists have described many species for some of these genera, e.g. (in Britain): Sigillaria 33, Lepidodendron 19, Alethopteris (pteridosperm leaves) 11, Calamites 8.

Some easily identified species occur over a wide area but only for a small part of the coal-forming period, and are thus useful as zone fossils.

References

1. ^ Cleal, C. J. & Thomas, B. A. 2005. Palaeozoic tropical rainforests and their effect on global climates: is the past the key to the present? 'Geobiology', 3, 13-31.
2. ^ ^{a b c} Sahney, S., Benton, M.J. & Falcon-Lang, H.J. (2010). "Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica" (PDF). Geology 38 (12): 1079–1082. doi:10.1130/G31182.1. http://geology.geoscienceworld.org/cgi/content/abstract/38/12/1079. 
3. ^ ^{a b} Plant Fossils of the British Coal Measures, by Christopher J. Cleal and Barry A.Thomas, publ. The Palaeontological Association, 1994, ISBN 0-901702-53-6

Further reading

• Prehistoric Park#Episode 5: The Bug House and Walking with Monsters#Episode Two are set partly in coal forest.

Links to reconstruction images of coal forest

• [1]
• [2] A fin-backed reptile, an amphibian, and a large dragonfly among sphenopsids, lycopods, and ferns
• [3]
• Coal forest solid reconstruction in a museum
• [4]
• [5] (800x800)