| Carboniferous period|
318 to 299 million years ago
Lasting some 19 million or so years, the Late Carboniferous or Pennsylvanian epoch was the high point of stem tetrapod evolution, especially during the Bashkirian and Moscovian ages. During this time the first reptiles and synapsids evolved and quickly diversified. By the end of the period these new forms, especially the synapsids, had supplanted the stem tetrapods as the dominant life form on land.
During the late Carboniferous period Laurussia and Siberia collide to form Laurasia; meanwhile Gondwana comes up from the south. The resulting Appalachian, Ouachita, Marathon, Ural, Variscan, and Hercynian orogenies formed some of the largest mountains of all time. As a result of the collision of Gondwana and Laurasia the supercontinent of Pangea comes into being.
On land, great forest swamps covered extensive equatorial areas. These forests consisted of diverse plants including tree ferns, which grew 15 meters in height, Calamites, a giant version of the modern "horsetail" plant, lycopods (e.g. Lepidodendron, which attained a height of 30 metres), the extinct group of plants called "seed ferns", and primitive conifer-like plants (Cordaites) that reached 40 meters in height
The extensive burial of biologically-produced carbon led to a buildup of surplus oxygen in the atmosphere; estimates place the peak oxygen content as high as 35%, compared to 21% today. This oxygen level resulted in insect and amphibian gigantism--creatures whose size is constrained by respiratory systems that are limited in their ability to diffuse oxygen. In this moist oxygen rich atmosphere flying insects were abundant, and some attained huge size, such as Meganeura, with a wing span of 70 centimetres.
Tetrapods were abundant, especially the "labyrinthodonts," so called because of the complex (labyrinthine) pattern of folded enamel in their teeth. They filled every available ecological niche, from fully aquatic eel-like forms, to large semiaquatic crocodile-like animals and small forms like modern day newts and salamanders, to terrestrial types similar to reptiles. Some types (the Aïstopoda) lost their legs altogether, superficially resembling snakes.
The earliest reptiles also evolved at this time, such as Hylonomus but remained relatively insignificant until the end of the period. Reptiles have a big advantage over stem tetrapods in that they do not have to return to water to breed; they can lay their eggs on dry land. So it is likely that with the appearance of reptiles the tetrapods were able to colonize the uplands for the first time, where they fed on an abundance of insects.
The name "Carboniferous" derives from the fact that most of the important coal producing strata are of this age. However, it is specifically in the Late Carboniferous or Pennsylvanian sub-period that this is so. During this time most of the world's coal deposits were laid down, the coal being formed from compressed layers of rotting vegetation.
The large coal deposits of the Carboniferous primarily owe their existence to two factors. The first of these is the appearance of bark-bearing trees (and in particular the evolution of the bark fiber lignin). The second is the lower sea levels that occurred during the Carboniferous as compared to the Devonian period. This allowed for the development of extensive lowland swamps and forests in North America and Europe. Large quantities of wood were buried during this period because animals and decomposing bacteria had not yet evolved that could effectively digest the new lignin. Those early plants made extensive use of lignin. They had bark to wood ratios of 8 to 1, and even as high as 20 to 1. This compares to modern values less than 1 to 4. This bark, which must have been used as support as well as protection, probably had 38% to 58% lignin. Lignin is insoluble, too large to pass through cell walls, too heterogeneous for specific enzymes, and toxic, so that few organisms other than Basidiomycetes fungi can degrade it. It can not be oxidized in an atmosphere of less than 5% oxygen. It can linger in soil for thousands of years and inhibits decay of other substances. Probably the reason for its high percentages is protection from insect herbivory in a world containing very effective insect herbivores, but nothing remotely as effective as modern insectivores and probably many fewer poisons than currently. In any case coal measures could easily have made thick deposits on well drained soils as well as swamps.
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page uploaded on Kheper Site on 27 May 1998, page uploaded on Palaeos Site 10 April 2002
last modified ATW030630
text content by M. Alan Kazlev 1998-2002
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