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In reply to Disinfo Agent's comment: Fair enough, but the language you used was inaccurate. They do not 'breathe through their skin'. While it's probably true that 'the amount of oxygen that they can absorb is proportional to their surface area', this must take into account the many ramifications of the trachea inside their bodies, not just their body's outer size...
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Disinfo Agent, your comment is correct. My original comment was inaccurate.
Perhaps the question is how to explain gigantism at that particular period of time. I would expect that there was period when all gigantic animals and plants become extinct and then gigantism returned.
I read Lane's book "Oxygen the Molecule that made the World". Lane suggested that increased O2 expanded the giantic insects, but there is a problem with oxygen toxicity. Animals die in high levels of oxygen.
The hypothesis that the explanation to this paradox is an increase percentage of oxygen in the atmosphere has some problems, related to oxygen toxicity. The Gaia hypothesis has the biosphere regulating the percentage of oxygen in the atmosphere.
http://www.nick-lane.net/Extract%20chapter%205.html
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THE SMALL MINING TOWN OF BOLSOVER in Derbyshire enjoyed an unexpected fifteen minutes of fame in 1979. While working a coal seam 500 metres beneath the surface, local miners dislodged a gigantic fossilised dragonfly with a wing-span of half a metre, rivalling that of a seagull. Experts from the Natural History Museum confirmed that the fossil dated to the Carboniferous period, about 300 million years ago. The giant was dubbed the Bolsover dragonfly, ….Gigantism was unusually common in the Carboniferous.
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DRAGONFLIES WERE NOT THE ONLY GIANTS of the Carboniferous - many other creatures attained sizes never matched again. Some mayflies had wingspans of nearly half a metre, millipedes stretched for over a metre, and the Megaranea spider, with a leg-span of nearly half a metre, would have chilled the marrow of Indiana Jones. Even more terrifyingly, scorpions reached lengths of a metre, dwarfing their modern cousins, the largest of which barely manages a fifth of that length. Among the terrestrial vertebrates, amphibians grew from newt-like proportions to reach body lengths of five metres….. …..in length and 14 cm across. In the plant world, ferns turned into trees, while the giant lycopods reached perpendicular heights of nearly 50 metres. Their only survivors today are the diminutive herbaceous ground or club mosses, such as the ground pine (Lycopodium obscurum), which rarely attain heights of over 30 cm.
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We must conclude that high oxygen is good, low oxygen is bad. At the end of such a lengthy and detailed analysis, this platitude may seem a bit of a lame conclusion. And yet. We saw in Chapter 1 that high levels of oxygen are toxic, causing lung damage, convulsions, coma and death, and we are assured that oxygen free radicals are at the root of ageing and disease. What is going on: is oxygen toxic or not? This paradox did not escape the notice of Barry Halliwell and John Gutteridge, authors of the standard text on free radicals in biology and medicine, who remarked laconically that 'the plants and animals existing in the Carboniferous times must presumably have had enhanced antioxidant defences, which would be fascinating to study if these species could ever be resurrected.' Yes indeed! How did they overcome oxygen toxicity? .... It is time to look in a little more detail at the strange spectre of oxygen toxicity, and what life does about it.
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