Understanding Media TC 101
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Date Created: 09/19/15
Insects 0f the Oxygeniferous By Shanti Menon DISCOVER Vol 16 No 09 l September 1995 l Ancient Life In the swampy forests of the Carboniferous Period 360 to 286 million years ago dragon ies with twoandahalffoot wingspans darted among the giant ferns May ies grew to canary size Cockroaches appeared suddenly as cockroaches do for the first time The number of insect families increased from 1 or 2 to more than 100 during the Carboniferous and many of the insects were huge and no one has been able to say exactly why Jeffrey Graham a researcher at the Scripps Institution of Oceanography has been pondering the problem since childhood I remember seeing models of giant dragon ies as a child and wondering how they could y Graham recalls Now Graham zoologist Carl Gans of the University of Michigan and their colleagues think they have an answer The ight of the giant dragon y they say along with the whole Carboniferous insect explosion may have been made possible by an oxygenrich atmosphere Graham and Gans s idea is a hypothesis based on a hypothesis Six years ago Yale geochemist Robert Bemer first suggested that the atmosphere in the Carboniferous was more oxygen rich than at any time before or sinceit was 35 percent oxygen Bemer estimated compared with 21 percent today Bemer attributed this to the rise of land plants in general and in particular to the vast and verdant swamps that characterized the Carboniferous All those swamp plants spit oxygen into the atmosphere and when they died they escaped the openair decomposition by bacteria that would have drawn oxygen back out of the atmosphere Instead they sank into the swamps ultimately forming the coal deposits that gave the Carboniferous its name There is no direct evidence of atmospheric oxygen levels 300 million years ago Bemer s hypothesis is based on a computer model But if there was extra oxygen around says Graham it was like a vitamin It was an ecological and evolutionary resource that animals utilized to enable them to do more The most spectacular beneficiaries he thinks were the insects For one thing the oxygenrich atmosphere was a denser atmosphere that provided more lift and thus made it easier for them to y The real explosion of ight occurs in the Carboniferous Graham says Wings were becoming better more proficient and ight was being perfected More important the excess oxygen made it easier for insects to breathe Unlike humans and other vertebrates insects do not have a circulatory system that actively transports oxygen to cells Instead oxygen diffuses passively into their tissues through branching tracheae that connect each and every cell to pores in their skin This limits an insect s size because the oxygen can only diffuse so far in a given amount of time But in an atmosphere that was 35 percent oxygen the gas would have diffused fasterthus enabling Carboniferous insects to grow larger Meganeura monyi the dragon y with a wingspan of two and a half feet had a body that was over an inch thick The largest dragon y today has a wingspan of only six inches and a body skinnier than a pencil
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