| Permian period|
254 to 251 million years ago
The End of an Era
The Changhsingian age began with the continuation of the successful therapsid communities of the preceeding Wuchiapingian age. As time progressed, massive vulcanism in what is now Siberia (the Siberian Traps), perhaps in association with other factors, resulted in dramatic greenhouse conditions, with increasing atmospheric carbon dioxide and methane, and decreasing amounts of oxygen. (Retallack 2005) The increasingly harsh conditions began to take their toll on the biota, culminating in the worst mass extinction in the histrory of advanced life on Earth.
Unlike the End-Cretaceous Extinction, in which dinosaurs and other animals remained common until the obvious asteroid impact, there seems to have been a gradual decrease in biodoversity, at least as far as terrestrial animals went, culminating in a sudden dramatic extinction that ravaged what was left of the already impoverished fauna. (Ward et al. 2005) Only those animls that were already pre-adapted for low oxygen conditions, such as burrowers like Lystrosaurus, were able to make it through. (Retallack et al. 2003)
Strangelove Ocean and Green Sky
Paleontologist and Science writer Peter Ward even proposes the theory (in his book Under a Green Sky), that the Permian mass-extinction, along with the other four major extinctions, were the result of runaway greenhouse effect, which heated the oceans and shut down the ocean conveyor belt. This is the phenomenon by which warm and hence poorly oxygenated surface water cools when it approaches the poles, taking in oxygen and sinking to the bottom, where it carries the oxygen rich water to the equator. At the equator it warms and rises, repeating the cycle.
Without this cycle, the oceans become berift of oxygen (this is called the "Strangelove Ocean", after the famous Stanley Kubric Cold War black comedy Dr Strangelove), higher life suffocates and dies, and anaerobic archaea and bacteria flourish. This is deadly for two reaons. First, some of these microrganisms (the methanogens) produces huge amounts of methane, further adding to the greenhouse effect. Others, the sulfate-reducing organisms, generate vast amounts of hydrogen sulfide, better known as rotten egg gas. Ward describes a nightmare scenario, with poisonous oceans belching methane, turning the sky green and hazy and poisoning plants and animals.
I am not saying it would have happened exactly the way Ward describes it, but certainly a Straneglove Ocean is a frightening possibility
For much of the Triassic, oxygen levels remained low, and according to Ward 2006, this favoured dinosaurs which - like birds wpould have had a more efficient aerobic metabolism, over mammals . Early Triassic survivors of the mass extinction like Lystrosaurus and Proterosuchus had stocky bodies and barrel-chests indicating increasing lung capacity, while therapsid carnivores like Galesaurus and Thrinaxodon had reduced lumbar ribs which, along with thickened thoracic ribs and higher thoracic vertebral spines may well indicate enlarged lungs and a muscular, mammal-like diaphragm, allowing more efficient respiration. (Retallack et al 2003 p.1148)
- See also: Permian extinction
- G. J. Retallack, R. M.H. Smith, and P. D. Ward, Vertebrate extinction across Permian-Triassic boundary in Karoo Basin, South Africa, Bulletin of the Geological Society of America, September 1, 2003; 115(9): 1133 - 1152.
- Retallack, G.J., 2005, Permian greenhouse crises, in Lucas, S.G. and Ziegler, K.E., ed., The nonmarine Permian. Bulletin New Mexico Museum of Natural History and Science 30, 256-269.
- Peter Douglas Ward, David W. Ehlert (Illustrator), Out of Thin Air: Dinosaurs, Birds, And Earth's Ancient Atmosphere, Joseph Henry Press 2006
- Peter D. Ward, Jennifer Botha, Roger Buick, Michiel O. De Kock, Douglas H. Erwin, Geoffrey H. Garrison, Joseph L. Kirschvink, Roger Smith, "Abrupt and Gradual Extinction Among Late Permian Land Vertebrates in the Karoo Basin, South Africa", Science 4 February 2005: Vol. 307. no. 5710, pp. 709 - 714
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