There is a Gaelic proverb, which loosely translated goes: “There are more ways of killing a cat than drowning it in butter”. That seems apt for mass extinctions, particularly the most severe, at the end of the Palaeozoic. A new hypothesis points the finger towards breathing problems, but not those likely from massive, ground-hugging emissions of sulphur dioxide from the Siberian flood basalts that coincide with the P-Tr extinction: “everyone knows” that they resulted in the universal coughing reflex in all surviving land vertebrates….. Raymond Huey and Peter Ward of the University of Washington reckon a major contributing factor for terrestrial extinctions was a fall in atmospheric oxygen (Huey, R.B. & Ward, P.D. 2005. Hypoxia, global warming and terrestrial Late Permian extinctions. Science, v. 308, p. 398-401).
For most of the Carboniferous and Early Permian Earth flipped in and out of glacial conditions that dominated the southern supercontinent of Gondwana. Tropical latitudes were cloaked in dense vegetation for the first time. Rapid sedimentation buried vast amounts of carbon in the form now taken by the world’s largest and most extensive coal deposits. Net carbon burial for 90 to 100 Ma resulted in extraordinary oxygen concentrations in the atmosphere. One line of evidence for that is the huge size of Carboniferous and Early Permian insect fossils, such as those of dragonflies. Insects do not breathe, but take in oxygen by a diffusive process through spiracles on the underside of their bodies. The more oxygen the larger they can grow. Carbon burial also links in with the global cooling that made the Carbonierous and Early Permian susceptible to astronomic forcing of glacial-interglacial cyclicity: CO2 fell.
The present-day oxygen concentration in the air is about 22%, whereas estimates for the Carboniferous Permian peak are around 30%. Most land animals today, including ourselves, have an altitude limit to permanent life of around 4 to 5 km, though the vast majority live much lower. In the Early to Middle Permian, the availability of oxygen for respiration corresponding to that at sea level today would have been around 6 km altitude, and at the top of a mountain the height of Everest breathing would be easy. The limit to altitude range of animals would have been temperature rather than oxygen availability. So, given sufficient warmth, the area available for animal life would have been very high. Estimates of the oxygen level at the end of the Permian are as low as about 16%. Even living at sea level would have demanded an ability to survive at about 2.7 km today, and at 6 km during the oxygen-rich Early and Middle Permian. Evolution of land animals during the 100 Ma long “global winter” would have adjusted to elevated oxygen availability, which Huey and Ward believe would have led to at least a limited altitude stratification of available ecosystems, governed by temperature. Their hypothesis is that declining oxygen forced extinctions by reducing the habitable range severely, and increased competition among those taxa able to live in the reduced, low-altitude land area: probably patches of “refugia”.
The decline in oxygen was accompanied by global warming. Permian and Triassic sedimentary records show a dramatic increase in red terrestrial sediments, coloured by iron oxide. Iron had been released and oxidised to insoluble iron(III), possibly by increased continental weathering, which would have sequestered oxygen by the formation of iron oxide coatings to sedimentary grains. Increased oxidation would also have encouraged biodegradation by aerobic bacteria, which may have run-away to help boost atmospheric CO2 levels. One testable outcome of such events is the rate of extinction during the Late Permian, which should have risen slowly, rather than plummeting at the P-Tr event. Another is that survivors might show signs of adaptation to low oxygen levels, and indeed some Triassic reptiles do. All in all, those times were stressful on land. Yet the extinctions were just as severe in marine ecosystems, where the fossil record is more complete. Less oxygen and warmer seas would have resulted in similar hypoxia for aquatic animals.