THE world鈥檚 worst mass extinction, 250 million years ago, was caused by ultraviolet radiation pouring in through a giant hole in the ozone layer, a new theory suggests.
In the final years of the Permian period herbaceous lycopod plants reproduced using pollen-like spores clumped into groups of four, rather than single spores. But modern lycopods only produce spores in fours when stressed or exposed to pollution.
Pollutants such as sulphuric acid spewed out by huge volcanic eruptions at the time would not have stayed in the air long enough to account for the abnormal pollen, says Henk Visscher, a specialist in fossil pollen at Utrecht University in the Netherlands.
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Instead, Visscher鈥檚 team suggests that hydrothermal fluids would have dissolved chemicals present in coal deposits, producing organo-halogen compounds such as methyl chloride and methyl bromide (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0404472101). These could have destroyed ozone in the stratosphere, exposing plants and animals at the time to extremely high levels of solar ultraviolet radiation.