At the end of the Permian period, 252 million years ago, 96 percent of all marine species and 70 percent of all life on Earth at the time became extinct. The event is called the Great Dying and was caused by catastrophic volcanism that formed the Siberian Traps. Now a group of researchers thinks they have identified another effect that might have precipitated the situation, at least on land: Ozone depletion.

According to a new paper published in Science Advances, the eruptions were so prolonged (about 1 million years) that they caused the amount of ozone in the atmosphere to diminish. With less ozone, plants and animals were more susceptible to the harmful ultraviolet rays (UV-B) of the Sun. The new research produces lab evidence suggesting that the lack of ozone made entire forests sterile.

"During the end-Permian crisis, the forests may have disappeared in part or fully because of increased UV exposure," lead author Jeffrey Benca, from the University of California, Berkeley, said in a statement. "With pulses of volcanic eruptions happening, we would expect pulsed ozone shield weakening, which may have led to forest declines previously observed in the fossil record."

"If you disrupt some of the dominant plant lineages globally repeatedly, you could trigger trophic cascades by destabilizing the food web base, which doesn't work out very well for land animals," he added.

In the last decade or so, evidence of fossilized malformed pollen from the late Permian has been discovered, so the researchers put forward the hypothesis that changes to the ozone layer could have been responsible for the mutation.

The researchers tested this idea in the lab. They used 60 dwarf clonal pines and exposed them to different amounts of UV-B rays. The trees received 7.5, 10, or 13 times more rays than normal trees for 56 days. The researchers discovered that pines exposed to the medium and high intensities had between 12 and 15 percent of pollen grains misshapen compared to the 3 percent in the control sample. And not only that. the trees had become sterile too.

"Unlike the relatively unprotected seed-bearing structures of affected end-Permian seed plants, today's pines have elaborate and very heavily reinforced, interlocking cone scales that shield their seeds from predators and the external environment," Benca explained. "Even so, these trees just ditched all their seed cones. Pinus mugo is an alpine species that should be quite resilient to increased UV levels."

The research is very interesting but it does not reveal the full picture of the Great Dying, as the ozone depletion doesn't explain the dramatic death of almost all marine species. But it does warn us about taking care of the ozone layer, which continues to deteriorate above cities today.