Wasteland Europe

时间:2019-03-07 10:17:08166网络整理admin

By Jeff Hecht EUROPE was turned into a desert after the great Permian extinctions 250 million years ago. Before the extinctions, huge conifer forests dominated the continent, which then basked in a dryish tropical climate. After the extinctions, trees did not return to European soil for 5 million years. The hiatus surprised palaeontologists. “The amazing thing is that it took so long,” says Henk Visscher of Utrecht University. The cause of the Permian extinctions remains a mystery, although something destabilised the ecosystem over tens or hundreds of thousands of years, says Visscher. Sea levels dropped and temperatures rose, probably caused by the greenhouse effect. Massive volcanoes spewed out some 2 million cubic kilometres of lava, which covered a large part of Siberia and may have helped to exterminate some species. The Permian extinctions wiped out some 95 per cent of species, far more than the asteroid impact that killed the dinosaurs 65 million years ago. Plants took between 10 000 and 100 000 years to recover from the dinosaur disaster. The recovery after the Permian has not been studied much before because fossils are scarce. The researchers collected and studied fossils of pollen and spores deposited at sites across Europe, North Africa and Israel. They found a spike of fungal spores at the peak of the extinction, marking the decay of vast amounts of plant material. For millions of years afterwards, the flora was dominated by lycopods, relatives of the club moss. These scrubby plants had leaves along an unbranched vertical stem with a cone of spores on top. Initially growing to a height of some 10 to 20 centimetres, they eventually peaked at 2 metres. As spore-bearing plants, lycopods usually need moist environments to reproduce, which suggests that they lived along rivers and streams, says David Dilcher, a palaeobotanist at the University of Florida in Gainesville, who worked with Visscher. Similar lycopods still thrive in wetlands. When conifers finally reappeared in Europe, the new species came in from elsewhere. Full recovery of the warm dry forest took half a million years after the conifers first reappeared. Today’s northern forests took only a few thousand years to grow again after the last ice age. The early conifer forests survived the Permian extinctions in Australia and Antarctica, which were then in cool southern latitudes, says Greg Retallack of the University of Oregon in Eugene. But most plants suffered severely. “The surprising thing is how long life seemed to struggle after this,