The discovery of a mass die- off that happened 250 million years ago could help explain what is wiping out the planet's species today. The event - more than likely volcanic - was catastrophic, possibly wiping out 75% to 80% of the world's species and destroying the largest land animals of their day, early mammal-like reptiles called dinocephalians.There have been six mass extinctions over the past 500million years - and some scientists believe we are in the middle of a seventh.Professor Bruce Rubidge, of the Evolutionary Studies Institute at Wits University, thinks that, by understanding what happened a quarter of a billion years ago could give us insight into what is happening on Earth today."We are losing two species an hour and although humans are largely to blame we need a full understanding of what is causing this, which we can get by looking into the past," he said.Rubidge said evidence of the mass extinction came from marine fossils in China, and research had accurately dated the event and revealed its effects on land species."We were working with the Massachusetts Institute of Technology, the best dating specialists, and came up with a date of 260.26million years," said Rubidge.The dating was arrived at by measuring the amount of uranium and lead found in zircon crystals recovered from a layer of volcanic ash in the Karoo.The ash layer suggests the cause of the extinction."This was a period of mass volcanic activity," he said.About 8million years later there was another epoch of widespread extinctions.The research, which has appeared in the latest issue of Proceedings of the Royal Society B, also entailed taking a census of dinosaur fossils found across the Karoo. The research was a culmination of 30 years of field work and collecting...