Mass extinctions that wiped out up to 90% of earth's flora and fauna were driven in large part by shifting ocean levels, according to a study published in Nature. Understanding what made many of the planet's living organism rapidly die out at least five times over the last half billion years remains one of the great challenges in paleontology and biology. Some theories point an accusing figure at the cooling effect of massive dust shrouds thrown into the atmosphere by volcanoes and asteroids crashing into earth, or the warming caused by rising levels of carbon dioxide. But the new study suggests that it was the ebb and flow of sea levels and sediment over geologic time, rather than cataclysmic events, that doomed tens of thousands of species to extinction. "The expansions and contractions of those environments have pretty profound effects on life on earth,' said Shanan Peters, a geologist at the University of Wisconsin-Madison and author of the study. With the exception of a colossal asteroid impact some 65 million years ago, the cause for the other mass extinctions is sharply contested. Even that one coincided with an abrupt retraction of oceans then covering much of North America and Europe that could have played a major role in the disappearance of dinosaurs. To test his hypothesis, Peters measured to two types of ancient shallow marine environments preserved in the rock record. One is composed mainly of calcium deposits produced by organisms with shells. The other is characterised by brown or muddy sand, rocky beaches and water that is greenish and cloud. "I looked at rates of extinction in the fossil record over the last 500 million years,' Peters said. "And then I compared them to the environmental changes that are encoded in the sedimentary rocks.' What Peters found was a very strong match, showing that the sometimes dramatic rise and fall of oceans levels correlated more consistently with mass extinctions that any other factor. AFP