"The expansions and contractions of those environments have pretty profound effects on life on Earth," he said. Since the advent of life on Earth 3.5 billion years ago, scientists think there may have been as many as 23 mass extinction events, many involving simple forms of life such as single-celled microorganisms. During the past 540 million years, there have been five well-documented mass extinctions, primarily of marine plants and animals, with as many as 75-95 per cent of species lost. For the most part, scientists have been unable to pin down the causes of such dramatic events. According to Arnold I. Miller, a paleobiologist and professor of geology at the University of Cincinnati, the new study is striking because it establishes a clear relationship between the tempo of mass extinction events and changes in sea level and sediment. "Over the years, researchers have become fairly dismissive of the idea that marine mass extinctions like the great extinction of the Late Permian might be linked to sea-level declines, even though these declines are known to have occurred many times throughout the history of life," he said. The clear relationship this study documents will motivate many to rethink their previous views," he added. Peters measured two principal types of marine shelf environments preserved in the rock record, one where sediments are derived from erosion of land and the other composed primarily of calcium carbonate, which is produced in-place by shelled organisms and by chemical processes.
"The physical differences between these two types of marine environments have important biological consequences," Peters explained, noting differences in sediment stability, temperature, and the availability of nutrients and sunlight. In the course of hundreds of millions of years, the world's oceans have expanded and contracted in response to the shifting of the Earth's tectonic plates and to changes in climate. As those epicontinental seas drained, animals such as mosasaurs and giant sharks went extinct, and conditions on the marine shelves where life exhibited its greatest diversity in the form of things like clams and snails changed as well.