http://news.nationalgeographic.com/news/2005/03/0309_050309_extinctions.html

Mystery Undersea Extinction Cycle Discovered

John Roach
for National Geographic News

March 9, 2005
Robert Rohde and Richard Muller are vexed. For the past 542 million years the number of animal species living in the world's oceans has risen and fallen in a repeating pattern, and the scientists haven't the foggiest idea why.

"I wish I knew what it all meant," said Muller, who is a professor of physics at the University of California, Berkeley.

The pattern includes a rise and fall of marine animal diversity every 62 million years and a weaker cycle of rising and falling marine diversity, which repeats every 140 million years. The researchers think that expanding and retreating glaciers may explain the 140-million-year cycle, but they are stumped over what drives the 62-million-year cycle.

The declines in the 62-million-year cycle correspond with some of the best known mass extinctions on Earth.