ScienceDaily (Jan. 27, 2011) — About 450 million years ago, Earth suffered the second-largest mass extinction in its history -- the Late Ordovician mass extinction, during which more than 75 percent of marine species died. Exactly what caused this tremendous loss in biodiversity remains a mystery, but now a team led by researchers at the California Institute of Technology (Caltech) has discovered new details supporting the idea that the mass extinction was linked to a cooling climate.
http://www.sciencedaily.com/releases/2011/01/110127141703.htm