55 million years ago, the world was literally a jungle. With such high temperatures, rainfall and humidity, life could proliferate. But 49 million years ago, something drastic happened: The Azolla Event, where ice was forming at the poles. The culprit? A kind of plant called Azolla. Within 800,000 years, photosynthesis and carbon sequestration from those plants reduced the atmospheric quantity of carbon dioxide from 3500 parts per million to 650.
[figure from Atmospheric carbon dioxide concentrations over the past 60 million years]
But the real question here is--did the cooling caused by the Azolla Event create a major extinction event of plant and animal species?