Tiny creatures called forams live on the seafloor, as they’ve done for hundreds of millions of years, long before there were mammals or dinosaurs. When they die, they leave a useful layer of microscopic skeletons. In those skeletons, traces of stable oxygen are woven into the matrix of the fossilized bone. One type is more common when the world is warmer; another when it’s cold. So, if you grind up a little pile of foram fossils, you can get a pretty good model of ancient weather.
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