Eocene–Oligocene Temperature Drop

Large temperature drop across the Eocene–Oligocene transition in central North America. 2007. A. Zanazzi et al. Nature 445: 639-642.

The largest climate change in central North America since the age of the dinosaurs 65 million years ago, a temperature drop of nearly 15^o F, is documented within the fossilized teeth of horses.

From the press release:

The Eocene-to-Oligocene transition (~33.5 MA) marks a massive extinction of life in both marine and land environments. Scientists believe the drop in temperature was likely due to changes in oceanic currents.

Researchers analyzed oxygen and carbon isotopes in the preserved teeth and bones of primitive fossil horses and a primitive cloven-hoofed mammal called an oreodont. Oxygen isotopes act as thermometers, telling scientists at what temperature they were formed; carbon isotopes act as barometers, revealing relative humidity.