During the PETM itself a strange thing happened to some mammals: They got dwarfish. Horses in the Bighorn shrank to the size of Siamese cats; as the carbon ebbed from the atmosphere, they grew larger again. It's not clear whether it was the heat or the CO2 itself that shrank them. But the lesson, says Gingerich, is that animals can evolve fast in a changing environment. When he first drove into the Bighorn four decades ago, it was precisely to learn where horses and primates came from. He now thinks that they and artiodactyls came from the PETM—that those three orders of modern mammals acquired their distinctive characteristics right then, in a burst of evolution driven by the burst of carbon into the atmosphere.
Learn more about the changes that happened during the PETM in the October issue of National Geographic magazine. Link
(Image credit: Ira Block)