By Katherine Harmon
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The first horses in North America would not have been able to hold their own in the Triple Crown. At just about 5.6 kilograms the Sifrhippus sandrae hoofed onto the scene some 56 million years ago about the size of a small dog.
But then a funny thing happened. In the next 130,000 years during the PaleoceneâEocene Thermal Maximum, these small equines got even smaller, reaching the tiny size of 3.9 kilograms -- some 30 percent lighter than their initial heft. Just 45,000 years later, however, the genus had bulked up to seven kilograms. (Read more...)