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An artist's impression of a Sifrhippus sandrae (right) with a modern Morgan horse. Warm-blooded animals got smaller at least twice in Earth’s history when carbon dioxide levels soared and temperatures spiked as part of a natural warming, a new study says. Three different species shrank noticeably about 54 million years ago when the planet suddenly heated up. One of them – an early, compact horse – got 14 percent smaller, going from about 7.7 kilograms to 6.6 kilograms, according to an analysis of fossil teeth in journal Science Advances. (Photo by AP Photo)
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