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Earliest Horses_500

This artist reconstruction compares Sifrhippus sandrae, right, with a modern Morgan horse that stands about 5 feet tall at the shoulder and weighs about 1,000 pounds. Sifrhippus, the earliest known horse, was the size of a small house cat (about 8.5 pounds) at the beginning of the Eocene about 56 million years ago. In a study appearing in Science Feb. 24, University of Florida researcher Jonathan Bloch used the fossil evidence and geochemical analysis data to show temperature was the most likely factor driving shifts in mammalian body size during a global warming event known as the Paleocene-Eocene Thermal Maximum.
University of Florida illustration by Danielle Byerley

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