image: Artist's reconstruction of Sifrhippus sandrae (right) touching noses with a modern Morgan horse (left) that stands about 5 feet high at the shoulders and weighs approximately 1,000 lbs. Sifrhippus was the size of a small house cat (about 8.5 lbs.) at the beginning of the Eocene (~55.8 million years ago) and is the earliest known horse. This image relates to a paper that appeared in the Feb. 24, 2012, issue of Science, published by AAAS. The paper, by Ross Secord at the University of Nebraska in Lincoln, Neb., and colleagues was titled, "Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum." view more
Credit: Illustration by Danielle Byerley, Florida Museum of Natural History