When the earliest horses appeared about 56 million years ago, they were about the size of a miniature schnauzer. Then, over the next 130,000 years, these "protohorses" became even smaller, shrinking to the size of a house cat (artist’s reconstruction of Sifrhippus sandrae, right, compared with a modern-day Morgan horse, which weighs a half-ton). The shrinkage, researchers have found, is due exclusively to warmer temperatures. In their analysis, the scientists estimated the body size of each protohorse by measuring its fossilized teeth, and they used the ratios of oxygen isotopes in the teeth of aquatic mammals that lived alongside the protohorses—a reliable paleothermometer—to estimate average annual temperature in the region. As global temperatures rose between 5° and 10°C during that period, the protohorses lost about 30% of their body mass, the team reports online today in Science. Other data, including analyses of the sediments surrounding the fossils, reveal that the climate got wetter—and the ecosystem was presumably more productive—as average temperatures climbed, nixing the notion that these animals shrank because of a reduced food supply. After the peak of the ancient warm spell, the creatures evolved back to a larger size as climate cooled, setting them on the evolutionary road that produced the saddle-worthy horses we know and love today.
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