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How many toes do horses have? A new study suggests a radical number: five

Equine scientists the world over will tell you: Horses have only one toe per foot. But a new study that traces their evolution back tens of millions of years suggests that they instead have five. Scientists have long acknowledged the existence of two remnant, vestigial toes left over from their multitoed ancestors—small bones fused to the side of each hoof. But after taking a closer look at an extinct horse called Mesohippus, which lived some 35 million years ago, they traced separate, detachable structures on its feet to two small ridges that run down either side of modern horse hooves, they report today in Royal Society Open Science. They saw something similar in Dinohippus, a 5-million-year-old horse relative, although in that case the bones were more fused, with suture lines showing where the two structures had joined together. The scientists see this as evidence that the ridges on modern horse hooves are vestiges of what were once distinct toes—and that horses have all five toes after all.