Special Report

Evolution World Tour: Ashfall Fossil Beds, Nebraska

Prehistoric rhinoceroses and horses died of volcanic ash inhalation 12 million years ago – their fossils are studied now as an example of natural selection

Three-toed horses such as Cormohipparion occidentale were adapted to forests prior to 12 million years ago. As the climate dried, one-toed horses became prevalent. (Roger Hubbard)
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The evolution of the horse is one of the most clearly documented in the fossil record. Ashfall, in particular, vividly shows that there was no single branch leading from smaller, prehistoric horses to larger, modern-day animals. Some of the species had three toes; others had just one (the precursor to the modern hoof). Voorhies cites another specimen, Pliohippus pernix, as a transitional species, in that it had three toes, but two were so small as to be useless. The forces of natural selection yielded several similar, co-existing adaptations that were neither identical nor equally successful. The evolution of the horse “was more like a bush, with numerous side branches,” says Voorhies. “And just one little twig has survived into modern times.”

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