ĭuring the Beagle survey expedition, the young naturalist Charles Darwin had remarkable success with fossil hunting in Patagonia. His sketch of the entire animal matched later skeletons found at the site. The tooth was sent to the Paris Conservatory, where it was identified by Georges Cuvier, who identified it as a browsing equine related to the tapir. The first Old World equid fossil was found in the gypsum quarries in Montmartre, Paris, in the 1820s. William Clark's 1807 expedition to Big Bone Lick found "leg and foot bones of the Horses", which were included with other fossils sent to Thomas Jefferson and evaluated by the anatomist Caspar Wistar, but neither commented on the significance of this find. In the 1760s, the early naturalist Buffon suggested this was an indication of inferiority of the New World fauna, but later reconsidered this idea. When the Spanish colonists brought domestic horses from Europe, beginning in 1493, escaped horses quickly established large feral herds. Wild horses have been known since prehistory from central Asia to Europe, with domestic horses and other equids being distributed more widely in the Old World, but no horses or equids of any type were found in the New World when European explorers reached the Americas. Left to right: Mesohippus, Neohipparion, Eohippus, Equus scotti and Hypohippus. History of research Extinct equids restored to scale. This ability was attained by lengthening of limbs and the lifting of some toes from the ground in such a way that the weight of the body was gradually placed on one of the longest toes, the third. At the same time, as the steppes began to appear, selection favored increase in speed to outrun predators. As grass species began to appear and flourish, the equids' diets shifted from foliage to silicate-rich grasses the increased wear on teeth selected for increases in the size and durability of teeth. The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist ground of primeval forests. Other species of Equus are adapted to a variety of intermediate conditions. This group of animals appears to have been originally specialized for life in tropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, modern horses are adapted to life in the climatic conditions of the steppes, which are drier and much harsher than forests or jungles. The perissodactyls arose in the late Paleocene, less than 10 million years after the Cretaceous–Paleogene extinction event. This means that horses share a common ancestry with tapirs and rhinoceroses. The horse belongs to the order Perissodactyla ( odd-toed ungulates), the members of which all share hooved feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. Much of this evolution took place in North America, where horses originated but became extinct about 10,000 years ago, before being reintroduced in the 15th century. Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal. The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, dog-sized, forest-dwelling Eohippus into the modern horse. Reconstruction, left forefoot skeleton (third digit emphasized yellow) and longitudinal section of molars of selected prehistoric horses Skeletal evolution Derivation of horses from an ungulate precursor This image shows a representative sequence, but should not be construed to represent a "straight-line" evolution of the horse.
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