| Evolution Of The Horse |
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is shaped throughout. The teeth are shown in Longitudinal Section . It should be noted that the animals shown are only selected examples - the evolution of horses was by no means a linear process. See the article for details.]] The evolution of the horse involves the gradual development of the modern Horse from fox-sized, forest-dwelling mammals. Zoologist s have been able to piece together a more complete picture of the modern horse’s evolutionary line than that of any other animal. OVERVIEW Horses belong to a group known as the Perissodactyls or 'odd-toed ungulates', which all share Hoofed Feet and an odd number of toes on each foot, as well as mobile Upper Lip s and similar Tooth structure. This means that they share a Common Ancestor with Tapir s and Rhinoceri . This group of animals appears to have been originally specialized for life in Tropical Forest s, but whereas tapirs and, to some extent, rhinoceri, retained their jungle specializations, horses instead adapted to life on dryer land in the much-harsher Climatic conditions of the Steppe s. The first ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist grounds of primeval Forest s. As the mainland was drying out, the steppes began to appear, and with it came large numbers of dry land Predator s. This in turn required the horse’s predecessors to possess more speed in order to survive. The ability to run faster was accomplished by the 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. On solid ground, pushing-off with a single toe and equipped at the last evolutionary link with a hoof, the horse was able to reach fast speeds. The modern horse is a single-toed, hoofed mammal, while its predecessors were multi-odd-toed animals. ''HYRACOTHERIUM'' The earliest animals to bear recognisably horse-like anatomy are found in the lower Eocene Epoch and are called '' Hyracotherium '', an animal approximately the size of a Fox with a relatively short head, 44 teeth with uneven, dull and bumpy molars, a short neck, a “springy”, arched back, and “wrist” and hock joints that are still low to the ground. Its limbs were relatively long, already showing the beginnings of adaptations for running. The forelimbs had developed five toes out of which only four were equipped with a small hoof; the fifth large “toe–thumb” was off the ground. The hind limbs had three out of the five toes equipped with small hooves, while the first and fifth toes did not touch the ground. During the Eocene, the ''Hyracotheria'' branched out into various types resembling the fox in size (from 250 Mm to 450 mm in height). Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers of North America, mainly in the Wind River basin of Wyoming . Similar fossils of horses have also been discovered in Europe. ''OROHIPPUS'' '' Orohippus '' (also called ''Protorohippus'') lived in the middle of the Eocene. It resembled ''Hyracotherium'' is size, but had a slimmer body, an elongated head, slimmer forelimbs and longer hind legs, all of which are characteristics of a good jumper. The outer toes of ''Hyracotherium'' are not present in the ''Orohippus''; hence on each forelimb there were four toes and on three on each hind leg. The first of the premolar teeth were dwarfed. ''MESOHIPPUS'' In the early stages of the Oligocene epoch, the North American environment was changing. During the still-warm and dry weather conditions, the forests were yielding to flatlands, which were home to Grass es and various kinds of brush. In some places these plains were covered in Sand , creating the type of environment resembling the present-day Prairie s. During the younger Oligocene, '' Mesohippus '' was one of the more widespread mammals in North America. It walked on three toes on each of its front and hind feet (the first and fifth toes remained but were small and not used in walking). The third toe was stronger than the outer ones and thus more weighted. Judging by its slim limbs, ''Mesohippus'', which was about 500 mm tall, was a fast animal. ''MIOHIPPUS'' AND ITS DESCENDANTS At the end of Oligocene epoch and the beginning of the Miocene epoch, '' Miohippus '' emerged, which was somewhat bigger than its predecessors. It branched out into two forms, one of which adjusted to the life in forests, while the other remained suited to life on the prairies. The forest form was ''Kalobatippus'' (or ''Miohippus intermedius''), whose second and fourth front toes were long, to suit travel on the soft forest floors. ''Kalobatippus'' travelled to Asia via the Bering Strait Land Bridge , and from there to Europe , where its fossils were formerly described under the name ''Anchitherium''. ''Kalobatippus'' is believed to be ancestral to another European form known as '' Hyohippus '', which became extinct near the beginning of the Pliocene . From the ''Miohippus'' that remained on the steppes are believed to be ancestral to '' Parahippus '', a North American animal about the size of a small Pony , with a prolonged Skull and a facial structure resembling the horses of today. Its third toe was stronger and larger, and carried the main weight of the body. Its four premolars resembled the molar teeth and the first were small and almost nonexistent. The incisive teeth of ''Parahippus'', like those of its predecessors, had a crown as humans do; however, the top incisors had a trace of a shallow crease marking the beginning of the core/cup. ''MERYCHIPPUS'' In the middle of the Miocene epoch, an animal called '' Merychippus '' was alive. ''Merychippus'' had wider molars than its predecessors, which are believed to have been used for crunching the hard grasses of the steppes. The hind legs, which were relatively short, had side toes equipped with small hooves, but they probably only touched the ground when running. ''Hipparion'' Three new equids are believed to be descended from the numerous varieties of ''Merychippus'': '' side, had a completely isolated Parapet . A complete and well-preserved skeleton of the North American ''Hipparion'' shows an animal the size of a small pony. They were very slim, rather like Antelope s, and were adapted to life on dry prairies. On its slim legs, ''Hipparion'' had three toes equipped with small hooves, but the side toes did not touch the ground. An American form of ''Hipparion'', also known as '' Neohipparion '', proliferated into many kinds of equids several of which managed to migrate to Asia and Europe during the Pliocene epoch. (The European ''Hipperia'' differs from the American ''Hipparion'' in the smaller body size – the best-known discovery of these fossils was near Athens .) Recent research suggests that ''Hipparion'' is an ancestor of the Zebra and the Donkey , rather than the horse. ''Pliohippus'' The form of Meryhippus that lead to present day horses was the Pliocene '' Pliohippus ''. It had long extra toes on both sides of the hoof, but they were externally barely visible as callused stubs. The long and slim limbs of ''Pliohippus'' reveal a quick-footed steppe animal. ''PLESHIPPUS'' '' Plesippus '' is an intermediary stage between ''Pliohippus and the present day horse. At the end of the Pliocene, the climate in North America began to cool down significantly and the animals were forced to move south. One group of the ''Plesippus'' species escaped to South America , and the other moved across the land bridge around the Bering Strait into Asia and Europe. A portion also remained in the southern part of North America. The Ice Age spread five times over Europe and North America and five times again receded; it is estimated that approximately one million years elapsed from the Ice Age (the Quaternary period) to our era. In South America a form named '' Hippidium '' developed from ''Pleshuippus''. ''Hippidium'' was relatively short-legged with an especially long nose that also formed the lower part of the skull. It continued to live on the South American Pampas for a long time, but eventually died out. ''EQUUS'', THE 'TRUE HORSE' The oldest species of true horse, ''Equus stenonis'', was discovered in Italy , and is believed to have evolved from ''Plesippus''-like animals at the end of the Tertiary or beginning of the Quaternary periods. ''Equus stenonis'' proliferated into two branches, one lighter in body mass and one heavier. ''Equus stenonis'' crossed into North American, where it is known as '' Equus Scotti ''; it did not differ in any way from the European form; however, some types (''Equus scotti'' var. ''giganteus'') exceeded the modern horse in size. However, all the horses in North America ultimately became extinct, perhaps due to some mass Contagion , but it is also theorized that humans hunted them to extinction, as the appearance of humans in the Americas occurred at about the same time as the extinction of most large mammals in the Americas. Divide between horses and zebras Recent studies by a team of geneticists headed by C. Vila indicate that the horse line split from the zebra/donkey line between 4 and 2 million years ago. '' Equus Ferus '', ancestor species to '' Equus Caballus '', appeared 630,000 to 320,000 years bp. ''Equus caballus'' was formed from several subspecies of ''Equus ferus'' by selective breeding widely over Eurasia for an extended time. The details of this process are currently a target of research by Archaeologist s and Geneticist s. Horses return to America At the end of the 15th Century , when the first Europeans came to America, there were no horses in the Americas; the Natives of modern-day Mexico and Peru did not even have a name for the animal. The Spanish imported horses back to America. Runaway horses and Cattle went wild on the pampas and proliferated into large herds, only to be caught again later and domesticated. DETAILS Toes The ancestors of the horse came to walk only on the end of the third toe and both side toes. Skeletal remnants show obvious wear on the back of both sides of Metacarpal and Metatarsal bones, commonly called the “splint bones”. They are the remnants of the second and the fourth toe. Modern horses retain the splint bones; it is often believed that they are a useless attachment, but they in fact play an important role in supporting the carpal joints (front knee) and even the tarsal joints (hock). It is not unheard of that foals are occasionally born with three toes equipped with hooves. This is called a phylogenetic Atavism caused by arrested development at a certain Embryo nic stage. Teeth Throughout the phylogenetic development, the teeth of the horse underwent significant changes. The type of the original Omnivorous teeth with short, "bumpy" molars, with which the prime members of the evolutionary line distinguished themselves, gradually changed into the teeth common to Herbivorous mammals. They became long (as much as 100 mm), roughly cubical molars equipped with a flat grinding surface. In conjunction with the teeth, during the horse’s evolution the elongation of the facial part of the skull is apparent, and can also be observed in the backward set eyeholes. In addition, the relatively short neck of the equine ancestors became longer with equal elongation of the legs. This is because they adapted to finding food by grazing on the steppes. Finally, the size of the body grew as well, not only due to plentiful food, but also to the increase in variety. EXTERNAL LINKS |
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