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EQUIDAE

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Originally appearing in Volume V09, Page 722 of the 1911 Encyclopedia Britannica.
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EQUIDAE , the See also:

family of perissodactyle ungulate mammals typified by the See also:horse (Equus caballus); see HORSE. According to the older See also:classification this family was taken to include only the forms with tall-crowned See also:teeth, more or less closely allied to the typical genus Equus. There is, however, such an almost See also:complete See also:graduation from the former to earlier and more See also:primitive mammals with See also:short-crowned cheek-teeth, at one See also:time included in the family Lophiodontidae (see See also:PERISSODACTYLA), that it has now become a very See also:general practice to include the whole " phylum " in the family Equidae. The Equidae, in this extended sense, together with the See also:extinct Palaeotheriidae, are indeed now regarded as forming one of four See also:main See also:groups into which the Perissodactyla are divided, the other groups being the Tapiroidea, Rhinocerotoidea and Titanotheriide. For the horse-See also:group the name Hippoidea is employed. All four groups were closely connected in the See also:Lower See also:Eocene, so that exact See also:definition is almost impossible. In the Hippoidea there is generally the full See also:series of 44 teeth, but the first premolar is often See also:deciduous or wanting in the lower or in both jaws. The incisors are See also:chisel-shaped, and the canines tend to become isolated so as in the now specialized forms to occupy nearly the See also:middle of a longer or shorter See also:gap between the incisors and premolars. In the upper molars the two See also:outer columns of the primitive tubercular molar coalesce to See also:form an outer See also:wall, from which proceed two crescentic transverse crests; the connexion between the crests and the wall being imperfect or slight, and the crests themselves sometimes tubercular. Each of the lower molars carries two crescentic ridges. The number of toes ranges from four to one in the fore-See also:foot, and from three to one in the See also:hind-foot. The paroccipital, postglenoid and See also:post-tympanic processes of the See also:skull are large, and the latter always distinct.

Normally there are no traces of See also:

horn-cores. The calcaneum lacks the facet for the fibula found in the Titanotheroidea. In the earlier Equidae the teeth were short-crowned, with the premolars simpler than the molars; but there is a See also:gradual tendency to an increase in the height of the crowns of the teeth, accompanied by increasing complexity of structure and the filling up of the hollows with See also:cement. Similarly the gap on each See also:side of the canine tooth in each See also:jaw continues to increase in length; while in all the later forms the See also:orbit is surrounded by a See also:ring of See also:bone. A third modification is the increasing length of See also:limb (as well as in general bodily See also:size), accompanied by a gradual reduction in the number of toes from three or four to one. All the existing members of the family, such as the domesticated horse (Equus caballus) and its See also:wild or See also:half-wild relatives, the b asses and the zebras, are included in the typical genus. In all these the crowns of the cheek-teeth are very tall (fig. I, b) and only develop roots See also:late in See also:life; while their grinding-surfaces (fig. 2, b and c) are very complicated and have all the hollows filled with cement. The summits'of the incisors are infolded, producing, when partially worn, the " See also:mark." In the skull the orbit is surrounded by bone, and there is no distinct depression in front of the same. Each limb terminates in one large toe; the lateral digits being represented by the splint-bones, corresponding to the lateral metacarpals and metatarsals of Hipparion. Not unfrequently, however, the lower ends of the splint-bones carry a small expansion, representing the phalanges.

Remains of horses indistinguishable from E. caballus occur in the See also:

Pleistocene deposits of See also:Europe and See also:Asia; and it is from them that the dun-coloured small horses of See also:northern Europe and Asia are probably derived. The ancestor of these Pleistocene horses is probably E. stenonis, of the Upper See also:Pliocene of Europe, which has a small depression in front of the orbit, while the skull is relatively larger, the feet are rather shorter, and the splint-bones somewhat more See also:developed. In See also:India a nearly allied See also:species (E. sivalensis), occurs in the Lower Pliocene, and may have, been the ancestor of the Arab stock, which shows traces of the depression in front of the orbit characteristic of the earlier forms. In See also:North See also:America species of Equus occur in the Pleistocene and from that See also:continent others reached See also:South America during the same See also:epoch. In the latter See also:country occurs Hippidium, in which the cheek-teeth are shorter and simpler, and the nasal bones very See also:long and slender, with elongated slits at the side. The limbs, especially the See also:cannon-bones, are relatively short, and the splint-bones large. The allied See also:Argentine Onohippidium, which is also Pleistocene, has still longer nasal bones and slits, and a deep See also:double cavity in front of the orbit, See also:part of which probably contained a gland. Onohippidium is certainly off the a b c See also:direct See also:line of descent of the See also:modern horses, and, on See also:account of the length of the nasals and their slits, the same probably holds See also:good for Hippidium. Species from the Pliocene of See also:Texas and the Upper See also:Miocene (Loup See also:Fork) of See also:Oregon were at one time assigned to Hippidium, but this is incorrect, that genus being exclusively South See also:American. The name Pliohippus has been applied to species from the same two formations on the supposition that the foot-structure was similar to that of Hippidium, but Mr J. W. Gidley is of See also:opinion that the lateral digits may have been fully developed.

Apparently there is here some gap in the line of descent of the horse, and it may be suggested that the See also:

evolution took See also:place, not as commonly supposed, in North America, but in eastern central Asia, of which the palaeontology is practically unknown; some support is given to this theory by the fact that the earliest species with which we are acquainted occur in northern India. Be this as it may, the next North American representatives of the family constitute the genera Protohippus and Merychippus of the Miocene, in both of which the lateral digits are fully developed and terminate in small though perfect hoofs. In both the cheek-teeth have moderately tall crowns, and in the first named of the two those of the See also:milk-series are nearly similar to their permanent successors. In Merychippus, on the other See also:hand, the milk-molars have short crowns, without any cement in the hollows, thus resembling the permanent molars of the under-mentioned genus Anchitherium. From the well-known Hipparion, or Hippotherium, typically from the Lower Pliocene of Europe, but also occurring in the corresponding formation in North See also:Africa, See also:Persia, India and See also:China, and represented in the Upper Miocene Loup Fork beds of the See also:United States by species which it has been proposed to See also:separate generically as Neo- 10 a, Hyracotherium (Eocene). d, Hipparion (Pliocene). b, Mesohippus (Oligocene). e, Equus (Pleistocene). c, Anchitherium (Miocene). hipparion, we reach small horses which are now generally regarded as a lateral offshoot from the Merychippus type. The cheek-teeth, which have crowns of moderate height, differ from those of all the foregoing in that the postero-See also:internal See also:pillar (the See also:projection on the right-hand See also:top corner of c in fig. 2) is isolated in place of being attached by a narrow See also:neck to the adjacent See also:crescent. The skull, which is relatively short, has a large depression in front of the orbit, commonly supposed to have contained a gland, but this may be doubtful. In the typical, and also in the North American forms these were complete, although small, lateral toes in both feet (fig.

3, d), but it is possible that in H. antilopinum of India the lateral toes had disappeared. If this be so, we have the development of a monodactyle foot in this genus independently of Equus. The foregoing genera constitute the subfamily Equinae, or the Equidae as restricted by the older writers. In all the dentition is of the hypsodont type, with the hollows of the cheek-teeth filled by cement, the premolars molariform, and the first small and generally deciduous. The orbit is surrounded by a bony ring; the ulna and See also:

radius in the fore, and the See also:tibia and fibula in the hind-limb are united, and the feet are of the types described above. Between this subfamily and the second subfamily, Hyracotheriinae, a partial connexion is formed by the North American Upper Miocene genera Desmatippus and Anchippus or Parahippus. The characteristics of the group will be gathered from the remarks on the leading genera; but it may be mentioned that the orbit is open behind, the cheek-teeth are short-crowned and without cement (fig. I, a), the gap between the canine and a b c d e 111 x, is the outermost incisor is short, the bones of the middle part of the See also:leg are separate, and there are at least three toes to each foot. The longest-known genus and the one containing the largest species is Anchitherium, typically from the Middle Miocene of Europe, but also represented by one species from the Upper Miocene of North America. The See also:European A. aurelianense was of the size of an See also:ordinary donkey. The cheek-teeth are of the type shown in a of See also:figs. 1 and z; the premolars, with the exception of the small first one, being molar-like; and the lateral toes (fig.

3, c) were to some extent functional. The summits of the incisors were infolded to a small extent. Nearly allied is the American Mesohippus, ,ranging from the Lower Miocene to the Lower Oligocene of the United States, of which the earliest species stood only about 18 in. at the See also:

shoulder. The incisors were scarcely, if at all, infolded, and there is a rudiment of the fifth metacarpal (fig. 3, b). By some writers all the species of Mesohippus are included in the genus Miohippus, but others consider that the two genera are distinct. Mesohippus and Miohippus are connected with the earliest and most primitive mammal which it, is possible to include in the family Equidae by means of Epihippus of the Uinta or Upper Eocene of North America, and Pachynolophus, or Orohippus, of the Middle and Lower Eocene of both halves of the northern hemisphere. The final See also:stage, or rather the initial stage, in the series is presented by Hyracotherium (Protorohippus), a mammal no larger than a See also:fox, See also:common to the Lower Eocene of Europe and North America. The general characteristics of this See also:pro-genitor of the horses are those given above as distinctive of the group. The cheek-teeth are, however, much simpler than those of Anchitherium; the transverse crests of 'the upper molars not being fully connected with the outer wall, while the premolars in the upper jaw are triangular, and thus unlike the molars. The incisors are small and the canines scarcely enlarged; the latter having a gap on each side in the lower, but only one on their hinder aspect in the upper jaw. The fore-feet have four complete toes (fig.

3, a), but there are only three hind-toes, with a rudiment of the fifth metatarsal. The vertebrae are simpler in structure than in Equus. From Hyracotherium, which is closely related to the Eocene representatives of the ancestral See also:

stocks of the other three branches of the Perissodactyla, the transition is easy to See also:Phenacodus, the representative of the common ancestor of all the See also:Ungulata. See also H. F. See also:Osborn, " New Oligocene Horses," See also:Bull. Amer. See also:Mus. vol. xx. p. 167 (19o4); J. W. Gidley, Proper Generic Names of Miocene Horses, p.191; and the See also:article PALAEONTOLOGY. (R.

End of Article: EQUIDAE

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