EQUIDAE, the horse family, a family of hoofed mammals, of the order Perissodactyla, of which the modern genus Equus is the type. It is customary to include in this family the entire series of Tertiary ancestors of the horse, the three principal progressive stages being distinguished as sub-families—Hyracotheriinae, An chitheriinae and Equinae. The Equidae, with the nearly related family of Palaeotheriidae, are included in the Hippoidea, one of the four primary groups into which the Perissodactyla subdivided early in the Eocene, the others being the Tapiroidea, Rhinocero toidea and Chalicotheroidea. The fundamental distinction lies in the composition of the molars. The primitive pattern of the upper molars in the Perissodactyla consists of six rounded cusps partially united into an outer and two obliquely transverse crests. In the Hippoidea this pattern is converted into two outer crescents and two incomplete oblique transverse crests. This primitive pattern is perfected in the Anchitheriinae and is further modified in the Equinae in connection with the progressive conversion of the transverse crests into a pair of inner crescents and one or two more or less isolated inner pillars. The molars are thus finally con verted into tall square prisms with a surface pattern of four cres cents and an inner pillar, which grow up from the jaw as they are worn off at the top. They do not, however, attain the final stage of hypsodonty and become rootless ever-growing teeth as do the molars of edentates, some rodents, etc., but close up the roots at about the sixth year (in the modern horse). As in all Perissodactyls the premolars except p i become progressively molariform. The front teeth in primitive Perissodactyla consist of a convex row of small spatulate incisors flanked by larger and more pointed canines, and separated by a considerable gap or diastema from the cheek teeth (premolars and molars). In the Hippoidea the incisors are progressively developed as a row of cropping teeth, the canines tend to be reduced or incisoform, the lower cheek teeth conform to the changes in the upper series, de veloping in the anchitheriine stage two obliquely set unsymmetric crescents, the posterior wing strong and subtransverse, the an terior wing weak, while in the Equinae the pattern is changed into a pair of symmetrical inward-facing outer crescents flanked by four supporting pillars on the inner side.
The feet in the Hippoidea undergo a marked reduction and spe cialization of the digits, commencing with the primitive hyra cotheriines with four well-developed toes in the fore and three in the hind foot, followed by the anchitheriines with three toes on each foot but the lateral digits much reduced, then by the Equinae with the lateral digits further reduced, and finally becoming ves tigial splints, the phalanges wholly lost, and the foot monodactyl. As the lateral digits are reduced the central (third) digit is en larged, the metapodials elongated into cylindrical tubular cannon bones, the central wrist and ankle bones enlarged and the lateral bones tend to acquire a footing on the median digit, or else dis appear. The ulna and fibula are reduced and consolidated with the radius and tibia, these bones are elongated, while the humerus and femur tend to assume a more horizontal position and to be caught up into the shoulder and flank of the body. The neck vertebrae and the facial part of the skull are very considerably increased in length, the orbits become enclosed by a bridge of bone separating them from the temporal fossa, the angular region of the lower jaw and the ridge above the cheek teeth are much de veloped, in order to provide adequate attachments for powerful masseter muscles used in grinding the food; but the coronoid process and the median sagittal crest over the brain-case are rela tively reduced, as the temporal jaw muscles are less used. The brain likewise increases in size and number of convolutions.
All these changes are features of adaptation of the equine phylum to swift-running for long distances in open and more or less level country, and to feeding chiefly upon the dry, hard grasses of the plains. They are paralleled in the evolution of va rious other races of similar habits and environment. In one family of the extinct Litopterna (q.v.) of South America a completely monodactyl running foot was developed, from the third digit as in the horses, but in the teeth these animals were less progressive than the horses, reaching only to a stage comparable with Meso hippus. In the ruminants a similar running foot is developed by consolidation of third and fourth digits ; and in many of them, especially the cattle, there is also a close parallelism in teeth to the modern horse, the upper molar having two outer and two inner crescents and a separate inner pillar ; but this is evolved out of different primary cusp elements than in the horses, and the pre molars do not become molariform in any ruminant. The kangaroo has likewise developed a nearly monodactyl foot, but for leaping instead of running.
The evolution of the Equidae has been traced through a closely connected series of intermediate stages back to Hyracotheriurn or Eohippus of the Lower Eocene, about the size of a fox, with four complete toes on the fore foot and three on the hind foot and with short-crowned semi-bunodont molars, premolars smaller and of simple pattern. In the Middle and Upper Eocene stages (Orohippus, Epihippus) the premolars except pi become pro gressively molariform and the molar crowns somewhat higher with more clearly defined crescents and cross-crests ; the lateral digits are slightly reduced but the outer fifth digit of the fore foot is still functional. In the Oligocene Mesojiippus and Miohippus the premolars except p i are fully molariform and the feet much more progressive, three toes on fore and hind, the lateral digits reduced and median digit enlarged, the fifth digit of the fore foot reduced to a small short splint. Parahippus of the Lower Miocene and Merychippus of the Middle and Upper Miocene show a pro gressive increase in the height of the cheek teeth, with correspond ing development of cement on the crown and change of pattern toward that of the later horses. The feet also progress, becoming fully unguligrade, the side toes further reduced and no longer resting on the ground, while the central metapodial becomes longer and more tubular.
Piioliippus of the Lower and Plesippus of the Upper Pliocene have the teeth further elongated and the pattern progressively nearer to that of Equus, the lateral digits reduced to splints, very long in the former, shortened up in the latter genus.
Equus proper appears at the base of the Pleistocene in America and was widely distributed during that period (see HORSE, fossil) surviving to-day in Asia and Africa.
Side branches of this phylum are Anchitlierium and Hypohippus of the Miocene and Pliocene, retaining the short-crowned teeth and other characters of Miohippus but increasing progressively to the size of a small horse (Hypohippus matthewi, Pliocene) ; and Hipparion of the Pliocene retaining the three-toed feet of Merychippus but with progressive teeth of a pattern somewhat diverse from the Equus line. Both these side branches originated in America but found their way to the Old World, Anchitherium in the Miocene, Hipparion in the Pliocene, as did Equus itself in the Pleistocene. The region of evolution and dispersal of the family appears on present evidence to have been North America. A third side branch, Hippidium, Onohippidium, is found in South America. It is of the size of Equus but has shorter legs and a peculiarly modified nasal opening in the skull ; the cheek teeth are very like those of Plioliippus.
Many of the later Tertiary Equidae have a deep pit or fossa on the skull in front of the orbit, which has been variously explained as indicating a proboscis (like tapirs) or a scent gland (like deer), but is now believed to have lodged an air-sac communicating with the front end of the nasal passages, remnants of which are found in modern horses.
BIBLIOGRAPHY.-Gidley, Bull. Amer. Mus. Nat. Hist., vol. xxiii. Bibliography.-Gidley, Bull. Amer. Mus. Nat. Hist., vol. xxiii. (1907) ; Lull, Amer. Jour. Sci., vol. xxiii. (19o7) ; W. Granger, Bull. Amer. Mus. Nat. Hist., vol. xxiv. (1908) ; W. D. Matthew and Chubb, Amer. Mus. Nat. Hist. Guide Leaflet, No. 36 (1913), end ed. 1921 ; H. F. Osborn, Mem. Amer. Mus. Nat. Hist., N.S., vol. ii. (1918) ; W. D. Matthew, Quar. Rev. Biol., vol. i., with bibliography (1926) .
(W. D. M.)