EVOLUTION OF THE HORSE IN NATURE LS and also a tiny remnant of the innermost or first digit. The proportions of the skull, the short neck and arched back and the limbs of moderate length, were very little horselike—recalling on the contrary, some modern ‘arnivorous animals, especially the civets (Viverride). The teeth were short-crowned and covered with low rounded knobs of enamel, suggesting those of monkeys and of pigs or other omnivorous animals, but not at all like the long-crowned complicated grinders of the horse. Commencing with the Hyracotherium, twelve stages have been recognized from as many successive formations, showing the gradual evolution of the race into its modern form, and each stage is characteristic of its particular geological horizon. Some of the stages have been found in several parts of the world, but by far the most complete and best known series comes from the Tertiary Badlands of the western United States. Besides the main line of descent which led into the modern horses, asses and zebras, there were several collateral branches which have left no descendants. Of some stages all parts of the skeleton have been found; of others only the jaws, or jaws and feet, are known. We can mention only the more important stages.
1 and 2.!. Hyracotherium and Eohippus. Lower Eocene. The Hyracotherium is the most primitive stage known, but only the skull has been found, so that it has not been determined exactly what the feet were like. The teeth display six rounded knobs or cusps on the upper molars and four on the lower ones, and these are just beginning to show signs of fusing into cross-crests. The premolar teeth have only one main cusp, except the third and fourth premolars (next the molars) in each jaw, which have two and three, respectively. The only specimens which have been found were in the London Clay or Lower Eocene of England and are preserved in the Fig. 4. Hohippus. Lower Eocene. et Viaeseum. Upper teeth, natural size. Short- : : crowned teeth, no cement, premolars The Hohippus is much better simpler and smaller than molars known. It comes from the Lower Eocene of Wyoming and New Mexico, and is very like the Hyracotherium? except that in some species the molar teeth have the cusps more clearly fusing into cross-crests; the last premolar is beginning to look like one of the true molars. The fore foot of this animal has four complete toes. The hind foot has three complete toes and the splints of the first and fifth digits can still be detected in some species. <A skeleton of Hohippus was found by Dr. J. L. Wortman in 1880 in the Wind River Badlands of 1These numbers refer to the stages in the direct line of descent of the modern horse.
2It is doubtful whether it is a distinct genus.
16 AMERICAN MUSEUM GUIDE LEAFLETS Wyoming, and was described by Professor Cope in 1885. It was until recently the only skeleton known of a four-toed horse, and is well known to the scientific world. It was purchased by the trustees of the American Museum in 1894 and is now on exhibition. a9 A second skeleton found by the Museum expedition of 1910, is of a larger species, but not advanced in any other respect. It is mounted in a grazing attitude, and probably represents the general appearance of Eohippus more truly than the Cope skeleton. A third imperfect skeleton . secured in 1911 is interesting as it shows the tiny splint bone of the inner digit of the hind foot, indicated by facets in other specimens but not actually found before. Crushed skulls, jaws and fragments of skeleton bones of thousands of individuals have been found in the Lower Eocene formations.
3. Orohippus. MuippiLe Eocene. In this animal the splints of the first and fifth digits of the hind foot have disappeared, but there are still four complete and usable toes in the fore and three in the hind foot. The crests on the molars are a little clearer and the last premolar has become almost like the molars, while the next to the last premolar is partially molariform. A skeleton of Orohippus, discovered by Mr. Walter Granger of the Museum expedition of 1906, in the Bridger Badlands of southwestern Wyoming, is exhibited next to the Hohippus skeleton. It shows an animal of the size of a small dog and proportioned much like the breed known as the whippet.
The Orohippus was of about the same size and proportions as some of the pigmy antelopes. or duikerboks (Cephalophus), which live in the open forests and brush country of East and South Fig. 5. Orohippus. Middle Eocene. Africa, and have gained their Upper teeth, natural size. Short-crowned name from the quickness with teeth, no cement, fourth premolar like which they can dive or “duck” molars into the tall grass and jungle on the approach of danger.
4. Epihippus. Upper Eocene. Of this stage of the evolution of the horse only incomplete specimens have been Fig. 6. Epihippus. Upper Eocene.
Upperteeth,naturalsize.Short-crowned teeth, no cement, third and fourth pre- found. The molar teeth have the molars like molars once round cusps almost completely converted into crescents and crests, while another tooth of the premolar series has become like the molars. The toes are still four in the fore foot and three in the hind
EVOLUTION OF THE HORSE IN NATURE 17 foot, but the central toe in each foot is becoming much larger than the side toes.
Paleotherium and Plagiolophus of the Upper Eocene of Europe form aside branch of the horse line. They were very abundant in Europe, but have not been found in the New World. One of these animals was thought by Professor Huxley to be a direct ancestor of the horse, but it is now considered to be merely a collateral relative. Some species of Paleotherium were of large size, equal to a tapir. They were first deseribed in the year 1804 by the celebrated Baron Cuvier from remains found in the gypsum quarries of Montmartre, Paris.
THREE-TOED HORSES 5, 6 and 7. Mesohippus and Miohippus. OuicoceNr. In this stage there are three toes on each foot, and a splint representing the fifth digit of the fore foot of the Eocene ancestors. The middle toe is now much larger than the side toes, which bear very little of the weight of the animal. Three of the premolars have now become entirely like the molar teeth, the crests on the crown are completely formed, and the outside crest in the upper molars has taken the shape of two crescents. In the Lower and Middle Oligocene are found several species of Wesohippus mostly of the size of a coyote, represented by two mounted skeletons, and various skulls, Jaws and feet. Inthe Upper Oligocene occurs MW iohippus intermedius as large asasheep, of which a fine skeleton is exhibited in the pavilion case. Several other species of larger or smaller size are represented by skulls and jaws.
The series of skeletons in the pavilion case shows in a very striking way the steady increase in size from the Lower Eocene to the Upper Oligocene, and the reduction of the side toes with concentration of the weight upon the middle toe.
Fig. 7. Mesohippus. Middle Oligocene. Upper teeth, natural size. Shortcrowned teeth, no cement, second, third and fourth premolars like molars 8a. Anchitherium and Hypohippus. Muocene. Anchitheriwn has been found only in Europe, and is still very incompletely known. It is much like Mesohippus, but is larger and has the crests of the teeth somewhat higher and more complete. Hypohippus is off the direct line of descent; its teeth are like those of Anchitherium, by which name it has
Fig. 8. Little three-toed horse (Mesohippus bairdi) from the Middle Oligocene of South Dakota. American Museum No. 1492 Four-toed horse (Hohippus venticolus) from the Lower Eocene of Wyoming. American MuseumNo.4832 IS
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20 AMERICAN MUSEUM GUIDE LEAFLETS been generally called, but the animal was much larger, equalling a Shetland pony in size. A complete skeleton of the Hypohippus (shown in wall case) was found near Pawnee Buttes, Colorado, in. 1901 by Mr. Barnum Brown, of the Whitney expedition. , Hypohippus first appears in the Middle Miocene, and survives along with more advanced stages of the three-toed horses. Itisalargerand more heavily proportioned animal; the wide flat hoofs and strong side toes enabled it to tread on soft ground, and the teeth are fitted for browsing rather than grazing. Hence Professor Osborn considers it as adapted for a forest life, and has named it the ‘‘three-toed forest horse.’? The restoration illustrates this supposed habitat and adaptation. Sand 9. Parahippus. Miocene. In Parahippus the tooth-crests are somewhat higher, and the transverse ridges on the upper molars are beginning to change shape so as to become a second pair of crescents inside the outer pair. In the valleys between the cross-crests and on the sides and base of the crown, a deposit of cement appears on the surface of the enamel. In some species there is a considerable amount, but usually it isa very thin coat. There is no cement on the milk teeth. The feet show a decided advance over the preceding stages, the side toes are very slender, and no longer touch the ground. The fifth digit of the fore foot is in some species a very short small splint, in others it is still further reduced to a little irregular nodule of bone. This stage first appears in the Lower Miocene, and several skulls and incomplete skeletons have recently been obtained from formations of this age. It survived however, although rare, as a contemporary of the later three-toed horses.
10. Merychippus. \MuIppDLE MioceNnrt. This stage marks the transition between the older short-crowned teeth without cement, and the long-crowned heavily cemented teeth of the Fig. 10. Parahippus. Lower Miocene.
Upper teeth, one-half natural size. Shortcrowned teeth, traces of cement. The teeth are larger than in Mesohippus [the later stages. The permanent drawing is on a smaller scale], the crowns molars are intermediate in length a little longer, and the inner “crests” of erown, and quite heavily cebegin to show the change to ‘“‘crescents”’ mented. but the milk molars are short-crowned and have little or no cement; they are not easily distinguishable from those of Parahippus. The side toes are slender and no longer reach the ground; in some species they are almost reduced to splints. Merychippus is common in the Middle and later Miocene, and many skulls, jaws and incomplete skeletons are contained in our collections, some of which are placed on exhibi-
EVOLUTION OF THE HORSE IN NATURE Zl tion. These very perfect specimens show a vestige of the fifth digit in the fore foot still preserved as a tiny nodule of bone at the back of the wrist.
11. Hipparion, Protohippus and Pliohippus. Upper Miocene AND PLIOocENE. These three closely related genera represent the latest stage of three-toed horses, before the side toes were reduced to splints. The teeth are long-crowned, both milk and permanent teeth being heavily cemented, and the side toes are extremely slender. They first appear in the Upper Miocene, probably directly descended from Middle Miocene species of Merychippus, and are best represented by the beautifully preserved skeleton of Hipparion whitney: from Little White River, Fig. 11. Merychippus. Middle Miocene. Upper teeth, one-half natural size. Moderatelylong-crownedteeth,wellcemented South Dakota, found by Mr. H. F. Wells of the Whitney expedition of 1902. This species, except for the very large head, had the graceful and slender proportions of the antelopes, but in Protohippus and especially in Pliohtppus the skeleton approached more nearly the stockier proportions of the modern horses. The Hipparion whitneyi is regarded by Professor Osborn as fitted to live in a semi-desert country, and in contrast to the Hypohippus, is called the ‘‘three-toed desert horse.”’ The restoration depicts this adaptation and environment. In this stage the crowns of the upper molars have become much longer, the two pairs of crescents on the upper molars are complete, with two half-separated cusps within the inner pair. And the valleys between the crests are completely filled with cement, so that with the wear of the teeth the edges of hard enamel are backed inside by dentine and outside by cement. In this way the surface of the tooth has a series of enamel ridges always projecting a little above the grinding surface, because the softer material on each side wears down into hollows, yet never breaking off, because they are braced so thoroughly on each side. This is a very efficient instrument for grinding hard grasses. The crowns of the teeth in these Miocene horses are by no means as long as in the modern horses; they must therefore have worn more slowly or worn out at an earlier age.
The feet in these genera have but one toe touching the ground. The side toes (second and fourth digits) are complete, but much more slender
22 AMERICAN MUSEUM GUIDE LEAFLETS than in the earlier stages and are apparently useless, as they cannot reach the ground. In some species they have almost disappeared.}) The fore feet of these Upper Miocene horses still retain the tiny nodule of bone at the back of the ‘‘knee”’ (the joint that corresponds to the wrist of a man), which is the last remnant of the fifth digit possessed by their remote ancestors. Hipparion is common in Europe and Asia as well as in North ‘America, but Protohippus and Pliohippus are not found in the Old World.
Fig. 12. Hipparion. Lower Pliocene. Upper teeth one-half naturai size. Long-crownedteeth,heavilycemented ONE-TOED HORSES 12. Pliohippus. PLIOCENE. In some species of Pliohippus, although not in all, the side toes are still further reduced to long slender splint-like bones which are enclosed within the skin, the little separate toes having disappeared. These are the first of the One-toed Horses. The splint is nearly as long as the cannon bone, much longer than it is in Equus. Two fine skeletons of Pliohippus inthe Museum collections (P. letdyanus, adult, and P. lullianus, young) admirably illustrate this stage.! They are of somewhat later geologic age than the Hipparion skeleton, and nearer to the modern horse in size, proportions and various details of structure. Yet they are so much like other species of Pliohippus in which the side toes are still complete that both are placed in the same genus. Pliohippus is therefore transitional from the threetoed to the one-toed stage. It may be that some of the later species of Hipparion were also one-toed, but at present there is no proof of it. 13a. Equus. PLEISTOCENE and RECENT. In this stage, that of the modern horse, the side toes have entirely disappeared and are represented by moderately long splints on the fore and hind foot. No trace remains on the fore foot of the little nodule which in previous stages represented the fifth digit, while on the inner side of the wrist the ‘‘trapezium,”’ 1Pliohippus was described in 1874 by Professor Marsh, as a one-toed ancestor of the horse; but his specimens do not prove that the side toes were certainly absent. These two skeletons found in 1917 by H.J.Cookand1916byF.R.Troxellshowthisconclusively,andwearethereforeabletodistinguish themasaseparatestageintheevolutionofthehorse.Inpreviouseditionsofthisguide,1903,1905, 1913, this stage was not distinguished, as conclusive proof from specimens had not been found.