Evolution of the horse

Evolution of the horse

Evolution of the horse http://hdl.handle.net/1874/30316 Evolution of the horse Alter Osborn SKELETON OF HORSE AND MAN Mounted for oomparative study...

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Evolution of the horse


Evolution of the horse Alter Osborn SKELETON OF HORSE AND MAN Mounted for oomparative study. Gift of the late William C. Whitney to the AmericanMuseum of Natural History

Evolution of the horse N NO. 36 OF THE CUIDE LEAFLET SERIES O? THE AMERICAN MUSEUM OF NATURAL HISTORY THIRD EDITION, REVISED AND ENLARCED Published under the direction of Henry Fairfield Osborn, President of the Boardof Trustees of the Museum. Curator Emeritus of Fossil Vertebrates MARY CYNTHIA DICKERSON, Editor

Evolution of the horse CONTENTS Part I. Evolution of the Horse in Nature Introduction

of the HorseIndex ...........

. . fossil horses of the aoe of man evolution of the horse Four-toed Horses . Three-toed Horses One-toed Horses Conclusions Meaning of the Change in Feet and TeethCause of the Evoltjtion ....Parallel Evolution in Other Racer Part II. The Horse under Domestication Origin of the Domestic Horse comparison of the skeleton of horse and man The Draft Horse...... Development of Great Size in the Draft Horse The Shetland Pony The Arabian Horse The Race Horse The Teeth of the Horse Structure of the Teeth Wear and Compensating Movement of the Ineisors Designation of the Teeth ..... Wolf Tooth of the Horse . . . . . Premolars and Molars of the Horse Eruption of the Teeth of the Horse Shedding the Teeth ....... Principal Literature on the 'Evoltjtion

Evolution of the horse FIG. 3. EVOLUTION OF THE HIND FOOT. PRINCIPAL STAGES(Right to Left) 1. Eohippus, Eocene Epoch; 2. Mesohippus, Oligocene; 3. Merychippus, Miocene; 4. Equus, Pleistocene and Modern Outer views showing middle and outer digits. Note progressive reduction of side toe (digit IV) from a well developed digit to a splint bone


Evolution of the horse POSSIL HORSES OF THE AGE OF MAN THE Age of Man, or Quaternary Period, is the last and by far the short-est of the great divisions of geological time. It includes the GreatIce Age or Glacial Epoch (Pleistocene), when heavy Continentalglaciers covered the northern parts of Europe and North America, and theRecent Epoch, of more moderate climate during which civilization has arisen. In the early part of the Quaternary Period, wild species of horse were tobe found on every continent except Australia. Remains of these true nativehorses have been found buried in strata of this age in all parts of the UnitedStates, in Alaska, in Mexico, in Ecuador, Brazil and Argentina, as wellas in Europe, Asia and Africa. All these horses were much like the livingspecies and most of them are included in the genus Equus. Mr. J.

W.Gidley, in charge of the American Museum expedition in northern Texas,discovered in 1899 a number of more or Iess complete skeletons of a speciesof fossil horse, Flquvs scotti, in an old river deposit on Rock Creek in DonleyCounty. These are the most complete specimens known of the extincthorses which inhabited this country at the beginning of the Ice Age. Amounted skeleton and several skulls have been placcd on exhibition, andtheir near resemblance to the modern animal appears at a glance. Thedifferenee from the domestic horse is chiefly in proportions: the skull isshorter with deeper jaws, the legs rather short and the feet small in pro-portion to the body. In these charaeters this fossil horse resembles anovergrown zebra rather than a domestic horse. We know nothing of itscoloring. It may have been striped, and in this case

would have been veryzebra-like; but there are some reasons for believing that it was not promi-nently striped. The bones are petrified, brittle and heavy, the animalmatter of the bone having entirely disappeared and having been partij'replaced by mineral matter. They are not much changed in color how-ever, and are so perfectly preserved that they look almost like recent bone. All the remains of these native horses which have been found in Americahave been petrified more or less completely. This means that they havebeen buried for many thousands of years, for true petrifaction is anexceedingly slow process. This condition serves as an easy means of dis-tinguishing them from bones of the domestic horse, found buried in theearth. These cannot in any case have been buried for more than three orfour centuries, and have not had time to

petrify. Remains of these fossil horses from various parts of the United Statesare shown in the counter cases. One very rich locality is on the NiobraraRiver in Nebraska, another is in central Oregon. Many separate teeth andbones have been found in the phosphate mines near Charleston, SouthCarolina; other specimens have come from central Florida, from southernTexas, Arizona, Kansas, Louisiana and even from Alaska. They are, infact, so often found in deposits of rivers and lakes of the latest geological12

Evolution of the horse THREE TOED HORSES 5, 6 and 7. Mesohippus and Miohippus. Oligocene. In this stagethere are three toes on each foot, and a splint representing the fifth digit ofthe fore foot of the Eocene ancestors. The iniddle toe is now much largerthan the side toes, which bear very little of the weight of the animal. Threeof the premolars have now become entirely like the molar teeth, the crestson the crown are completely formed, and the outside crest in the uppermolars has taken the shape of two crescents. In the Lower and MiddleOligocene are found several species of Mesohippus mostlv of the size of acoyote, represented by two mounted skeletons, and various skulls, jaws andfeet. In the Upper Oligocene occurs Miohippus mtermcdius as large as asheep, of which a fine skeleton is exhibited in the pavilion case.

Severalother species of larger or smaller size are represented by skulls and jaws. The series of skeletons in the pavilion case shows in a very strikingway the steady increase in size from the Lower Eocene to the Upper Oligo-cene, and the reduc-tion of the side toeswith concentrationof the weight uponthe middle toe. . c


Fig. 7. Mesohippus. Middle Oligocene. Upper teeth, num and Hyponip- natural size. Short-erowned teeth, no cement, second,pus. MlOCENE. An- third and fourth premolars like molarschitherium has been found only in Europe, and is still very incompletely known. It is muchlike Mesohippus, but is larger and has the crests of the teeth somewhathigher and more complete. Hypohippus is off the direct line of descent;its teeth are like those of AnchUherium, by which name it has been generallycalled,

but the animal was much larger, equalling a Shetland pony in size.A complete skeleton of the Hypohippus (shown in wall case) was found nearPawnee Buttes, Colorado, in 1901 by Mr. Barnum Brown, of the Whitneyexpedition. Hypohippus first appears in the Middle Miocene, and survives alongwith more advanced stages of the three-toed horses. It is a larger and moreheavily proportioned animal; the wide flat hoofs and strong side toesenabled it to tread on soft ground, and the teeth are fitted for browsingrather than grazing. Hence Professor Osborn considers it as adapted for aforest life, and has named it the " three-toed forest horse." The restorationillustrates this supposed habitat and adaptation. 8 and 9. Parahippus. Miocene. In Parahippus the tooth-crests aresomewhat higher, and the transverse ridges on the upper

molars are begin-ning to change shape so as to become a second pair of crescents inside the 17

Evolution of the horse Fig. 8. Little three-toed horse (Mesohippus bairdi) from the Middle Oligocene ofSouth Dakota. American Museum. No. 1492 Four-toed Horse {Eohippus renticolus) from the Lower Eocene of Wyoming. AmericanMuseum, No. 4832 ,

Evolution of the horse 20

horses. 10.

AMERICAN MUSEUM GUIDE LEAFLETS outer pair. In the valleys between the eross-crests and on the sides andbase of the crown, a deposit of cement appears on the surface of the enamel.In some species there is a considerable amount, but nsually it is a very thincoat, There is no cement on the milk teeth. The feet show a decidedadvance over the preceding stages, the side toes are very blender, and nolonger touch the ground. The fifth digit of the fore foot is in some species avery short small splint, in others it is still further reduced to a little irregularnodule of bone. This stage first appears in the Lower Miocene, and severalskulls and incomplete skeletons haverecently been obtained from formationsof this age. It survived however, although rare, as a contemporary of thelater three-toed

Merychippus. Middi.e Miocene. This stage marks the transi- tion between the oldershort-crowned teeth with-out cement, and the longcrowned heavily cementedteeth of the later stages. Fig. 10. Parahippus. Lower Miocene. Upperteeth, one-half natural size. Short-crowned teeth,traces of cement. The teeth are larger than inMesohippus [the drawing is on a smaller scalel, theerowns a little longer, and the inner "crests"begin to show the change to "crescents" The permanent molars areintermediate in length ofcrown, and quite heavilycemented, but the milkmolars are short crowned and have little or no ce-ment; they are not easilydistinguishable from thoseof Parahippus. The sidetoes are slender and no Fig. 11. Merychippus. Middle Miocene. Upperteeth, one-half natural size. Moderately

long-crowned teeth, well cemented longer reach the ground; insome species they are al- most reduced to splints.Merychippus is common in the Middle and later Miocene, and many skulls, jaws and incompleteskeletons are contained in our collections, some of' which are placed onexhibition. These very perfect specimens show a vestige of the fifth digitin the fore foot still preserved as a tiny nodule of bone at the back of thewrist. 11.

Middle Miocene species ofMerychippus, and are best represented by the beautifully preserved skeleton

Hipparion, Protohippus and Pliohippus. Upper Mioceneand Pliocene. These three closely related genera represent the latest stageof three-toed horses, before the side toes were reduced to splints. The teethare long-crowned, both milk and permanent teeth being heavily cemented,and the side toes are extremely slender. They first appear in the Upper i Miocene, probably directly descended from

Evolution of the horse 22

AMERICAN MUSEUM GUIDE LEAFLETS stages, but progressively smaller, is now generally absent entirely. Thecrowns of the teeth are much longer, and of the two half-separated innercolumns on the upper molars, one has disappeared, the other has increasedin size and changed in form. The skull has lengthened and the animal ismuch larger. It is well represented among the fossil horses by the skeleton and skullsof Equus scotti already noticed. Fig. 13. Equus. Pleistocene. Upper teeth, one-half natural size. Verylong-crowned teeth, heavily cemented 126. Hippidium. Pleistocene. South America. The feet are likethose of Equus, except that they are short and stout. The teeth are likethose of Pliohippus, from which it is supposed to be descended. The skullis large and long with very long

slender nasal bones. A cast of the skeletonpresented by the Museo Nacional of Buenos Ayres, Argentine Republic,is on exhibition. CONCLUSIONS THESE are the principal stages in the evolution of the modern horsefrom the little four-toed Eohippus. Intermediate between themare numerous minor stages, the earlier species of each genus beingmore primitive, the later species more advanced. The series of upper molarteeth shown in Figs. 19-20 show no wide differences from one stage to thenext. But between most of them several intermediate species are known.This gradual change is seen not merely in one or a few selected parts, but inevery bone, every tooth, every portion of the skeleton. Elsewhere in thehall may be found similar although less complete series leading up fromanimals very close to Eohippus into the modern

tapir and rhinoceros.The conclusion is unavoidable that horse, rhinoceros and tapir, three raceswidely different to-day, are derived through progressive changes from acommon ancestral type. New species may have appeared suddenly, butthe race in its broader lines is the product of gradual evolution, and diverseraces may be traeed back to a more ancient common stock.1 1 Scientiflc criticisms of " Darwinism" are concerned with the way in which newspecies have appeared, whether by inftnitesimal gradations or by finite "mutations,"appreciable although usually small steps. The broader lines of evolution are not affectedby these criticisms.

Evolution of the horse 24 FIG. 15. SKELETON OF SHORT-LIMBED HORSE HippidiumFrom Pleistocene of Argentine Republic. From a cast in the American Museum

Evolution of the horse 20 AMERICAN MUSEUM GUIDE LEAFLETS is regulated by the distance of the centre of gravity from the point ofattaohmcnt, as that of a pendulum is by the height of the bob. To increasethe length of lower leg and foot therefore gives the animal greater speed;but it puts an inereased strain on the ankles and toe-joints, and thesemust be strengthened correspondingly by convertingthem from ball-and-soeket joints to "ginglymoid"or pulley joints. Additional strength, likewiseat the expense of flexibility, is obtained by theconsolidation of the two bones of the f ore-arm (ulna and radius) and of the leg(tibia and fibula) into one, the shaftof the smaller bone practically dis-appearing, while its ends becomefused solidly to its largerneighbor. The increase in length of limb renders it neces- sary for the grazing

animal that the head and neck sho in-crease inlength FIG. 17. EVOLUTION OF THE FORE LEG. PRINCIPAL STAGES In the four-toed horses the radius and ulna are separate and their shafts ofabout equal size. In the earlier three-toed horses the ulna is separate but its shaftconsiderably reduced. In the later three-toed horses the ulna is partly Consoli-dated with the radius and its shaft is reduced to a thin thread. In the one-toedhorses the ulna is more eompletely consohdated with the radius and its shaft h^sentirely disappeared

Evolution of the horse A/ter Osborn FIG. 21. EVOLUTION OF THE HIND FOOT OF THE HORSE Side views of six stages, Eohippus, Mesohippus, Miohippus, Merycfiippus, Hipparion,Equus

Evolution of the horse CAUSE OF THE EVOLUTION THE evolution of the horse, adapting it to live on the dry plains,probably went hand in hand with the evolution of the plains them-selves. At the commencement of the Age of Mammals the westernpart of the North American continent was by no means as high above sealevel as it now is. Great parts of it had but recently emerged, and the Gulfof Mexico still stretched far up the valley of the Mississippi. The climateat that time was probably very moist, warm and tropical, as is shown by thetropical forest trees, found fossil even as far as Greenland. Such a climate,with the low elevation of the land, would favor the growth of dense forestsall over the country, and to such conditions of life the animals of the be-ginning of the mammalian period must have been adapted. During theTertiary the continent was steadily rising

above the ocean level, and at thesame time other influences were at work to make the climate continuallycolder and drier. The coming on of a cold, dry climate restricted andthinned the forests and caused the appearance and extension of open,grassy plains. The ancient forest inhabitants were forced either to retreatand disappear with the forests, or to adapt themselves to the new conditionsof life. The ancestors of the horse, following the latter course, changed withthe changing conditions, and the race became finally as we see it to-day, oneof the most highly specialized of animals in its adaptation to its peculiarenvironment. At the end of the Age of Mammals the continents stood at ahigher elevation than at present, and there was a broad land connectionbetween Asia and North America, as wcll as those now existing. At thistime the horse became cosmopolitan, and

inhabited the plains of all thegreat continents, excepting Australia. It is a question whether the direct ancestry of the modern horse is to besearched for in western America or in the little known interior plains ofeastern Asia. It is also unknown why the various species which inhabitedNorth and South America and Europe during the early part of the Age ofMan should have become extinct, wdiile those of Asia (horse and wild ass)and of Africa (wild ass and zebra) still survive. Man, since his appearance,has played an important part in the extermination of the larger animals;but there is nothing to show how far he is responsible for the disappearanceof the native American species of horse. 31

Evolution of the horse PARALLEL EVOLUTION IN OTHER RACES IT is interesting to observe that while the evolution of the horse wasprogressing during the Tertiary period in North America anothergroup of hoofed animals, the Litopterna. now extinct, in South Americaevolved a race adapted to the broad plains of Argentina and Patagonia andsingularlv like the horse in many ways. These animals likewise lost thelateral toes one after another, and concentrated the step on the central toe;they also changed the form of the joint-surfaces from ball-and-socket to FIG. 22. THREE-TOED PSEUDO-HORSE Fore and hind feet, one-half natural size. Diadiaphorus of the Miooene of SouthAmerica, although so closely like the three-toed horses in the feet, has a whollydifferent skull and teeth, and belongs to a different order, the Litopterna, peeuliar toSouth America and now extinct. From

specimens in the American Museum 32


Evolution of the horse Part II. THE HORSE UNDER DOMESTICATION ITS ORIGIN AND THE STRUCTURE AND GROWTH OF THE TEETH By S. H. Chubb WHEN animals are living under perfectly natural eonditions, theirphysical structure is slowly modified by climate, topographyof habitats and food supply as well as by many other forces.In the struggle for self-preservation among the competitors by which theyare surrounded, they develop weapons of defence, or acquire speed mechan-ism or other means of escape. Thus what we may call natural, as opposedto artificial evolution, is controlled by a great variety of forces, while in acondition of domestication wTe have development directed in certain linesby man's intelligence. ORIGIN OF THE DOMESTIC HORSE IN the light of researches made by Professors Ridgeway, Osborn andEwart, there seems to be little doubt that the domestic horse has beenderived from several wild types which have since become extinct aswild species. Of two at least we may be reasonably

sure: one of Europeor northwestern Asia, which has been called the Norseman's horse; theother from northern Africa, which Professor Ridgeway has called Equuslibycus. There is abundant evidence to prove that in the late Quaternary, duringand after the Glacial Period, but nevertheless many thousands of years ago,prehistorie man chased and killed wild horses, using their flesh for food andpossibly their skins for raiment. This period was foliowed by a second,during which wild horses were captured, broken to rude harness and driven.The rearing action of the horse skeleton in the group on exhibition in theMuseum (Frontispiece) is designed to express unwilling subjection, and theposition of the man, as if holding a bridle, intellectual control. Theperiod of domestication passed insensibly into a third, that of artificialdevelopment, during which the horse was modified, and is stil! being modi-fied in various directions. Under man's protection and management, changes are brought about indomestic animals with considerable

control and much more rapidly thanunder perfectly natural eonditions. Through training and careful selec-tion in breeding, speed has been greatly increased in the race horse, weightand strength have been developed in the draft horse, while the Shetlandpony has been reduced to a most diminutive size. 37

Evolution of the horse Fig. 26. The draft horse pulling a heavy load. Gift of the late William C. Whitney.The subject from which the skeleton was prepared was presented by Mr. George Ehret Fig. 27. Arabian stallion 'Nimr." Mounted in the position of watching a herd. Subjectpresented by Mr. Randolph Huntington

Evolution of the horse THE HORSE VN DER DOMESTICATION :!9 The intimate relations which have thus existed between the horse andman have influenced both, and it is generally acknowledged by studentsof mankind that the subjugation of the horse and his adoption, both as arneans of transportation and as an aid in agriculture, have been factors ofthe greatest importance in the later development of the human race. COMPARISON OF THE SKELETON OF HORSE AND MAN WHILE considering these two skeletons (Frontispiece), it will beworth while to look for a moment into their structure. Theyare so placed as to facilitate comparison. The one representinga typical or average type of horse, the other a man of about six feet inheight and proportionately heavy. The limbs of the horse, moving onlyforward and backward, have much less freedom of motion than have those ofman. Note the ball-and-socket joint of man at the shoulder and hip, therotary motion of the forearm and the flexibility at the wrist and ankle, andcompare

with the restricted movement at the shoulder of the horse, thehingelike joint at the elbow, and the limited movement at the wrist (knee)and ankle (hoek). The most pronounced differences however, are found inthe head and feet. Compare the skull of man, which has an enormouslydeveloped brain and reduced facial portion, with that of the horse whichhas a comparatively small brain, the face, mouth, and teeth monopolizingalmost the whole skull. The feet, instead of having five toes as in man, arereduced to a single digitl and they are very much elongated as a strikingspeed specialization. And yet a careful «tudy will reveal a most strikingsimilarity between the two subjects in general structure, the differencesbeing simply modifications of a common plan. THE DRAFT HORSE THE Norseman's horse of Europe seems to have been an animal withlarge head, convex forehead and rough coat, of rather clumsyappearance, and of little speed. It may have been a near relativeof the Przewalsky horse which still lives as a wild species in

northwesternMongolia, the only true wild horse known at the present time. Amongother evidences of this prehistorie type, are the many sketches found in thesouth of France and elsewhere, made by the very primitive cave-dwellers,which seem clearly to depict this type of horse, and it is no doubt this strainwhich predominates in our heavy draft horse of to-day although of coursethis horse has been greatly increased in size and strength by long and careful breeding. 1 See page 21.

Evolution of the horse 40 AMERICAN MUSEUM GUIDE LEAFLETS An attempt has been made toexpress the strength acquired inthis breed in the mounted skele-ton of the draft horse (Fig. 26).This skeleton has been so mount-ed as to show the action of thebones when the animal is draw-ing a heavy load. We mustimagine that the shoulders restagainst a collar upon which thehorse is exerting its energy. Notethat the head and body are low-ered, the hind legs are doing thegreater amount of work, the forelegs acting as supports, though aportion of the weight of the bodyhas been thrown against the col-lar. From the rear view (Fig. 28)note that the right hip joint ismuch nearer the median linethan is the left. The joint isalso lower, and indeed, the entireright side of the pelvis has beenlowered and thrown well to theleft, so that when at the momentof greatest strain the right leg isextended, the pressure upon theanterior part of the body, andthence upon the collar, is appliedin a line which coincides withthe main axis of the body. Atthe

next moment the left leg issupposed to be exerted, the right isreleased, the pelvis then swingsto the right, the curve in thebackbone becomes reversed andthe pressure is again applied (thistime from the left) along themain axis of the body as before;and so on from step to step. Fig. 28. The draft horse. Rear viewphowing action of pelvis and backbone

Evolution of the horse DEVELOPMENT OF GREAT SIZE IN THE DRAFT HORSE THE skeleton of a very large draft horse is exhibited to show theextreme development of size in this breed. It has been photo-graphed with that of a Shetland pony to show two extremes indomestie horses (Fig. 25). The following is a table of measurements of thetwo specimens. Giant Draft Horse

Shetland Pony i Height at shoulders..........6 ft. 1 in. (181 hands) 2 ft. 9| in. (81 hands) Weight in life...........................2370 lbs. 170 lbs. Bulk of humerus......................118J cu. in. 91 cu. in. Bulk of femur.........................188 cu. in. 131 cu. in. The pose chosen for the mounting of this specimen is one of inaction,designed to express quiet restfulness, in contrast to the running action ofthe race horse, to be mentioned later. It shows also a habit peculiar tohorses, that of alternately resting the hind legs. Almost the entire weightof the hind quarters is supported on the extended left leg, while the righthind leg rests in a more flexed position and hangs perfectly

lax. The pelvisalso seems to hang, as it were, from the left hip joint, tilting very much tothe right and slightly twisting the vertebral column. A peculiar function of the patella (kneecap) in the horse is shown in theleft knee joint or stifle. While the limb is extended and supports the ani-mal's weight, the patella rests on a projecting articulation of the femur, sothat the knee is locked in an extended position by a very strong ligamentwhich holds the patella at a fixed distance from the tibia below, thus sus-taining the weight required of it with comparatively little muscular exertion. An abnormal peculiarity of this specimen is a pair of supernumerarymolar teeth, which appear at the posterior end of the upper set. Having noopposing teeth in the lower jaw upon which to wear, they have protrudeddown into the gum and must have caused the animal considerable discom-fort. THE SHETLAND PONY IT is highly probably that the Shetland pony is derived from practicallythe same wild stock as the very much larger draft horse, but has

beenreduced in size by ad verse conditions to which it was subjected in thecold and barren Shetland Islands with their limited area, and also byhaving been systematically bred for the smallest possible size. Figure 25 shows the skeleton of ar exceptionally small pony which wassecured through the Whitney fund. This pony was bred in Scotland andobtained through the kindness of Professor J. C. Ewart of the Universityof Edinburgh. The skeleton represents the grazing action. It will be seenthat the downward reach of the head and the slow, lax step modify the posi-tion of almost every bone in the body. 41


Evolution of the horse 48 AMERICAN MUSEUM GUIDE LEAFLETS of the cow and other simple teeth. The root of B is not as yet fully grown,but presents a large pulp cavity which will be reduced to a narrow nervecanal as the root grows longer, and tapers to a point, leaving only a verysmall foramen for the passage of the nerve. Figure 30C shows the perma-nent incisor of a colt two years old, which erupts and replaces its deciduouspredecessor at about three and a half years. The cul-de

mentioned above, is very deep, running down almost the full length of the crown, theroot not yet ha ving begun to develop. There is already considerable cementin the cul-de-sac, and a very slight deposit beginning on the external surfaceabout the upper end of the crown. A series of lower incisor sections andalso vertical sections of incisor teeth, exhibited in the wall case, will explainthe structure more clearly. WEAR AND COMPENSATING MOVEMENT OF THE INCISORS WHAT is frequently spoken of as " growth" in the

horse's teeth isnot growth, but rather movement. It is true that growth doescontinue in some few of the teeth until the horse is ten ortwelve years old, but only in the completion and closing of the roots(see above), the crowns being all fully developed, including grinders as wellas incisors, at about five and one-half years. From the time the tooth beginsits service it continues to move out of the alveolar cavity, which fills withnew bone, this movement counteracting the rather rapid wear of the cuttingor grinding surface, so that in extreme old age some of the teeth may beworn down to the very points of the roots. One of their most remarkablespecializations is the extremely long crown which instead of being entirelyabove the gum line, as with the more simple teeth, human for instance,extends far down into the alveolar cavity, as much as 3f inches in thelarge premolars, as a reservoir of grinding material. Figure 31 shows aseries of upper incisors to explain the manner in which the tooth is worn asit proceeds from the alveolar cavity, the

wearing surface altering in con-tour as it progresses along the crown of the tooth which constantly changesin shape toward the root. A is the permanent tooth of a colt two years old, before it has replacedits deciduous predecessor or has received any wear, so that the externalenamel is continuous with that of the cul-de-sac or internal enamel. 'Thecement now lines the cul-de-sac and is beginning to be deposited on theexternal enamel. As yet the tooth has no root, but presents a wide-openpulp cavity at the base. B is the tooth of a horse about four years old. It has recently replacedits deciduous predecessor, being in use only about six months. The foldededge of enamel is worn through, exposing the dentine beneath, and sepa-

Evolution of the horse Fig. 32. Designation of teeth A.

Lower jaw of colt 9 months and 12 days old, marked to show designation of deciduousteeth B.

Lower jaw of horse 5 years old marked to show designation of permanent teeth the designation of deciduous and permanent teeth. A is the lower jaw ofa colt nine months and twelve days old, in which the deciduous dentitionis all in use, and though the eruption of the molars has not yet begun, thedevelopment of one at least is well under way. In B, a horse about fiveyears old, the permanent teeth are all in place. It must be borne in mindthat when a subject has a reduced dentition the teeth are numbered notnecessarily as they appear in the jaw, but according to what is believedto be their true relation in the typical dentition, having forty-four teeth.For instance, in the human subject where there are but two premolarsknown among dentists as bicuspids, the first one in the lower jaw would beexpressed p.m. 3. The second would be p.m.\, assuming that p.m. y andp.m. 2 are the

missing teeth. The teeth of specimens on exhibition, Nos.10 and 36, are labeled in the manner described. THE WOLF TOOTH OF THE HORSE IN the horse we may say that p.m. j (Fig. 37B) is now in the last stageof elimination. Just in front of the upper premolars there is fre-quently found a very small abortive tooth commonly known as the si

Evolution of the horse 52

AMERICAN MUSEUM GUIDE LEAFLETS "wolf tooth." It does also occur in the lower jaw but is very rare. It isabsolutely non-functional, and is in f act rather detrimental, as it sometimesgives rise to more or less irritation. So that in the horse the first functionalpremolar is p.m. f. The wolf tooth, though interesting, is rather hardto study owing to its irregularity. When speaking of the permanent teethit is generally designated as p.m. -, which would be correct if it really be-longed to the permanent set; this however, I am inclined to doubt, althoughit is frequently found long after the shedding period is over. Nevertheless,it seems highly probable that it is a deciduous tooth, which ha ving no suc-cessor, is not shed. In this case it would seem that the permanent toothhad been eliminated earlier in the course of evolution than the deciduousone. But until we have more conclusive evidence on this point probablyit will be as well to include the wolf tooth among the teeth with which wefind it. In figure

32^4 there is shown the unusual occurrence of the wolftooth in the lower jaw. In looking over the fossil ancestors it seems thistooth began to show signs of elimination in the late Oligocene Period,which is estimated to be about two million years ago. At this rate we mayreasonably hope for some few social reform» before the horse is entirely ridof this rather undesirable appendix. PREMOLARS AND MOLARS OF THE HORSE WHEN we compare the molar tooth with its most intricate struc-ture, with the incisor, the latter seems perfect simplicity.Without here discussing the evolutionary history of the tooth,it is extremely interesting to consider these special adaptations, which aremerely gradual modifications of a simple form brought about without anyradical or sudden changes, but slowly developed in an orderly and shallwe say, orthodox manner. The molar of the horse as we find it to-day, is a wonderful structure,presenting on its grinding surface a most complicated system of sharpenamel edges almost as hard as glass, supported

on one side by dentineand on the other by cement. These materials being somewhat softer thanenamel, wear down slightly in advance, leaving the enamel edges sufficientlyexposed to give a perfect self-sharpening, grinding surface, but at the sametime not projecting so far beyond the general plane as to be in danger ofchipping off. When we compare such a tooth as this with a perfectly simple one,composed of dentine and having a short, either cone-shaped or flattenedcrown, covered merely with a smooth shell of enamel without cement, it isdifficult at first sight to see any relation or similarity between them, or torealize that the highly specialized organ is simply a development of the

Evolution of the horse 54 AMERICAN MUSEUM GUIDE LEAFLETS figure 33C. This is a premolar of a horse eightor nine years old. A careful measurement of theexposed enamel in this tooth reveals the interest-ing fact that if these edges were straightened outin a continuous line, it would be fourteen inchesin length. In a tooth of this size on the simple"cylinder" plan, it is obvious we would haveonly about four inches of enamel, hence we canwillingly concede nature's wisdom in this com-paratively "modern improvement." This enamel pattern however, presents a moresimple aspect as the tooth wears down toward thebase of the crown, so that in old age when thegrinding surface is near the root, there is lessenamel, and the self-sharpening surface is muchless perfect. The antero-posterior diameter ofthe crown is also reduced toward its base. Figure 34 shows an upper right grinder (p.m. -)of a horse ten or eleven years old. The tooth iscut in five sections, and reveals on each cut sur-face, the pattern of enamel

which will be presentedon the naturally worn grinding surface at variousages. Section A is a diagram of the natural grindingsurface as it appeared in life, and shows a verycomplicated pattern of enamel, and large culs-de-sac almost completely filled with cement. Section C which is cut one and one-fourthinches farther along the crown, shows great sim-plification of enamel pattern, and the culs-de-sacare much smaller. The five branches of the pulpcavity (h) are open, but would have been filledwith new dentine before the natural grinding sur-face reached this point (Page 49) which takesplace when the horse is approximately twenty-five years old. Section E. The lower border of enamel whichmarks the base of the crown where this section D enameldentinecementttaiurfal cavity Fig. 34. Upper grinder (p. m. -) of horse 10 or 11years old cut in 5 sections; showing pattern of enamelat various ages Branches of the pulp cavityPulp cavity C, E,

Evolution of the horse 56

AMERICAN MUSEUM GUIDE LEAFLETS The three posterior teeth or molars, incline very considerably to thefront, while the other three, the premolars, lean slightly toward them (Fig.37), so that when pressure is applied on their grinding surfaces they areforced together like stones in the arch of a building. And as the teeth weardown to a point of smaller diameter, the last molar in extreme old ageassumes an almost horizontal position (Fig. 37C). The grinders from a lateral view present a most formidable lookingdental battery. It gives some idea of their development when we find thata full set of premolars and molars, taken from a fair-sized horse of four orfive years, will weigh about four pounds, the premolars being a little largerthan the molars, a fact which is quite unusual, as in the great majority ofmammals the premolars are very much inferior in size and development, aswas also the case in the ancestral horse.1 We see from the lateral view as well as from the grinding surface, themore highly

specialized characters of the permanent tooth. Compare thevery long crowns of figure 37^4 with the deciduous molar (Fig. 36), withits short crown, clearly defined base line and flaring roots. Wear and movement of premolars and molars can be studied veryreadily from the skulls on exhibition, three of which are shown in figure 37.For the present we will consider only the lower teeth, which will applyin a general way to the upper as well. The growth of the tooth begins atthe grinding surface, inside the alveolar cavity, and proeeeds toward theroot, the tooth constantly moving upward, erupting and beginning to wearoff before even the crown is complete. Figure 37/1 shows the skull of a horse about five years old. The decidu-ous set has been shed and the permanent teeth are all in use, though thewolf tooth (p.m. j) is not present in this specimen. The crown of p.m. 3, the last tooth in thegrinding set to reach the wearing line, is nowfully grown, having already lost about half aninch from wear. It now measures three anda half inches, makirig a

total length of fourinches to be slowly moved upward and wornaway. The roots are just beginning to developat the base of the crown. The next tooth backof this, m. j, is the first permanent tooth toappear, hence the oldest one in the mouth.The crown is worn down much shorter than Fig. 36. Deciduous molar that of p.m. 5, and the roots are quite evident, (d. m. 3) of colt 11 months though still incomplete, In figure 37B, showing a specimen about old. Natural size. a, crown; b, base of crown; c, root 1 See page 15.

Evolution of the horse Fig. 37. Dental battery of adult horse A.

SkuU 5 years old. Permanent teeth all in use B.

SkuU 8 years old. Crowns reduced in length by wear and roots grown longer. Vestigialp. m.i present in this individual C.

SkuU 39 years old. Crowns almost worn away. Lower molars incline forward (Thecanines are absent in the female) 57

Evolution of the horse INDEX Page numbers of illustralionfs are set in heavy face type Meaning of change in f eet and teeth, 25-29Merychippus, 10, 11, 20, 29, 30Mesohippus, 10, 11, 17, 31; intermedius, 17; bairdi, 18, 28Miohippus, 17, 28, 30Molars, 9, 52, 56" Nimr," 38, 43Norseman's horse, 37, 39, 42One-toed horses, 21-22Orohippus, 16, 28Osborn, H. F., 13, 17, 21, 37, 43Owen, Richard, 14Palmotherium, 16Parahippus, 17, 20, 28Plagiolophus, 16Plateau type, 13PUohippus, 20-21, 22Prehistorie man, 13, 37, 39Premolars, 52, 55Prolohippus, 20-21Pseudo-horses, 32, 33Quagga, 13 Race horse, 37, 43-44, 45Ridgeway, William, 37, 42Shetland pony, 36, 37, 41-42; com- pared with draft horse, 36, 41Skeleton of horse and man, 2, 39Steppe type, 13"Sysonby," 44, 45Teeth of colt, 59Teeth of horse, 9, 44-60, 57; designa- tion, 50-51; eruption of, 58-60; evolution of, 28; pattern of enamel, 54; shedding, 60, specialization, 47, structure, 46-48; wearing of, 47,48,53Teeth of Equus, 22, of Eohippus, 15, of Epihippus,

16, of Hipparion, 21; of Merychippus, 20, of Mesohippus, 17; of Orohippus, 16; of Parahippus, 20Thoatherium, 33Three-toed horses, 17-21Touchstone, S. F., 43Whitney, William C., 2, 38Wolf tooth, 51-52Wortman, J. L., 15Zebra, 11, 13, 31 Adaptation to environment, 28, 31 Age determination by teeth, 49 Anchitherium, 17 Arabian horse, 13, 38, 42-43 Ass, 11: African wild, 13, 31 "Byerley Turk," 43 Cuvier, Baron, 16 Darley Arabian, 43 Dental battery of adult horse, 57 Dental star, 47, 49 Desert type, 13 Diadiaphorus, 32, 33 Domestic horse, Origin of, 37 Draft horse, 36, 37, 38, 39-41, 40 Eohippus, 10, 11, 15-16, 28, 30; venti-colus, 18 Epihippus, 16, 28 Equus, 10, 11, 12, 13, 21-22, 30; asinus,9, 13; burchelli, 9; caballus, 29; com-plicalus, 29; grevyi, 9; hemionus, 9;libycus, 37; przewalskii, 13, 39; scotti,12, 22, 23; sterwnis, 29; zebra, 9 Equus Beds, 13 Evolution, Cause of, 31; Parallel, 32-33 Evolution of the Horse, 8, 14, 34-35;of fore foot, 10; of fore leg, 26; ofhind foot, 11, 30; of hind leg, 27 Ewart, J. C, 37, 41 Foot, Evolution

of, 10, 11 Forest type, 13 Fossil horses, 12-13 Four-toed horses, 14-16 Geological range of ancestors of horse,25 Gidley, J. W., 12 Godolphin Arabian, 43 Granger, Walter, 16 Hipparion, 20, 29, 30; whitneyi, 19, 21 Hippidium, 13, 22, 24 Huntington, Randolph, 38, 43 Hypohippus, 17 Hyracotherium, 14, 15, 27 Keene, James R., 44 "Kismet," 43 Leg, Evolution of, 26, 27 Libyan horse, 42-43 Litopterna, 32 83

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