Fig. 125.
Fig.125.—Asiatic Wild Ass.Equus onager.×1⁄20.
Opinions as to the number of species of Asses differ. On the most liberal estimate there are three Asiatic and two African species. The best known of the Asiatic Wild Asses is the Onager,E. onager. It is of a uniform yellowish, "desert" colour, with a dark stripe along the middle of the back, and is found in Persia, the Punjab, and the country of Cutch. The creature is of great swiftness; it has been stated to be untameable, but Mr. Tegetmeier makes the absolutely opposite statement that the Ass occasionally "becomes so tame as to be troublesome"! The Syrian Wild Ass,E. hemippus, hardly, if at all, differs from this.
Fig. 126.
Fig.126.—Nubian Wild Ass.Equus africanus.×1⁄20.
The Kiang,E. hemionus, seems to have more claims to distinctness. In the first place it has a more limited and adifferent distribution; it is confined to the high tablelands of Thibet at an elevation of 15,000 feet and upwards. In correlation with this habitat it has a thicker and more "furry" coat, which is, moreover, of a darker shade than that of the Onager. This coat is shed in the summer, and replaced by one which is not so dark in hue. It is an interesting fact that the African Wild Asses approach to the zebra type in having at least traces of stripings. There are apparently two species. The best known, the Nubian Ass,E. africanus, is probably the parent of the domestic donkey. It has a dorsal longitudinal stripe, and another across the shoulder—in legend the marks of the Saviour. The matter of the name of this Ass seems difficult to decide. It has been called alsoE. asinusandE. taeniopus. It has been observed that this animal has a great aversion to water, and a delight in rolling in the dust—both of which characteristics argue a desert existence. But on the other hand the Kiang will plunge boldly into streams, yet it would seem to be the descendant of a purely desert form. The Ass is a longer-livedanimal than the Horse. Mr. Tegetmeier calls attention to a donkey living in 1893 which had been ridden fifty-five years previously. The Horse, on the other hand, lives not much more than twenty-five years.
A second species of African Wild Ass,E. somalicus,[150]is distinguished by its greyer colour, by the absence of the shoulder stripe, by the very faint development of the dorsal stripe, and by the presence of numerous cross stripes upon the legs. It has, too, smaller ears, and a longer and more flowing mane. Mr. Lort Phillips, an experienced naturalist and traveller, saw a herd of these Wild Asses in Somaliland, which he regarded as being of quite a new species. A living example in the Zoological Society's Gardens led Mr. Sclater to an identical conclusion, which was supported, as he pointed out, by the fact that this Ass has a different range to the African or Nubian Wild Ass.
Of the Zebras three species are usually allowed; these areE. zebra, the "Mountain" or "Common" Zebra,E. burchelli,E. grevyi, as well asE. quagga. Professor Ewart thinks that the Common Zebra, Burchell's, and the Quagga are not very distinctly marked off from each other. No one, however, has any doubt of the distinctness ofE. grevyi. This latter differs from the rest in its larger size, in the large head and ears, and in the marked hairiness of the ears. It would seem to be a primitive type of Zebra, if the fact that the occasional reversion of hybrids to a parent form be allowed; for Professor Ewart found a cross-bred Zebra to present several characteristics in the face-marking of this, the finest of the Zebra tribe. Only four specimens ofE. grevyihave been exhibited alive in Europe—two in Paris, and two in the Zoological Society's Gardens in London. The latter were presented to Queen Victoria by King Menelek of Abyssinia. The species was named by Professor A. Milne-Edwards in honour of a late President of the French Republic, from an example also sent by King Menelek.
The Common Zebra has closer and darker stripes than Burchell's, but not quite so close as inE. grevyi. It has also a very characteristic arrangement of stripes on the withers in the form of a gridiron. This latter is wanting in both the other species. InE. grevyi, in fact, this part of the back is white.E. zebrahas also a dewlap in front.E. burchellihas fewer and broaderstripes, and between them lie in many cases shadow-stripes of a faint brown.
Fig. 127.
Fig.127.—Burchell's Zebra.Equus burchelli.×1⁄20.
All these animals, and the Quagga too, are absolutely confined to Africa. Mr. R Crawshay,[151]in describing what he considered to be a new variety, remarked upon the curiosity ofE. burchelli. "They remain out in the sun on the plains all day long, not retiring into covert at all. They are then an intolerable nuisance to any one in pursuit of other game; indeed this may be said of them at all times. If once they notice you, they draw in and mob you in their curiosity—only, however, when one takes no interest in them, for when they fancy they are the object of the intruder's attention, no animals are more watchful and cunning in safeguarding themselves. If only their curiosity were manifested in silence it would not so much matter, but it vents itself in snorts and thundering stampedes, which puts every beast within earshot on thequi vive."
Whether Burchell's Zebra[152]can be further subdivided into species or sub-species appears to be doubtful. Dr. Matschie considers thatEquus boehmimay be regarded as a valid form, and in addition to this two sub-species,E. burchelli grantiandE. burchelli selousi, have been proposed for what are at most local races. But it is at present far from certain whether their distribution favours this subdivision.
The Quagga was more striped than is sometimes represented in illustrations. According to Dr. Noack, from whose paper[153]upon the animal I quote here, the transverse stripes reached back as far as the buttocks; they were, however, completely absent from the legs. The animal is, as every one knows, probably completely extinct. In the year 1836 it was still abundant; in 1864 the last specimen ever exhibited was received by the Zoological Society. Mr. W. L. Sclater thinks that it may have survived in the Orange River Colony as late as 1878, but admits that any certainty is difficult, as it was frequently confounded by the Boers with Burchell's Zebra. Its rarity is emphasised by the fact that it is not mentioned in the recent work of that most skilful of hunters, Mr. F. Selous. Gaudry places the Quagga nearest of all living Equidae to theHipparion gracileof Pikermi.
Fossil Equidae.—The existing Equidae all belong to the genusEquus, though there are some who would (quite unnecessarily) divide off the Zebras as a genusHippotigris. The genusEquusitself goes back in time to the Pliocene, during which epoch there lived in IndiaE. sivalensis, the same species according to some with theE. stenonisof Europe. None of these species, Old World or New, are easily to be separated fromE. caballus. But many names have been given to them. It is of course perfectly conceivable that they may have differed among themselves as much as do the existing Zebras and Asses, the separation of which would be hardly possible did we know their bones only. There are, however, extinct genera, undoubtedly related so closely toEquusas to be placed in the same family, though clearly separable as genera.Hipparionis one of these genera; its remains are known from Europe, Asia, and North Africa, from beds of Miocene and Pliocene times. A large number of different species have been described. It was a beast of about the size of a Zebra. The principal characters are that each foot has three toes, of which, however, the two side ones are smaller than the central toe. There is a marked round fossa on the maxillary bone, a feature shared by the South AmericanOnohippidium.[154]The pattern ofthe molar teeth is, too, a little different from that ofEquus.Protohippusof the North American Pliocene is also three-toed, but the two additionally-developed toes are smaller than inHipparion. Other forms are dealt with below in connexion with the ancestry of Perissodactyles. It is a curious fact aboutHipparion, which is not now regarded as on the direct line of equine descent, that the edges of the enamel plates of the molars may show a complicated folding very like that presented by that clearly terminal form of Perissodactyle life, the giganticElasmotherium. This is indicative of high specialisation, which ended in extinction.
Ancestry of the Horses.—TheLophiodontidaeand thePalaeotheriidaeare two of the most interesting extinct families of Perissodactyles; for among them we find what would appear to be the ancestral forms of both the existing Tapirs and Horses. The Rhinoceroses also would seem to be derivable from the Palaeotheriidae. The very vagueness of the characters of these creatures, considered from a classificatory point of view, has led to much diversity in their placing. This though gratifying to the evolutionist is tiresome to the writer who wishes to give a methodical account of their various characters. It will be best perhaps not to attempt an accurate placing or to reconcile conflicting opinion, but to give some salient features of osteology which lead to the belief in their relationship to existing groups of Perissodactyles. A book upon the history of mammals would be incomplete without some account of that well-ascertained series of forms which seem to connect these primitive Perissodactyles with the modern Horse.Equus, in fact, is not only the "show horse" of the doctrine of evolution, but also the "stalking horse."
In the Eocene of both Europe and America are met with a number of forms from which we may start.Hyracotherium, regarded on the one hand as the type of a sub-family of the Equidae themselves, and on the other as a member of the family Lophiodontidae, was a small-sized animal, three feet or so in length; it possesses the complete Eutherian dentition with a slight diastema. The orbits are not separated from the temporal fossa; the fore-limbs were four-toed, the hind three-toed, with moderately long metapodia, especially on the hind-feet. The shoulder bladehas a well-marked coracoid process. The radius and ulna are separate; so too are the tibia and fibula.Eohippus, belonging to the same sub-family, is slightly more primitive; for the hind-feet have a rudiment of digit I.Orohippusis a little nearer to the Horses in that the molar teeth have acquired a little further advance towards the equine type. Instead of the tubercles of the teeth remaining for the most part separate, they have fused into a set of ridges, of which, however, the pattern is less complex than in the modern Horses. In other respectsOrohippusis much likeHyracotherium.Pachynolophusseems to be but a synonym.
The next stage is shown byMesohippus, a Lower Miocene form, usually referred to the neighbourhood ofPalaeotherium. It has nearly lost one of the toes of the fore-foot, a rudiment only remaining; the metapodials, at any rate of the fore-feet, seem to be slightly increased in length. The orbit is not encircled by bone, but there is a strong process from the frontal, which nearly meets the zygomatic arch.
Anchitherium, from the Upper Miocene, is not far removed in structure from the last-mentioned form; it is a trifle nearer the existing Horse in several points. The ulna is further reduced and fused with the radius below: the rudiment of digit V is still more rudimentary; the two lateral digits are smaller in proportion to the central one than they are inMesohippus; the fibula is fused below with the tibia. From this form toEquusis a small series of steps, characterised by the still further reduction of all the digits except III, by the still further reduction of the already rudimentary ulna and fibula, and by the increasing depth of the molar teeth, which are of course, inEquus, hypselodont.
Another interesting conclusion may seem to follow when we consider the geographical range of the ancestral Horses.HyracotheriumandPachynolophusoccurred both in the Old and New World. From them may have arisen the Horses of both hemispheres. After that point there is a division.Mesohippusis American, and we get atEquusin that continent throughDesmatippusandProtohippus. On the other hand there are no remains known ofMesohippusin Europe; and unless subsequent researches prove the existence ofMesohippus, we have to rely upon forms which are placed withAnchitheriumandHipparion.
It seems that in America the next genus in the direct line of equine descent toMesohippusisMiohippus. It is smaller insize thanAnchitherium, to be considered immediately. The odontoid process of the axis is just beginning to assume the characteristic spout-like shape of that of the existing Horse and many modern Ungulates. The median digit of both fore- and hind-limbs has become greatly enlarged as compared with the corresponding digit of earlier forms.
It is held, however, thatAnchitheriumis not on the direct line of descent either in America or in Europe, in both of which it occurs. Its teeth are in some respects less Horse-like than in some of the more ancient genera, to which the converse would be expected on the descent theory. Its hoofs are much elongated and flattened, a mark of specialisation and not appropriate to a creature holding an intermediate position in the equine series. Both the American (A. equinum) and the European species (A. aureliense) are of very large size, larger than its successors, and such "alternations in bulk are unlikely."
The genusDesmatippusof Professor Scott[155]fills in the gap betweenMiohippusandProtohippus. The molars and premolars are brachyodont, but there is a thin deposit of cement in the tooth valleys, leading towards the more complete filling of these valleys with cement, which is found inProtohippus. This genus of Horses, of which there is at present but one species,D. crenidens, was three-toed, and "the lateral digits, so far as can be judged by fragmentary remains, were still fairly developed, and though much more reduced than inMiohippus, appear to be somewhat less so than inProtohippus."
To recapitulate, the following is the probable series of equines in America—Mesohippus,Miohippus,Desmatippus,Protohippus.
The development of the limbs of the Horse shows a most interesting series, of stages, which correspond in part to the ancestral forms which palaeontology seems to prove to be the line of the descent of our existing Equidae. This matter has recently been elucidated by Professor Ewart, who details the following facts and comparisons:—
In the youngest embryo (about 20 mm. in length) the humerus is somewhat curved, and considerably longer than the radius and carpus taken together. The first-named bone is shorter in the adult, and the proportions of that bone in the young as well as its curvature are suggestive of that ancientUngulatePhenacodus(see p.202). In the next stage (an embryo of 25 mm.) the humerus has slightly decreased in proportionate length, and has come to be more like that ofHipparion. In both of these embryos it should be noted that the ulna is complete and separate from the radius. In the second of the two it has more distinctly acquired the form which it will possess in the adult. The second metacarpal—one of the splint bones of the adult—is tipped with a small nodule of cartilage, which is clearly the representative of one or more of the phalanges belonging to that digit.
Fam. 2. Tapiridae.—The Tapirs may be distinguished from the Horse and from the Rhinoceros tribe by a few characters, which are as follows:—
The dentition is generally the full one of forty-four teeth. The premolars in the more ancient forms are unlike the molars, but like them in more recent forms. The molars of the upper jaw have two crests parallel and united by an outer crest. The fore-feet have four, the hind-feet three toes.
The family is fully as ancient as that of the Equidae, but the specialisation of the toes never advances so far. The modern representatives of the order are, so far as the feet are concerned, in the condition of very early representatives of the equine stock. Nor do the teeth of the Tapirs ever reach the complicated pattern of that presented by at least the modern Horses, or indeed of the Palaeotheres. Apart from this it is not an easy matter to distinguish accurately between these several families, including the Lophiodontidae, which, as already mentioned, is placed nearer to the Tapiridae than to the Palaeotheriidae. Indeed the differentiation of these two families, the Tapiridae and the Lophiodontidae, seems to be a matter of the greatest difficulty. The difficulty is well emphasised by the fact that naturalists disagree most profoundly as to the relations of various genera of extinct Tapir-like animals. For Mr. Lydekker the genusLophiodonincludes also the American generaIsectolophusandSystemodon, which are placed by Zittel in the sub-family Tapirinae as opposed to Lophiodontinae, which containsLophiodonandHelaletes. The existing Tapirs can be differentiated from the existing Horses with great ease, as the following account of the existing genera will show.
Fig. 128.
Fig.128.—American Tapir.Tapirus terrestris.×1⁄10.
The genusTapirusis now met with only in South and Central America, and in the Malay Peninsula and the islands of Java and Sumatra. This animal is in many respects the most ancient of existing forms referable to the Perissodactyle order. It has four toes on the front-feet, though only three on the hind-feet. The number of teeth is 42—nearly the typical Eutherian number. The Tapirs are always moderately-sized animals, entirely covered with hair, and usually of a brownish-black colour. The Malayan Tapir is, however, banded broadly with white—a single band; the young of the Tapir is spotted, and striped with white. The nose and upper lip conjoined are produced into a short trunk, precisely comparable with that of the Elephant. As in the Rhinoceros—and in this both contrast with the other existing Perissodactyle genusEquus—the temporal fossa is not separated from the orbit by bone. Of existing Tapirs there are at any rateT. terrestris,[156]T. roulini(the "Tapir Pinchaque" of Cuvier),T. dowiandT. bairdiin America (the last two being sometimes separated into a distinct genus,Elasmognathus, on account of the prolongation of the ossified mesethmoid), andT. indicusin the East. The tapir, probablyT. terrestris, is described by Buffon as "a dull and gloomy animal." It is certainly mainly nocturnal in habit. The nameterrestriswas given by Linnaeus, who placed it in the same genus asHippopotamus amphibius; hence the epithet applied to the Tapir. But as a matter of fact it loves marshy neighbourhoods, and is in a way amphibious. This does not of course apply to the AndesianT. roulini, which inhabits the cordillera of Ecuador and Colombia. The distribution of existing Tapirs is, as is so often the case, restricted when compared with that of their extinct congeners and allies. In Europe the remains of the genusTapirusare abundant from Pliocene strata, and its remains are there known from as far back as the Miocene. The genus is thus one of the very oldest forms of Mammalia at present inhabiting the earth.
Fig. 129.
Fig.129.—Malayan Tapir.Tapirus indicus, young. ×1⁄10. (FromNature.)
The Malayan Tapir is to be distinguished from the American (T. terrestris—the other species have not been dissected) by the greater development of the valvulae conniventes in the intestine, the absence of a moderator band in the heart, and the less elongated caecum, which is sacculated by only three bands, there being four inT. terrestris.[157]The animal frequents the most retired spots among the hill woods, by which habit it seemslargely to escape the Tiger, its most formidable foe in those regions of the world. Its quickness of senses enables it also to slip away with rapidity. It can proceed at a great pace when disturbed, and can readily push its way through obstacles. The young animal, like that of the American species, is dark brown with yellowish spots. It is stated by Mr. H. N. Ridley that the young animal lies during the hot part of the day under bushes, in which situation "its coat is so exactly like a patch of ground flecked with sunlight that it is quite invisible." It is interesting to note that here, as with some other animals, it is the young that are especially protected by such mechanisms. Moreover, some of the spots are round and some are more elongated, so that the resemblance to spots of sunlight which come in a direct and in a slanting direction is greatly increased. Even the colours of the adult are not so conspicuous when it is in its native haunts as might be supposed. The breaking up of the ground colour into tracts of two different colours prevent it from striking the eye so plainly as if it were of one colour throughout. "When lying down during the day it exactly resembles a grey boulder, and as it often lives near the rocky streams of the hill jungles, it is really nearly as invisible then as it was when it was speckled."[158]
Fam. 3. Rhinocerotidae.—This family is to be distinguished from the preceding by a number of characters, which though not universal are general. In the first place, there are commonly horns, or a horn, consisting of what appears to be an agglomeration of hair-like structures fixed upon a roughened patch of bone on the surface of the nasals. The incisors are diminished or defective, and the upper canines are often wanting. The molars and premolars are alike. The fore-feet are four- or three-toed, but are functionally tridactyle; the hind-feet are three-toed. The skeleton in this family is massive, and the limbs relatively short. The skull, as in the Tapirs, has a confluent orbit and temporal fossa. The upper lip is generally more or less prehensile; the body is as a rule—to which the Pleistocene Hairy Rhinoceros is of course an exception—rather sparsely covered with hair. In this feature the Rhinocerotidae contrast both with the Tapiridae and the Equidae. The family in reality contains but one existing genus, though three have been instituted, viz.Rhinoceros,Ceratorhinus, andAtelodus. As there are so few existing species the subdivision of animals which agree in so many and such highly-characteristic features seems to be an unnecessary procedure. The existing Rhinoceroses are but a fragment of the total number of known forms from past epochs. The family is very markedly on the wane.
The genusRhinocerosis characterised by its heavy build and thick, almost smooth, skin—smooth, that is to say, so far as concerns the slight development of hair—which is often thrown into folds. There is one or there are two horns on the fore-part of the head, which are, as has already been pointed out, structuressui generis, and not exactly comparable with the horns of other living Ungulates. There are three nearly equal toes on both fore- and hind-limbs. The canine teeth of existing species have disappeared; the incisors are, or are not, present; the molars and premolars are three and four in each half of each jaw.
The visceral anatomy of the Rhinoceros has been much investigated so far as concerns the Asiatic forms. A curious feature, which serves to discriminate some of the Asiatic species from others, is to be seen in the small intestine. InRh. indicus[159]this gut is furnished with numerous long cylindrical narrow outgrowths "like tags of worsted"; in the alliedRh. sondaicusthese tags are present, but are flatter and broader; while in the two-hornedRh. sumatrensisthere are no tags at all, but only smooth valve-like folds. Another mark by which these species can be distinguished depends upon the variation in the presence or absence of certain glands imbedded in the integument of the foot—the so-called "hoof glands." These occur inRh. indicusandRh. sondaicus, but are absent inRh. sumatrensis.
Sir W. Flower[160]studied some years since the skull features which serve to differentiate the existing forms.
InRh. sumatrensisthe two long downward processes of the squamosal bone, termed respectively post-glenoid and post-tympanic, do not unite below the auditory meatus. In this the species in question agrees with the African forms but not with the one-horned Asiatic species, where the two processes completely fuse. Again, another character, though perhaps less important,is the sloping backwards instead of forward of the occipital crest in all two-horned species, whether African or Asiatic.
The Asiatic Rhinoceroses have, what the African animals have not, functional incisor teeth throughout life. It has been proposed on these and other grounds to separate generically the African and Asiatic forms.
Fig. 130.
Fig.130.—Indian Rhinoceros.Rhinoceros indicus.×1⁄40.
The Asiatic Rhinoceroses include three well-differentiated species, in all of which the skin is much thrown into folds.Rh. indicusis the largest form. It is one horned, and has enormous folds of skin at the neck and hanging over the limbs. So like artificial armour is this thick plating, that Albrecht Dürer may be excused for having given the beast the appearance of being actually mail-plated in a sketch which he made of a specimen sent over to the King of Portugal in 1513. This particular beast, one of if not the first sent over to Europe, proved so intractable in disposition that the king sent it as a present to the Pope. But "in an access of fury it sunk the vessel on its passage"! The horn of this and of other species was held until almost our times to have medicinal and other more curious values. So recently as 1763 it was gravely asserted that a cup made of its horn would fall to pieces if poison were poured into it. "When wine is poured therein," wrote Dr. Brookes in the year referred to, "it will rise, ferment, and seem to boil; but whenmixed with poison it cleaves in two, which experiment has been seen by thousands of people." John Evelyn also wrote of a well in Italy which was kept sweet by a Rhinoceros' horn. This species seems to be long-lived, even in captivity; a specimen now to be seen in the Zoological Society's Gardens has been there since the year 1864.
Rhinoceros sondaicus, the Rhinoceros of the Sunderbunds, has a much wider range than the last species or Indian Rhinoceros. This is unknown out of India itself, and is there limited to a small region; the Sondaic form is found in Bengal and in the Malayan Islands. It is a smaller species, and the armour has a tesselated appearance. The female generally, if not always, is hornless.
Fig. 131.
Fig.131.—Sumatran Rhinoceros.Rhinoceros sumatrensis.×1⁄15. (FromNature.)
The Sumatran species,Rhinoceros sumatrensis, is to be distinguished from the last two by its two horns. It is also coveredby a much thicker coat of hairs, which are sometimes blacker and sometimes redder. On account of its two horns it has been proposed to separate it from the other Oriental species into a distinct genus,Ceratorhinus. The animal has much the same range as the last species, but extends to Borneo. A variety of this species with hairy ears, from Assam, has been separated as a distinct form, under the name ofRh. lasiotis, by Mr. Sclater. The animal upon which that species was founded was until quite recently living in the Zoological Society's Gardens.
Fig. 132.
Fig.132.—Hairy-eared Rhinoceros.Rhinoceros lasiotis.×1⁄30.
There are only two certainly-known species of Rhinoceros in Africa. These are the White Rhinoceros (Rh. simus) and the Black Rhinoceros (Rh. bicornis). The origin of the names is not easy to understand, since the "white" animal is, if anything, darker in colour than the Black Rhinoceros. It is stated, however, that in past years the specimens ofRh. simusfound in the south-west of Cape Colony were "paler and whiter in colour than those in the north-east." At present there are no grounds for distinguishing the species by their colour characters. But they are plainly distinguishable on other grounds.Rhinoceros simushas a square upper lip, and in relation to this crops the herbage upon the ground.Rh. bicornishas a prehensile upper lip projecting beyond the lower, and in a corresponding fashion feeds principally upon the branches of shrubs, It has been pointed out by Mr.Coryndon[161]that the calf ofRh. simus"always runs in front of the cow, while the calf ofRh. bicornisinvariably follows its mother." Both animals of course have two horns, and upon the varying proportions of the horns a large number of "species" have been made in the past. It is stated that the longest horn of the "White Rhinoceros" known measures 56½ inches; while that ofR. bicornisis shorter, 40 inches being apparently the maximum. But the animal is smaller.
Fig. 133.
Fig.133.—Head ofRhinoceros bicornis.
The possible third African species ofRhinoceros[162]has been provisionally named after Mr. Holmwood, and is based upon two horns 41 and 42 inches long, which may be abnormal horns ofRh. bicornis; but they are thinner and have a smaller pedicel.
Extinct Rhinocerotidae.—The existing Rhinoceroses are thus confined to Africa, to certain parts of the continent of Asia, and to some of the large islands lying to the south of that continent. But formerly the genus, and allied genera, had a wider range. As far back as the Miocene we meet with remains of Rhinoceroses closely allied to existing forms. The more ancient forms have, as is natural, more ancient characters. Thus inRh. schleiermacheriof the Miocene, canines appear to have been present. The MioceneAceratherium, primitive in the absence of horns as itsname denotes,[163]had also canines and, in one species, six incisors in the lower jaw. ThisAceratheriumhad, moreover, four toes in the fore-feet. In the Miocene and later the Rhinoceros existed in Europe and America. There was even a purely northern form, theRh. tichorhinus, which possessed a woolly covering and had the same range as the Mammoth. This Rhinoceros was two-horned.
The post-Pliocene and EuropeanElasmotheriumwas a colossal rhinocerotine creature. This great beast had two horns and a body 15 feet long. Its limbs are not known, and as the teeth are different from those of Rhinoceroses in general, it may not have belonged to this group at all, though Osborn is inclined to derive it fromAceratherium, admitting at the same time that the evidence is "decidedly slender." The teeth in fact are like those of a Horse in being hypselodont and prismatic in form. As to the two horns, they were apparently not exactly like those of typical Rhinoceroses; there was an enormous horn posteriorly, supported on a huge boss of bone, and in front of this a roughened spot suggests a smaller or at least a much more slender horn.
It is important to notice that fossil Rhinoceroses belonging to the restricted genusRhinoceroswere in Europe invariably two-horned; it is only in India, where they still exist, that one-horned forms are met with in a fossil state.
Fig. 134.
Fig.134.—Skeleton ofHyracodon nebrascensis. ×1⁄12(After Scott.)
The Rhinoceroses of America were mostly hornless.Diceratheriumis an exception; but in many cases it had two parallel not successive horns, and these were, to judge from the slight prominences, but feeble in development, and perhaps hardly exactly comparable with the formidable weapons of the Old-World forms.Aceratherium tridactylum, with indications of paired horns, may be ancestral toDiceratherium. The American forms have weak and slender nasals in correspondence with the absence of horns; the sagittal crest is retained in contradistinction to the great flattened surface of the skull in the horned Rhinoceroses.Aceratheriumof both divisions of the globe probably represents the ancestral group of the horned and the hornless forms. This being the case it is highly interesting to note a distinct convergence in the quiteseparate American genera towards the European horned genera. A genus sometimes united withAceratherium, but still differing from it in some points, isAphelops(Teleoceras).[164]This animal is more nearly approximated to "the modern standard" of Rhinoceroses than is its possible ancestorAceratherium. The skeleton in general is more robust, even surpassing that of modern forms, and approaching theHippopotamus. There is a reduction in the upper incisors, which are limited to two pairs, and the lower molarsare reduced to five. The lower incisors are only two. The sagittal crest is less marked; the fifth digit is reduced to a tiny nodule representing the metacarpus. It had a small nasal horn. There are numerous other details of likeness to modern Rhinoceroses in this creature, which has only community of descent with them from the older hornless forms, such asAceratheriumandCaenopus. In the genusPeracerasthe upper incisors are as completely gone as in the living African Rhinoceroses.
Fig. 135.
Fig.135.—Skeleton ofAphelops (Teleoceras) fossiger. ×1⁄15. (After Osborn.)
The most ancient rhinocerotine types[165]are the Hyracodonts and the Amynodonts. They both date from the Eocene, and became extinct in the succeeding Oligocene.Hyracodon[166](Fig. 134) was "an agile, light-chested, and rather long-necked" type, resembling a Horse in build. There were no horns present, but the hoofs were more like those of the Horses than of the existing Rhinoceroses. These animals were apparently plain dwellers and defenceless, which is held to account for their compact hoofs and outward similarity to a Horse. The genus is Oligocene. The dental formula is I 3/3 C 1/1 Pm 4/3 M 3/3.
It is surmised by Professor Scott that the number of dorso-lumbar vertebrae was twenty-three or twenty-four. The radius and ulna are complete and separate bones, but the latter is somewhat reduced. There are four metacarpal bones, of which, however, the fifth is much reduced. The animal is only three-fingered. The tibia and the fibula are distinct, and show no tendencies towards fusion; but the fibula is much reduced. There are only three metatarsals and three toes. Had this line, which is to be regarded as a side branch of the Rhinoceros stem, not died out, it would probably have resulted, thinks Professor Scott, in monodactyle—very Horse-like types. It is later than the next genus to be described,Hyrachyus, of which it is possibly a descendant. An intermediate type,Triplopus, appears to bind togetherHyracodonandHyrachyus.
InHyrachyus agrariusthe skull is long and narrow, the facial region being markedly longer than in existing Rhinoceroses. The mastoid portion of the periotic bone is widely exposed upon the outer face of the skull, which is, as has been said, not the case with the existing genusRhinoceros. The dentition is the complete Eutherian dentition of forty-four teeth. The uppermolar teeth are strikingly like those of the genusRhinoceros. The fore-feet are pentadactyle, but functionally tetradactyle; the hind-feet tridactyle. The ulna is less reduced than inHyracodon, and the dorso-lumbar vertebrae are twenty-five.
Fig. 136.
Fig.136.—Skeleton ofMetamynodon planifrons. ×1⁄22. (After Osborn and Wortman.)
The Amynodonts were short, heavy types, probably marsh-haunting in habit, and possibly with a proboscis like that of the Tapir. The orbit is higher than it is in the purely terrestrialHyracodonts, and it is suggested that when swimming it was raised above the surface as with the Hippopotamus. "This feature," observes Professor Osborn, "with the long curved tusks, undoubtedly used in uprooting, suggests the resemblance between the habits of these animals and those of the hippopotami." There were no horns in the Amynodonts. The face is shorter than in the Hyracodonts, and the mastoid is covered as in recent Rhinoceroses. The canines are very strongly developed into tusks, but the incisors show signs of disappearance. We know of the generaAmynodon,Metamynodon, andCadurcotherium. All except the last, which is European, are American in range.
Fam. 4. Titanotheriidae.—These Oligocene Ungulates, often attaining to large dimensions, are nearly peculiar, so far as is at present known, to the North American Continent, and are at least most abundant in it.[167]Many generic names, such asTitanotherium,Brontotherium,Brontops,Titanops, andMenodus, have been given to them; but a recent study of the entire material accessible for description or already described has led Professor Osborn to the opinion that there was but a single genus, to which the nameTitanotheriummust be applied. Of this genus there are some thirty well-characterised species, of which the gradual evolution can be traced from the lowest strata of the White River beds where their remains occur. An entire skeleton ofT. robustumenables us to understand the osteology of these forms and to compare them with other Perissodactyles. This animal was more than 13 feet long, standing some 7 feet 7 inches in height. It seems to have presented during life the aspect of a Rhinoceros with perhaps a touch of Elephant. The skull is not unlike that of a Rhinoceros in general dimensions and shape; but there are a pair of apparent horn cores anteriorly, which are smaller in the more ancient forms and acquire a large size, a forward direction with a divergence of the two in the later forms. A glance at the accompanying figures of skulls (Fig. 137) of early and later Titanotheres will exhibit the changes in this particular which the skulls underwent in the lapse of time occupied by the deposition of these Oligocene beds. The nasals are short in the later, longer in the more early species, such asT. helocerasandT. coloradense. The zygomatic arch projects much, and is "shelf-like" in the later forms, the skull thus getting an immense breadth, which,together with the long and divergent horn cores, must have given to the living animal a most bizarre appearance. It is an interesting fact that this animal, though a Perissodactyle, agrees with the Artiodactyla in the nineteen dorso-lumbar vertebrae, of which seventeen bear ribs.