A history of land mammals in the western hemisphere

by William Berryman Scott

The †lophiodonts of the Eocene are represented by very fragmentary material; so far as that material goes, it does not show much change from the White River genus, except that the premolar teeth were smaller and simpler, the limbs and feet retaining the same characteristics of length and slenderness. The Wasatch genus (†Heptodon) had a similar lightness of limb and narrowness of feet, these characters thus appearing at the very beginning of the family history, so far as their North American career is concerned.

5. Rhinocerotidæ. True Rhinoceroses

The history of the great group of rhinoceroses and rhinoceros-like animals is a very long and complicated one, inferior in its completeness only to that of the horses. The complexity of the story arises from the large number of phyla into which the families are divisible, and, despite the great wealth of material and the admirable preservation of much of it, it is extremely difficult to find a clew through the mazes of this labyrinthine genealogy. From the standpoint of the existing geographical distribution of animals, few mammals could seem more foreign and exotic to North American life than do the rhinoceroses, and yet for a very long time that continent was one of the chief areas of their development, so far, at least, as that development can be followed. It is even probable, though not clearly demonstrable, that the family originated here and subsequently spread to the Old World, but not to South America, where no member of it has ever been found. The later history of the rhinoceroses ran its course in the Old World entirely, and the highest specializations within the family are to be found there; in North America these animals are not known to have persisted beyond the lower Pliocene, and if they did survive, it was only as a few stragglers in out of the way places.

The modern rhinoceroses are restricted to Africa, southern Asia and some of the larger Malay islands, Borneo, Sumatra and Java, and within these wide geographical limits are to be found the terminal representatives of at least three separate and quite distinct phyla, the African, Indian and Sumatran genera respectively (Opsiceros, Rhinoceros, Dicerorhinus). It will be advisable to begin the study of this peculiarly interesting family with a brief examination of its modern members, even though none of these are found in the western hemisphere.

Fig. 172.—Skull of the Javan Rhinoceros (R. sondaicus). Note the single upper incisor, and the rough surface on the nasal bones for the attachment of the single horn.

All the existing rhinoceroses are large and massive animals, ranging from four feet to six feet six inches in height at the shoulder, and all have solid dermal horns, except in most females of the Javan species[6] (R. sondaicus). The Indian and Javan species have a single horn on the nose, while those of Africa and Sumatra have, in addition to the nasal horn, a second one on the forehead. The horns, thus, do not form a transverse pair, but are placed in the median line of the head, one behind the other; it should also be noted that these horns are solid, dermal structures, made up of agglutinated fibres or hairs and not having a bony core formed by outgrowths of the skull, as do the horns of most ruminants, such as oxen, sheep and antelopes, which are therefore called “hollow-horned” (Cavicornia). The skull, however, betrays the presence of horns by the extremely rough areas which serve for their attachment and thus the presence or absence of these weapons may be readily determined in the case of an extinct species of which only the skeleton remains. The skin is very thick and coarse, typically “pachydermatous,” and is quite naked in most of the species; but in the Sumatran form there is a sparse coat of hair, which is quite thick in the young animal. In the Indian Rhinoceros unicornis the enormously thick skin has conspicuous and regularly arranged folds, which make the creature look as though encased in armour; the ears and tail are tufted with hair. In the African and Sumatran genera the folds are obscurely marked and not definitely arranged, giving the body a smoother appearance. All the existing species, except one, are browsers and feed upon leaves and twigs, and they frequent forests and marshes where their food is abundant. Not that these and other browsing animals do not occasionally eat grass, but it is not their principal diet. The exception noted is the largest of all the living species, the Broad-Lipped Rhinoceros (erroneously called “White”) of Africa, Opsiceros simus, which is strictly a grazing animal and therefore frequents more open country than the other African species, O. bicornis.

There are considerable differences in proportions and general appearance among the various species, but they all have short necks, very long and massive bodies, short and heavy limbs and short, columnar feet, which look much like those of elephants, but have only three toes each. In all but two of the living species the upper lip is prehensile and characteristically pointed and can be used to pick up very small objects, like the “finger” on an elephant’s trunk; in the Sumatran species (Dicerorhinus sumatrensis) the lip, though pointed, is horny and inflexible, while in the African O. simus it is broad and straight-edged.

The teeth of the modern rhinoceroses are extremely characteristic and may always be recognized at a glance. In the African genus (Opsiceros) there are no front teeth, all the incisors and canines being lost; the other genera have on each side a single large and trenchant upper incisor, in shape like a broad, obliquely edged chisel, which shears against a still larger elongate and tusk-like lower incisor, that is procumbent and points directly forward. The Indian Rhinoceros (R. unicornis) is said to use its tusks as weapons in very much the same fashion as the Wild Boar. Between the large lower tusks there is a pair of very small incisors, which can have little or no functional value; the third lower incisor has been suppressed, as have also the canines of both jaws. The dental formula then is: i 1/2 or 0/0, c 0/0, p 4/4, m 3/3, × 2 = 28 or 34 (see [p. 93]). The premolars, except the first, though somewhat smaller than the molars, have essentially the same pattern. The upper molars have moderately high crowns, yet they are purely brachyodont, except in the grazing, broad-lipped African species (O. simus), in which they may fairly be called hypsodont. The external wall of the tooth is broad and nearly smooth, not divided into cusps, as it is in the horses and tapirs, and the two transverse crests, which in the tapirs are directly transverse, are very oblique. In all the existing species additional complications are given by the short spurs, which project inward from the outer wall or from the transverse crests. The lower molars are formed each of two crescents, one behind the other, but their arms or horns are angulate, not curved as they are in other perissodactyls which have crescentic lower teeth.

The upper surface of the skull is very concave in the antero-posterior direction and very broad over the cranium, where there is no sagittal crest. The nasal bones are immensely thick and strongly arched, with the convexity upward; both this arching of the nasals and the fore-and-aft concavity of the skull are devices for giving a strong and solid attachment to the great nasal horn, for the attachment of which these bones have an extremely rough surface, and in the two-horned species, a second roughened area on the forehead marks the place of attachment of the frontal horn. The bones of the cranium are very thick, but lightened by the many chambers which traverse them. The articulation of the lower jaw with the skull is in some respects unique among mammals; the postglenoid process is a long spike, which fits inside of a bony lump (the postcotyloid process) behind the condyle of the lower jaw, and the posterior margin of the latter is greatly thickened. The neck is short and stout, the trunk very long, broad and deep, the long and strongly arched ribs and the widely expanded hip-bones providing space for the great mass of viscera. The bones of the limbs are short and very massive; the humerus has a very prominent deltoid ridge and the femur an unusually large third trochanter; the bones of the fore-arm and lower leg are separate, as in the massive ungulates generally. The foot-bones are likewise extremely short and heavy, and the number of digits is three in each foot. Each of the five or more existing species has its skeletal peculiarities, every portion of the bony structure showing characteristic features; but these are only minor modifications of the general plan and may be neglected in any comprehensive account of the living representatives of the family.