A series of genera, of disputed systematic position, is represented to-day by the so-called Musk-Ox (Ovibos moschatus), which is now exclusively North American, but in the Pleistocene ranged over northern Asia and Europe as far west as Great Britain. The Musk-Ox, which is at present found only in the extreme north, is a heavy, short-legged animal, three and a half to four feet high, and six feet or more in length; the body is covered with a dense coat of woolly hair overlaid by a thatch of long, straight hair, which gives the animal a very shaggy appearance. The horns are broad at the base, especially so in old males, in which they meet in the middle line and cover much of the head as with a horny casque; they curve downward and then upward and forward, with the tips directed toward the front; in the females and young males the horns are very much smaller.
This series cannot be traced back of the Pleistocene, in which epoch it was not only far more widely distributed, but also very much more diversified, no less than three extinct genera, in addition to the existing one, having been found in the North American Pleistocene. One of these (†Symbos), which extended from Alaska to Arkansas, had horns which were smaller and shorter than in the modern genus, and, even when fully developed, did not meet in the middle line of the head. The other two genera, from California (†Euceratherium and †Preptoceras [Fig. 116, p. 203]), are of great interest as showing affinities to the Musk-Ox and also to sheep and to certain antelopes, such as the Takin (Budorcas) of northern India and Tibet. They serve to connect the musk-oxen with other Cavicornia, but the origin of all these animals is to be sought in Asia.
In Recent North America there are four or five species of sheep (Ovis) which are confined to the mountainous and broken areas of the western part of the continent and extend from Alaska to Mexico. The “Bighorn” or Rocky Mountain Sheep (Ovis canadensis) is characterized by great, spirally coiled horns in the rams, in the ewes the horns are very much smaller and nearly straight; the other species differ but slightly from this type. The species O. canadensis has been found in the Pleistocene, but nothing further is known of its history. Evidently, the sheep were late immigrants.
“The geographical distribution of wild sheep is interesting. The immense mountain ranges of Central Asia, the Pamir and Thian Shan of Turkestan, may be looked on as the centre of their habitat.” “Sheep are essentially inhabitants of the high mountainous parts of the world, for dwelling among which their wonderful powers of climbing and leaping give them special advantages. No species frequent by choice either level deserts, open plains, dense forests or swamps. By far the greater number of species are inhabitants of the continent of Asia, one extending into North America [should read, four or five] one into Southern Europe and one into North Africa.... No remains that can be with certainty referred to the genus [Ovis] have been met with in the hitherto explored true Tertiary beds, which have yielded such abundant modifications of Antelopes and Deer.”[10]
The only other division of the family which is represented in North America is that of the bisons, of which the fast vanishing remnant of a single species[11] (Bison bison) is all that is left of what was once an extensive and varied assemblage. The bisons differ from the true oxen in the form and structure of the skull, in the shoulder-hump, which is produced by the very long spines of the dorsal vertebræ and in consequence of which the back slopes downward from the shoulders to the croup. They differ further in the character of the hair, which is short and woolly on the body and hind quarters, very long and shaggy on the head and neck. In the Pleistocene of North America there were at least seven recognizable species of bisons, which ranged over the continent from Alaska to Florida, though it is not probable that they were all contemporary. One of the earliest and by far the largest of these was the gigantic B. †latifrons, a specimen of which in the American Museum of Natural History measures six feet across the horns in a straight line; this was a Mississippi Valley species and extended from Ohio to the Gulf of Mexico and westward to Kansas and Texas. Another gigantic species (B. †crassicornis) lived in Alaska in association with a second and smaller species (B. †occidentalis) which ranged as far south as Kansas. B. †occidentalis, though smaller than the preceding species, was larger than the existing one and was remarkable for the great size of the hump. The bisons were migrants from the Old World and are the only members of the great ox-tribe that ever reached America. At present the Old World has but a single species of Bison (B. bonasus), which has been saved from extermination only by the most rigid protection.
Neither sheep nor bison extended their range to South America; both are and have been essentially northern groups and seem to have been unable to cross the tropics.
From the foregoing account, confused as it unavoidably is, one thing at least stands out clearly, that North America played a very insignificant rôle in the evolution of the Pecora, and has only two peculiar groups, the Prong Buck and the American types of deer, and of these, the probable American ancestry does not extend back of the lower Miocene and perhaps not so far. Even in the Old World the story, so far as it has been deciphered, is by no means clear and consistent, which is no doubt due to the fact that the regions from which Tertiary mammals have been obtained are so small in comparison with those that have yielded nothing. Certain broad outlines of the history may, nevertheless, be discerned.
The suborder Pecora at an early date became divided into the two great branches of the Cervicornia and Cavicornia, the former giving off the giraffe series, which in the Miocene and Pliocene ramified and extended through Asia and southern Europe, though now confined to Africa. In the lower Miocene of Europe the muntjac-like deer and the antelopes, the first of the Cavicornia, were already well distinguished. From the primitive antelopes arose not only the wonderful assemblage of modern antelopes, but also the goats and sheep and the great and varied ox-tribe. From the middle Oligocene forms it may obviously be inferred that both Cervicornia and Cavicornia united in a single trunk, or, traced in the other direction, diverged from a common stock, to which also the suborder of the Tragulina goes back.