Deer are the only members of the Pecora which inhabit South America, where there are several genera of them, all much more nearly allied to North American than to Old World forms. No record of the presence of the family in the southern continent has been found in beds older than the Pleistocene, but in view of the degree of specialization which they have there undergone, it is probable that the immigration took place in the Pliocene.
SECTION CAVICORNIA. HOLLOW-HORNED RUMINANTS
In the animals of this second and far larger section of the Pecora there are bony outgrowths of the skull, from the frontal bones, outgrowths which are permanent and non-deciduous; these are the horn-cores, which are tapering and unbranched. The horn-core is, in turn, covered with a sheath of horn, likewise unbranched and permanent, but growing from year to year until the maximum size is attained, a process which is familiarly illustrated in the growth of a calf. Among Recent Cavicornia there is but one exception to the rule that the horny sheath is non-deciduous and unbranched and that one is the Prong Buck (Antilocapra americana). In the Cavicornia it is the very general rule that both sexes are horned, though the females commonly have smaller horns and in several genera of antelopes the does are hornless. There is almost as great variety in the shape and size of the horn as of the antler; we find small, medium-sized and enormously large horns, which may be straight, simply curved, complexly curved, spiral, lyrate or twisted. The antelopes have many types of horns, as have the sheep and goats, the oxen, buffaloes and bisons; but only a few of them are exemplified in the western hemisphere, which now, as in the preceding geological periods, is singularly poor in representatives of the Pecora.
9, 10. Antilopidæ and Antilocapridæ. Antelopes
Two very different kinds of antelopes are found in North America at the present time; one of them, the erroneously named Rocky Mountain Goat (Oreamnos montanus), is evidently a late immigrant from the Old World, and fossil remains of it have been found in the Pleistocene cave-deposits of California. This animal is a member of the true antelope family (Antilopidæ) and belongs to the chamois group of mountain-antelopes; it has no near relatives among other American mammals, living or extinct.
The Prong Buck, or Prong-horned Antelope (Antilocapra americana), occupies a very isolated position, so much so that a distinct family, the Antilocapridæ, has been created for its reception. It differs from all other Cavicornia in having a branched horn, though the bony core is simple, and in annually shedding and renewing the horny sheath; the horn is directly over the eye; there are no dew-claws and all traces of the bones of the lateral digits have completely disappeared. The grinding teeth are thoroughly hypsodont. The genus occurred in the older Pleistocene, where it was associated with the last of the †deer-antelope, or †Merycodus series (†Capromeryx), and which, so far as it is known, would seem to connect the two families, though this is doubtful. A middle Miocene genus (†Dromomeryx [Fig. 128, p. 237]) would be a more probable ancestor of the Prong Buck, if it were not for the long, unfilled gap of the upper Miocene and the whole Pliocene. †Dromomeryx had erect horn-cores placed directly above the eyes as in the modern genus, but low-crowned grinding teeth; it was the most ancient American cavicorn yet known. It remains to be determined by future exploration, whether this middle Miocene genus was actually the ancestor of Antilocapra, or merely an anticipation of it.
In the lower Pliocene have been found the remains, very incomplete, of several antelopes, which appear to have been immigrants from the Old World, but are too imperfectly known for any definitive reference. One resembles the flat-horned, or goat-horned, antelopes of the European Miocene and Pliocene. Others had spirally twisted horns like those of the Recent strepsicerine, or twisted-horn antelopes of Africa and Asia, but may, nevertheless, be referable to the Antilocapridæ.
Antelopes even penetrated to South America, and three genera of them have been reported from the Pleistocene of the Brazilian caverns and the Argentine pampas, but they were less successful in establishing a foothold than were the deer, and form no part of the modern Neotropical fauna.