CHAPTER IX

HISTORY OF THE ARTIODACTYLA

The artiodactyls are and for a very long time have been a very much larger and more variegated group than the perissodactyls, and the Old World has been and still is their headquarters and area of special development, where they are represented in far greater number and variety than in the New; the perissodactyls, on the other hand, flourished especially in North America, as was shown in the preceding chapter. At the present time the artiodactyls are the dominant ungulate order, far outnumbering all the others combined, and include an assemblage of varied types, which, when superficially examined, appear to be an arbitrary and unnatural group. What could seem more unlike than a dainty little mouse-deer, no larger than a hare, a stag, a camel, a giraffe, a bison and a hippopotamus? Yet, in spite of this wonderful diversity of size, proportions, appearance and habits, there is a genuine unity of structure throughout the order, which makes their association in a single group altogether natural and proper, especially as these structural characters are not found united in any other group.

It would be superfluous to enumerate all of the diagnostic characters which, on the one hand, unite all the living and extinct artiodactyls and, on the other, distinguish them from all other hoofed animals, and it will suffice to mention a few of the more significant of these features.

As the name implies, the artiodactyls typically have an even number of toes in each foot, four or two; though this rule may be departed from and we find members of the order with five digits or three, just as the tapirs and nearly all the Eocene genera of perissodactyls had four toes in the manus. Much more important is the fact that the plane of symmetry, which in the perissodactyls bisects the third digit and is therefore said to be _mesaxonic_, passes between the third and fourth digit and is _paraxonic_. The third and fourth digits always form an equal and symmetrical pair and are the “irreducible minimum,” beyond which the number of toes cannot be diminished. A single-toed artiodactyl would seem to be an anatomical impossibility; at all events, such a monstrosity was never known. Hence the term “cloven” or “divided” hoof, which seems to take the solid hoof of the horse as the norm; but “cloven or divided,” while expressing the appearance of the foot with sufficient accuracy, is erroneous, if taken to mean the splitting of what was once continuous.

[Illustration: FIG. 186.—Left fore-arm bones of the Domestic Pig (_Sus scrofa_). _R._, radius. _U._, ulna. _ol._, olecranon.]

[Illustration: FIG. 187.—Left manus of Pig. _S._, scaphoid. _L._, lunar. _Py._, pyramidal. _Pis._, pisiform. _Td._, trapezoid. _M._, magnum. _U._, unciform. _Mc. I_, second, _Mc. II_, third, _Mc. III_, fourth, _Mc. IV_, fifth, metacarpals.]

[Illustration: FIG. 188.—Left pes of Pig. _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cb._, cuboid. _Cn. 2_, _Cn. 3_, second and third cuneiforms. _Mt. II-V_, second to fifth metatarsals.]

[Illustration: FIG. 189.—Bunodont upper molar of peccary (_Tagassu_).]

[Illustration: FIG. 190.—Selenodont upper molar of deer (_Odocoileus_).]

Especially characteristic of the order is the structure of the ankle, or “hock-joint” of the hind limb. The ankle-bone, or astragalus, has a double pulley, the upper and lower ends being of quite similar shape; its lower end is almost equally divided between the cuboid and navicular, which are made concave to receive it. This type of astragalus is altogether peculiar to the artiodactyls, all of which possess it; it is unlike that of any other mammal whatever and may be recognized at a glance. The calcaneum, or heel-bone, has a large convex facet, by means of which it articulates with the fibula, or external leg-bone; there is no such articulation in the perissodactyls. The lower end of the calcaneum is narrow and fits into a step cut in the cuboid, which is thus every whit as peculiar and characteristic as the calcaneum and astragalus. The femur never has the third trochanter, which is always present in the perissodactyls. Another respect in which the artiodactyls differ from all perissodactyls except the horses is in the much more complex mode of articulation between the vertebræ of the lumbar and posterior dorsal regions, which the former display, and even the horses have no such elaborate arrangement. Finally, another very marked difference from the perissodactyls is in the teeth, for the premolars and molars are never alike, and only in very rare instances does the last premolar assume the molar-pattern. Of this pattern, there are two principal kinds, one exemplified by the peccaries, in which the crown supports a series, fundamentally two pairs, of conical cusps, and called _bunodont_, and the other, to be seen in all the ruminating animals, in which the crown is composed of two pairs of crescents and is therefore said to be _selenodont_. The bunodont was the primitive type, whence the other was derived, and many transitional forms are known.

The classification of the immense horde of living and extinct genera and species which are referable to the artiodactyls is an extremely difficult problem, which has found no thoroughly satisfactory solution and will not until much more is learned concerning the history of the order and conflicting opinions can be reconciled. The most important American families and genera are given below, though the arrangement is but tentative.

Suborder A. ARTIODACTYLA †PRIMITIVA. (Extinct genera of doubtful affinities)

I. †TRIGONOLESTIDÆ.

_†Trigonolestes_, low. Eoc.

II. †LEPTOCHŒRIDÆ.

_†Leptochœrus_, low. Oligo. _†Stibarus_, low. Oligo.

III. †DICHOBUNIDÆ. _†Homacodon_, mid. Eoc. _†Bunomeryx_, up. Eoc.

IV. †ANTHRACOTHERIIDÆ.

_†Anthracotherium_, low. Oligo. _†Bothriodon_, do. _†Arretotherium_, do.

V. ?†OREODONTIDÆ.

† _Protoreodon_, up. Eoc. _†Merycoidodon_, low. Oligo. _†Eporeodon_, up. Oligo. _†Promerycochœrus_, up. Oligo. to up. Mioc._†Merycochœrus_, Mioc. and low. Plioc. _†Pronomotherium_, up. Mioc. _†Mesoreodon_, low. Mioc. _†Merychyus_, low. Mioc. to low. Plioc. _†Leptauchenia_, low. Oligo. to low. Mioc. _†Cyclopidius_, mid. Mioc.

VI. †AGRIOCHŒRIDÆ.

_†Protagriochœrus_, up. Eoc. _†Agriochœrus_, Oligo.

Suborder B. SUINA. Swine-like Animals

VII. TAGASSUIDÆ, Peccaries.

_†Helohyus_, mid. Eoc. _†Perchœrus_, low. Oligo. _†Thinohyus_, up. Oligo. _†Desmathyus_, low. Mioc. _†Prosthennops_, up. Mioc. and low. Plioc. _†Platygonus_, mid. Plioc. to Pleist. _Tagassu_, Recent, Pleist. in S. A.

VIII. †ENTELODONTIDÆ. †Giant Pigs.

_†Parahyus_, low. Eoc. _†Achænodon_, mid. and up. Eoc. _†Arcæotherium_, low. Oligo. _†Boöchœrus_, up. Oligo. _†Dinohyus_, low Mioc.

Suborder C. TYLOPODA. Camels and Llamas

IX. CAMELIDÆ.

_†Protylopus_, up. Eoc. _†Eotylopus_, low. Oligo. _†Poëbrotherium_, Oligo. _†Pseudolabis_, low. Oligo. _†Protomeryx_, up. Oligo. and low. Mioc. _†Oxydactylus_, low. Mioc. _†Miolabis_, mid. Mioc. _†Protolabis_, mid. and up. Mioc. _†Alticamelus_, mid. Mioc. to low. Plioc. _†Stenomylus_, low. Mioc. _†Procamelus_, up. Mioc. and low. Plioc. _†Pliauchenia_, up. Mioc. to mid. Plioc. _Camelus_, Pleist. _Lama_, Plioc. to Recent, S. A.

X. †HYPERTRAGULIDÆ.

_†Leptotragulus_, up. Eoc. _†Leptoreodon_, up. Eoc. _†Leptomeryx_, low. Oligo. _†Hypertragulus_, Oligo. _†Hypisodus_, low. Oligo. _†Protoceras_, low. Oligo. _†Syndyoceras_, low. Mioc.

Suborder D. PECORA. True Ruminants

XI. CERVIDÆ. Deer.

_†Blastomeryx_, low. Mioc. to low. Plioc. _Cervus_, Pleist. and Rec. _Rangifer_, Pleist. and Rec. _Alce_, Pleist. and Rec. _†Cervalces_, Pleist. _Odocoileus_, Pleist. and Rec., N. and S. A. _Mazama_, Pleist. to Rec., S. A.

XII. †MERYCODONTIDÆ. †Deer-Antelopes.

_†Merycodus_, mid. Mioc. to low. Plioc. _†Capromeryx_, Pleist.

XIII. ANTILOCAPRIDÆ. Prong-Bucks.

_Antilocapra_, Pleist. and Rec. _?†Dromomeryx_, mid. and up. Mioc.

XIV. BOVIDÆ. Antelopes, Sheep, Goats, Oxen, etc.

_†Neotragocerus_, _†Ilingoceros_, _†Sphenophalus_, low. Plioc. _†Preptoceras_, _†Euceratherium_, _†Symbos_, Pleist. _Ovibos_, Pleist. and Rec. _Bison_, Pleist. and Rec.

This list of families and genera, portentous as it is, would be greatly increased by the addition of the Old World forms, which outnumber those of the western hemisphere.

SUBORDER SUINA. SWINE-LIKE ANIMALS

The history of the American types of pig-like forms is, in one sense, very full and complete in that the successive genera may be traced back to the Eocene, but, in another sense, the story is exasperatingly imperfect, because so much of the material is fragmentary. Of most of the genera, nothing is known but teeth and jaws, and these, though sufficient for identification, tell but little of the structural changes which it is desirable to know. It is merely a question of time, when more adequate material will be obtained.

1. _Tagassuidæ. Peccaries_

The peccaries, or American swine, are now chiefly of Neotropical distribution, extending into the Sonoran region only as far as Arkansas; but this has been true only since the Pleistocene, for nearly the entire history of the family has been enacted in North America. In many points of structure the peccaries of the present day are more advanced and specialized than the far more varied and diversified true swine of the Old World, for it is a singular fact that such a long-lived and persistent stock as the peccaries should have given rise to so few variants and side-branches. Existing peccaries all belong to a single genus (_Tagassu_) and are relatively small animals, of unmistakably pig-like character and appearance, but far smaller than the Wild Boar (_Sus scrofa_) of Europe, or the Wart Hog (_Phacochœrus æthiopicus_) of Africa, to mention only two of the Old World swine.

[Illustration: FIG. 191.—Dentition of the Collared Peccary (_Tagassu tajacu_) left side. _i 3_, external incisor. _C_, canine, _p 2_, second premolar (the first is lost), _m 1_, first molar.]

One characteristic and thoroughgoing difference between the peccaries and the swine is the shape of the canine tusks. In the former, the tusks, though very effective weapons, are not very large and are straight and have a vertical direction, while in all the true swine the upper tusk is curved upward and outward, projecting strongly from the side of the jaw, and the great, curved lower tusk wears against its anterior side. The peccaries further have smaller and simpler molars, each with four principal, conical cusps (quadrituberculate pattern) arranged in two transverse pairs, with numerous very small cuspules around and between them, obscuring the plan. In the true swine the teeth are much larger and covered with innumerable wart-like cusps, large and small, seldom arranged according to any definite plan.

In the following particulars the modern peccaries show advance over the Old World swine: (1) the last lower premolar has taken on the molar-pattern, a very exceptional feature among the artiodactyls; (2) the ulna and radius are coössified; (3) there are but two functional digits in each foot; the fore foot has, in addition, two complete, but very reduced and slender, lateral digits and the hind foot only one, whereas in all the pigs of the eastern hemisphere there are four functional toes in each foot; (4) in the hind foot the two functional metatarsals, the third and fourth, have coalesced to form a “cannon-bone,” a structure which is not found in any other family of the suborder; (5) the stomach is complex, approximating that of a ruminant.

In the North American Pleistocene the predominating kind of peccary was a genus (_†Platygonus_) which was more advanced than the existing form (_Tagassu_), and, to all seeming, better fitted to survive, though for some inexplicable reason it failed to do so. It was a considerably larger animal, with proportionately longer and heavier legs. Its molar teeth are of special interest because they reproduced a type which has been so often repeated and independently acquired in so many different groups of mammals. In this molar the two conical cusps of each pair were fused into a high, transverse ridge or crest. Precisely the same modification took place among the true swine in the genus _†Listriodon_ of the French middle Miocene. _†Platygonus_ first appeared in the middle Pliocene, and its predecessor in the lower Pliocene and upper Miocene showed the crests of the molars in process of formation. In the latter stage it was accompanied by a true peccary with tuberculated teeth, which differed from the modern species in the simplicity of the hindmost premolar, which had not taken on the molar-pattern. If the feet and limbs of this upper Miocene peccary were known, they would doubtless prove to be much more primitive than those of _Tagassu_, but they still await discovery.

Little can be said of the peccaries of the middle and lower Miocene other than to record the fact of their presence in those formations, but those of the upper Oligocene (John Day) are, however, represented by well-preserved skulls, which show that more than one phylum of the family had arisen, though there was no great difference between them; they were considerably smaller animals than those of the Pliocene and Pleistocene. Still smaller was the White River genus (_†Perchœrus_) of which some fragmentary skeletons have been obtained. Although an undoubted peccary, this animal was not far from what the common progenitor of the peccaries and the true swine might be expected to resemble. The molars were quadrituberculate without the numerous accessory cuspules of the modern genus; the bones of the fore-arm were separate and the feet had four functional digits each, while there was no cannon-bone in the pes, the metatarsals remaining free.

No peccaries have yet been found in the Uinta, but probably this is a mere accident of collecting. It is, however, possible that the White River genus was not of American derivation, but an immigrant from the Old World. In the middle Eocene, or Bridger stage, this series is known only from teeth and jaws and a very few scattered foot-bones, and these, though probably referable to the family, cannot be definitively assigned to it without more complete material. Several species, larger and smaller, of the genus _†Helohyus_ occurred in the Bridger, where they were not uncommon, considering the general rarity of artiodactyls in that stage. Thus, the peccaries, though none of them were large, followed the usual law of mammalian development, and, beginning with very small forms, increased in size with each succeeding geological stage down to the Pleistocene.

2. _†Entelodontidæ. †Giant Pigs_

The †giant pigs, a most remarkable group of swine-like forms and of as yet unknown origin, appeared for the last time in North America in the lower Miocene, where the genus of that date (_†Dinohyus_) was the largest of known suilline animals, the hippopotamuses excepted. In nearly every part of the skeleton these great beasts displayed an unusual and aberrant kind of development. The incisors were long and pointed, and the canines formed stout and heavy, though not very long, tusks, which in shape were more like those of a bear than those of either peccaries or swine. The premolars were very simple, of compressed conical and trenchant shape, and occupied a very long space in the jaws, while the molars were relatively small and quadrituberculate, the crowns covered with very thick, coarsely wrinkled enamel. The skull was immensely elongate, especially the facial region in front of the eyes, while the brain-case was so absurdly small as to give the skull a reptilian aspect, when viewed from above. Evidently, these great pigs were profoundly stupid, in this respect rivalling the †titanotheres of the White River (p. 311). Beneath each eye-socket was a long, descending, bony flap, or process, and on the under side of the lower jaw were two pairs of prominent knobs, the function of which, as of the flaps beneath the eyes, is quite problematical. The eye-sockets themselves were completely encircled in bone, a rare character in the suborder.

The neck was short, as in the pigs generally, the body not very elongate and the tail of moderate length; at the shoulders, the spines of the dorsal vertebræ were very long, making a decided hump, and in the lumbar and posterior dorsal region the processes for articulation between the vertebræ were extremely elaborate. For one of the pigs, the limbs were very long and gave quite a stilted look to the animal. As in the modern peccaries, the fore-arm bones were indistinguishably fused together and the feet had only two toes each, the only members of the suborder in which digital reduction had proceeded so far, though the existing peccaries approximate this condition. There were, however, nodular vestiges of two other digits, which prove the derivation of this form from at least a four-toed type; no cannon-bone was formed. In view of the size of the animal, the hoofs were surprisingly small, which suggests that the weight was chiefly borne upon a pad. _†Dinohyus_ was a very large animal, six feet or more in height at the shoulder.

[Illustration: FIG. 192.—Right manus of †entelodont (_†Archæotherium ingens_) from lower White River beds. Princeton University Museum.]

[Illustration: FIG. 193.—Skull of White River †entelodont (_†Archæotherium mortoni_). Princeton University Museum. For restoration, see Fig. 137, p. 260.]

In the upper Oligocene were very large species of another, but closely similar, genus (_†Boöchœrus_) though somewhat smaller than those of _†Dinohyus_, and the species of the upper White River beds (_†Archæotherium_) were little, if at all, smaller than those of the John Day. A number of specimens in the museum of Princeton University throw a welcome light upon the habits of these strange creatures. In one, the external, or third, upper incisor tooth has a deep, triangular notch worn in its postero-external face, and the lower canine has a well-defined groove worn on the posterior side at the base of the crown; other individuals show less distinct marks of similar kind. (See Fig. 194.) It is out of the question to suppose that these grooves and notches could have been produced by abrasion with other teeth, for no other teeth could reach the worn areas, and it is altogether probable that they were made in digging up roots. The root, held firmly in the ground at both ends and looped over the teeth which pulled until it broke, and being covered with abrasive grit, would wear just such marks as the teeth actually display.[7] While the †entelodonts were thus rooters, they were doubtless omnivorous, like other pigs, and did not disdain a meal of carrion when they could get it. It is likely that the heavy canine tusks were also used as weapons, both in defence against the attacks of carnivores and in fighting between the males of the same species. It must have been in some such encounter that the animal represented by a complete skeleton in the Princeton Museum received its broken rib; that the fracture was made during life is demonstrated by the large callus growths on the broken ends, but the pieces did not knit.

[Illustration: FIG. 194.—Specimen showing characteristic grooves of wear in the anterior teeth of †entelodont (_†Archæotherium_) from upper White River beds. Princeton University Museum.]

In the middle and lower substages of the White River the genus (_†Archæotherium_) was the same as in the upper substage of these beds, but the species were all smaller and some of them very much so, not exceeding an ordinary pig in size. Throughout the series, as we now have it, from the lower Oligocene into the lower Miocene, there is very little change except in size, all the essential features of structure remaining the same; the genera are therefore distinguished by modifications of very secondary importance, and it is a question whether all the species should not be included in a single genus. The European genus _†Entelodon_, which gives its name to the family, is so like the American forms that by most writers the White River species are referred to it. It is of interest to note that the †giant pigs have also been found in the marine Miocene of New Jersey, one of the few records of the Tertiary land mammals of the Atlantic seaboard.

At present, the †entelodonts proper cannot be traced back of the lower White River beds, nor are they found in any more ancient formations in Europe. It is, therefore, probable that they were immigrants in both of these continents, presumably from Asia.

[Illustration: FIG. 195.—Skull of †short-faced pig (_†Achænodon robustus_) from the Bridger Eocene. Princeton University Museum.]

The whole Eocene of North America had a series of pig-like animals, called the †achænodonts or †short-faced pigs, which seem to have been related to the †entelodonts. They ended their career in the Uinta just before the appearance of the †entelodonts, and it would be natural to suppose that the latter were descended from them. If, however, the principle that an organ or structure once lost can never be regained, is valid, then there can be no relation of ancestor and descendant between the two groups, for of the †achænodonts, even their most ancient representatives had lost the first premolar, giving the formula _p_ 3/3, while in the †entelodonts it is constantly _p_ 4/4. The †achænodonts, which are much less fully known than the †entelodonts, had teeth very similar in form to those of the latter; and their most conspicuous feature was the shortness of the face and jaws, as contrasted with the extreme elongation of these parts in the †entelodonts, nor did they have the bony flaps under the eyes or the knobs on the lower jaw which gave such a fantastic appearance to the †entelodont skull. Little is known of the skeleton except that there were four functional digits in the manus. The Uinta and Bridger genus (_†Achænodon_) was larger than the Wasatch form (_†Parahyus_), which was an immigrant, probably from the same region as afterwards sent out the †entelodonts to America and Europe; this would account for the similarity and probable relationship of the two subfamilies.

SUBORDER ARTIODACTYLA †PRIMITIVA. †PRIMITIVE ARTIODACTYLS

No doubt, this suborder is an artificial assemblage of unrelated families, a sort of waste-basket, into which are thrown the groups of which no other disposition can be made in the present state of knowledge. As information becomes more complete, the various families will be redistributed among the groups with which they had a genuine relationship.

3. _†Anthracotheriidæ. †Anthracotheres_

This family was abundantly represented in Europe from the middle Eocene through the Oligocene, in Asia persisting even into the Pliocene, and were abundant in the Oligocene of Egypt. Migrants from the Old World reached America in White River times, but speedily died out, as they did not survive into the upper Oligocene. The most fully known of these animals is an American species of a European genus _†Bothriodon_. Almost complete skeletons of this genus have been obtained in the channel sandstones of the upper White River substage. In size and proportions, _†Bothriodon_ was not unlike a domestic pig, but had a very long head with slender, pointed snout; it had also a short neck, long body, short limbs and feet. The primitive character of this genus is made clear by many features of its structure; the molar teeth were extremely low-crowned and their cusps were so imperfectly crescentic in form as to be called _buno-selenodont_, as indicating their transitional nature, and the upper molars had five cusps instead of four, a very primitive feature. Another very significant character was the five-toed manus; the first digit, or pollex, was much smaller than the others.

[Illustration: FIG. 196.—_†Bothriodon brachyrhynchus_, upper White River stage. Restored from a skeleton in the museum of Princeton University.]

The second genus of the family which had American representatives was _†Anthracotherium_, which was much like _†Bothriodon_, but even more archaic in character; the molars could hardly be called selenodont at all.

4. _†Oreodontidæ. †Oreodonts_

This was one of the most characteristic of North American artiodactyl families, and its members were exceedingly abundant throughout the upper Eocene, the whole Oligocene and Miocene, ending their long career in the lower Pliocene. In distribution the family was exclusively North American, and no trace of it has been found in any other continent. In the course of their long history the †oreodonts underwent many transformations and branched out into several distinct phyla, yet through all these changes they remained singularly conservative, for the transformations, some of them sufficiently bizarre, affected chiefly the teeth and skull, the remainder of the skeleton changing but little. The †oreodonts were all small or of moderate size, none of them surpassing the Wild Boar in stature, nor was there any decided increase in size from stage to stage. One and all, they were strange beasts. Dr. Leidy, who first described and named most of the genera, spoke of them as combining the characters of camel, deer and pig, and called them “ruminating hogs,” a conception expressed in the names which he gave to some of them, such as _†Merychyus_ and _†Merycochœrus_, both of which mean ruminant swine.

The general proportions of most of the species were quite as in the peccaries, though, for the most part, with much longer tails; they had a short neck, elongate body, short limbs and feet. In one genus (_†Mesoreodon_) of the lower Miocene a rudimentary collar-bone has been found, and probably all of the more ancient genera possessed it, but only by an unusually lucky chance would so small and loosely attached a bone be preserved in place. As the collar-bone is superfluous in hoofed animals, in which the limbs are used only for locomotion and move in planes parallel with that of the backbone, it is almost universally absent in them, and in only one other group of ungulates, the extinct †Typotheria of South America, has its presence been demonstrated. In all of the †oreodonts the bones of the fore-arm and lower leg remained separate. The teeth were in continuous series, and there was a peculiar feature in the dentition common to nearly every one of the genera. On casual examination, one would say that the animals had four lower incisors on each side and that the lower canine closed behind the upper one, a most exceptional arrangement. More careful study shows that the apparent fourth incisor was the canine, a transformation which has also taken place in all of the ruminants except the camels, and the tooth which had assumed the form and function of the lower canine was really the first lower premolar; this latter change is not found among the ruminants, but was repeated in a few other extinct families.

[Illustration: FIG. 197.—Head of _†Merycochœrus proprius_, lower Miocene to lower Pliocene. Restored from a skull in the American Museum of Natural History.]

Only two genera of †oreodonts (_†Merychyus_ and _†Merycochœrus_) survived into the lower Pliocene. Both had the proportions common throughout the family, but _†Merychyus_ was much more slender and lightly built, its lateral digits were reduced in size and very thin and it had hypsodont grinding teeth; while _†Merycochœrus_ was of larger size (about that of a large domestic pig) and stouter build and had low-crowned teeth; its head, however, had a very different appearance, given by the possession of a short proboscis, the presence of which is indicated by the greatly reduced nasal bones; the jaws and face were also much shortened. The eye-sockets presented obliquely forward and upward, instead of laterally, as is usual among mammals, and were placed high in the head. This position of the eyes and of the entrance to the ear renders it probable that _†Merycochœrus_ was largely aquatic in its habits. Both genera had short, four-toed feet, as was general throughout the family and in no genus did the reduction of digits proceed beyond the loss of the first of the original five, the pollex and hallux.

The two genera above described, representatives of two distinct phyla within the family, held over, as it were, from the upper Miocene without essential change. The phylum of the hypsodont and slender _†Merychyus_ went back, with only minor modifications, into the upper substage of the lower Miocene, but cannot as yet be traced to an Oligocene ancestry; it is therefore still impossible to say just where and when it branched off from the main stem of the family. Future discoveries in the Oligocene will no doubt clear up this problem. The real terminal and most highly specialized member of the _†Merycochœrus_ phylum and the most extraordinary member of the entire family was confined to the upper Miocene. The extreme peculiarity of this genus (_†Pronomotherium_) was displayed only in the head, which was an exaggeration of the _†Merycochœrus_ type, the face being excessively shortened and the nasals so reduced as to show that the proboscis was much better developed than in the parent genus. The shortening of the face and the great vertical height of the skull and lower jaw gave a decided likeness to the skull of a great ape, though the proboscis would mask any such resemblance in the living head. _†Merycochœrus_ itself went back to the upper division of the lower Miocene, but in the lower division it was replaced by an ancestral genus, _†Promerycochœrus_, which had an elongate face and jaws and no proboscis; but in other characteristic features, such as the extreme thickness and roughness of the zygomatic arches, it was like its descendant. _†Promerycochœrus_ contained the largest known species of †oreodonts, some of them equalling a Wild Boar in stature, and its remains are found so abundantly in the middle and lower Miocene and upper Oligocene, that there must have been great herds of these animals over the plains. Probably it was itself derived from some of the larger species of _†Eporeodon_ of the upper White River beds, but there is a gap in the history, due to the fact that the lower part of the John Day is almost barren of fossils and the connecting link has not been recovered.

[Illustration: FIG. 198.—Head of _†Pronomotherium laticeps_, upper Miocene. Restored from a skull in the Carnegie Museum, Pittsburgh.]

It is an interesting and significant fact that ancestral and derivative genera may continue to live side by side in the same region. _†Promerycochœrus_, it is believed, gave rise to _†Merycochœrus_, but survived with it into the middle Miocene. _†Merycochœrus_, in its turn, produced _†Pronomotherium_, and, so far from being replaced by the latter, actually outlived it and persisted into the lower Pliocene.

[Illustration: FIG. 199.—_†Promerycochœrus carrikeri_, lower Miocene. Restored from a skeleton in the Carnegie Museum, Pittsburgh.]

A third phylum of the †oreodonts, which appeared for the last time in the middle Miocene (genus _†Cyclopidius_), was a series of small and very small species, of which the skull was almost as peculiar as that of _†Pronomotherium_, but in a different fashion. The face was very much shortened and on each side a great vacuity reduced the nasal bones to mere splints; the elevated position of the eye-sockets, which projected above the forehead, and of the tubular entrance to the ear is an evidence of an aquatic or amphibious mode of life, such as is illustrated by the hippopotamuses, which can float almost completely submerged, with only the ears, eyes and nostrils above the surface of the water. The tympanic bullæ (see p. 66) or bony chambers into which the ear-tubes opened, were of relatively enormous size and added much to the unusual appearance of the skull. The incisors were very small and the grinding teeth narrow and completely hypsodont, this and the _†Merychyus_ series being the only two phyla of the family in which the hypsodont molar was fully acquired. The remainder of the skeleton differed but little from the type common to the whole family, except for a somewhat shorter tail.

[Illustration: FIG. 200.—Skull of _†Leptauchenia nitida_, upper White River.]

The animals of this series were common in the middle and lower Miocene and in the upper substage of the White River, but have not been found in the intermediate John Day. This may have been a matter of geographical distribution, these creatures not extending west of the main ranges of the Rocky Mountains. In the upper White River the genus _†Leptauchenia_ is extremely common, but below that level they suddenly and completely vanish and, as in the case of the _†Merychyus_ phylum, it is not yet practicable to determine the point in time or space of their branching off from the main stem of the family. Were the †oreodonts not entirely confined to North America, we should, as a matter of course, explain the seemingly sudden appearance of _†Leptauchenia_ as due to immigration, and it is entirely possible that the series did actually originate in some part of North America which has left no record of its Eocene or Oligocene terrestrial life. On the other hand, no one can imagine that everything that can be known of the mammals of the middle and lower White River has already been learned, and at any time the sought-for ancestor of _†Leptauchenia_ may be found in those beds.

[Illustration: FIG. 201.—_Leptauchenia nitida_, upper White River. Restored from a skeleton in the Museum of Princeton University.]

The fourth phylum may be regarded as the main or central stem of the family and was the one which underwent the least change, though it probably gave rise to all the other phyla, which branched off from it at various stages in its history. This series terminated in the middle Miocene and comprised several genera, all very much alike, in the lower stages of that epoch. One of these genera (_†Mesoreodon_) displayed a very remarkable peculiarity of structure in the ossification of the great cartilage of the larynx, which seems to point to the possession of uncommon vocal powers. It is impossible to say whether this feature was confined to the single genus, or was general in the family, for only in rare instances would so extremely delicate a structure be preserved. In the John Day the genus _†Eporeodon_, which was very abundant, was the representative of this phylum, and the same, or a closely similar, genus lived in the latter part of the White River stage.

In the middle and lower White River substages †oreodonts are the commonest of fossils, so that the collector soon wearies of them (see Fig. 136, p. 259); they must have lived in great herds in the forests and along the streams. There were several species, varying principally in size, the largest about as long as a wolf, but with shorter legs, and the smallest not so much as half of that size. All belonged to a single genus, for which the rigid law of priority compels us to use a most cumbrous name (_†Merycoidodon_), the widely used term _†Oreodon_ being a synonym. This genus was the central stock of the family, from which most, if not all, the others were directly or indirectly derived, though, as previously pointed out, we cannot in all cases trace the connection. In these White River animals the grinding teeth were very low-crowned and had considerable resemblance to those of a deer; the molars were typically selenodont and made up of two pairs of crescentic cusps. The skull differed little from that of the succeeding genera of this phylum; the neck was short, body and tail long. An especially interesting fact is that the fore foot had five digits, the first, or pollex, very small and of no functional value, but complete in all its parts; the hind foot was four-toed. In all of the subsequent genera of the family the number of digits was uniformly four in both manus and pes.

[Illustration: FIG. 202.—Skull of _†Merycoidodon culbertsoni_, middle White River. (After Leidy.)]

In the Uinta stage of the upper Eocene lived the most ancient and primitive member of the family yet discovered, the genus _†Protoreodon_, which is in every respect what the ancestor of the White River genus should be. The functional transformation of the lower canine into a fourth incisor and the replacement of the canine by the first lower premolar had already taken place, but the molars were much more primitive than those of the White River and succeeding genera; the crescents were thicker and less complete, plainly indicating their derivation from conical cusps, and a small fifth cusp was present between the anterior pair of the upper molars, as in the †anthracotheres and other European families of the Artiodactyla †Primitiva. Before the discovery of _†Protoreodon_, the character of its molars was predicted by Dr. Schlosser, of Munich. The skull resembled that of the White River genera, except that the eye-socket was open behind, and there was no glandular pit in front of the eye. The skeleton is but partially known, but it has been ascertained that there were five toes in the manus and probably also in the pes.

[Illustration: FIG. 203.—Skull of _†Protoreodon parvus_, Uinta Eocene. Princeton University Museum. N.B. This skull is actually much smaller than that shown in Fig. 202.]

Nothing has yet been discovered in formations older than the upper Eocene which can be regarded as ancestral to the †oreodonts, and this is not surprising in view of the extremely meagre and unsatisfactory nature of our information regarding the artiodactyls of the Bridger. On the whole, however, it seems rather more probable that the Uinta genus was an immigrant (whence, we cannot say) than that the Bridger will ever yield the desired ancestral forms. So long as the early Tertiary mammals of northern and central Asia remain unknown, this and many similar problems can find no definitive solution. The question of relationship with other families is bound up with that of the origin of the †oreodonts; many characters point to a connection with the †anthracotheres and, from the standpoint of present knowledge, that appears to be the most probable affinity; but, on the other hand, there are structural features which suggest relationship with the primitive camels. Between these and other alternatives, only the recovery of the middle and lower Eocene forms can finally decide.

Reviewing the long history of the oreodont family from the evolutionary point of view, we find a course of development which differs in several respects from that exemplified by most of the families previously considered:

(1) There was a general increase in size, though it was far from steady, and almost every genus had larger and smaller species, and in some of the phyla the species were far larger than in others. The members of the _†Leptauchenia_ phylum were very small and no member of the family ever attained to more than moderate size.

(2) The upper molars early lost the fifth cusp, and after that there was little change in the dentition, except that in the _†Merychyus_ and _†Leptauchenia_ phyla the grinding teeth became hypsodont.

(3) There was great variety in the modifications of the skull, each phylum having its own peculiarities. The orbit, which was open behind in the Uinta _†Protoreodon_, was closed in the White River and all succeeding genera. In the _†Merycochœrus_ series, the skull first enlarged, with little change in proportions, then elongated the facial region, then shortened the face and so reduced the nasals as to indicate the presence of a proboscis, culminating in the grotesque, ape-like skull of _†Pronomotherium_. In the _†Leptauchenia_ phylum the skull became depressed and flattened and the face was invaded by great openings, or vacuities; the tympanic bullæ were enormously inflated and the orbits and ear-openings raised, presumably in adaptation to an amphibious mode of life. These were the extremes of change within the family; the other phyla need not be considered.

(4) At an early stage the digits were reduced from five to four, first in the pes and then in the manus, and there reduction ceased; though in _†Merychyus_, especially in the upper Miocene species, the lateral digits were very slender and, had this series survived, it would probably have led to didactyl forms.

[Illustration: FIG. 204.—Left manus of †oreodonts. _A_, _†Merycoidodon culbertsoni_, White River. _B_, _†Merycochœrus proprius_, upper Miocene.]

In other respects there was very little difference in the skeletons of the various phyla and herein lies the peculiarity in the history of the family, great variety in the form of the skull, and, relatively speaking, hardly any change in the body, limbs or feet. In the horses, rhinoceroses and †titanotheres the modifications of the successive genera affected all parts of the structure, but in the †oreodonts, except for the loss of one digit in manus and pes and variations in the length of the tail, the skeletons of the latest genera did not differ in any important respect from those of the earliest. Such a combination of mutability and plasticity in the skull with extreme conservatism in the remainder of the bony structure is an exception to the usual mode of development, though something of the same sort has already been pointed out in the case of the tapirs (p. 325) and will recur in that of the elephants (Chap. X).

5. _†Agriochœridæ. †Agriochœrids_

This family, one of the strangest and most aberrant of ungulate groups, was very closely allied to the †oreodonts and by many authorities is included in the same family. The history of the successive steps of discovery, by which the structure of these extraordinary animals was gradually made plain, is much the same as in the case of the even more peculiar perissodactyl family of the †chalicotheres (p. 356). The various parts, found scattered and at long intervals of time, had been referred to no less than three different mammalian _orders_! for, until the discovery of †chalicothere skeletons gave the clue, no one imagined that such discordant parts could belong to the same animal.

[Illustration: FIG. 205.—Skull of _†Agriochœrus latifrons_, White River. (After Wortman.)]

[Illustration: FIG. 206.—_†Agriochœrus antiquus_, White River. Restored from a skeleton in the American Museum of Natural History.]

The †agriochœrids had a very much shorter career than the allied family of the †oreodonts, extending only through the upper Eocene and the Oligocene (Uinta to John Day, inclusive); and only two genera of the family are yet known, _†Agriochœrus_ of the John Day and White River, and _†Protagriochœrus_ of the Uinta. In the former the teeth were not in a continuous, closely crowded series, but there were open spaces behind the upper canine and first lower premolar; the same exceptional character of the lower teeth which was found in the †oreodonts was repeated in the present family, the canine assuming the form and functions of an incisor and the first premolar those of the canine; the upper incisors were extremely small and were shed in the adult, just as in the true ruminants. The molars had the selenodont pattern, but the upper molars were very different in shape from those of the †oreodonts, resembling rather those of the †anthracothere _†Bothriodon_ (see p. 370). Another difference from the †oreodont dentition was that the last lower premolar had acquired the molar form and the last upper one nearly so, a very unusual feature among the artiodactyls. The skull was almost exactly like that of the White River †oreodonts, save in a few details; the face was somewhat longer, the orbit was open behind and there was no glandular pit on the face in front of the eye. The neck was short and the body long, and the backbone in the region of the loins very stout, the vertebræ of this region having much resemblance to those of the great cats, as though _†Agriochœrus_ were an agile and powerful leaper. Another likeness to the cats was in the very long and heavy tail, which was much longer than in the †oreodonts, and its vertebræ were hardly distinguishable from those of a Leopard. The limbs were relatively longer than those of the †oreodonts and the separate bones had a suggestive likeness to those of carnivores, and, more specifically, of cats. The feet, save in one particular, were not only artiodactyl, but also characteristically †oreodont in structure and, as in the earlier members of that family, there were five toes in the manus and four in the pes. The exception was that, instead of narrow and slender hoofs, the feet were armed with sharp, though not very large claws, which were not comparable in relative size to the great claws of the †chalicotheres.

Altogether, a strange jumble of incongruous characters was united in this skeleton. Were only the skeleton known without the skull, one would be tempted to call it that of a carnivorous artiodactyl, but the teeth make such a suggestion absurd, since they could have been used only for masticating a diet of soft vegetable substances. No flesh-eater has, or ever had, teeth in the remotest degree like these, which were of characteristically herbivorous type. How such a creature lived and what were its habits, are questions to which no satisfactory answer has been found.

[Illustration: FIG. 207.—Right manus of _†Agriochœrus latifrons_, White River. (After Wortman.)]

_†Protagriochœrus_ of the upper Eocene is, unfortunately, known only from very imperfect and fragmentary specimens, which, however, are sufficient to determine some significant points. These remains show that, while the two families of the †agriochœrids and the †oreodonts were already distinct in the Uinta, they were decidedly nearer together than they became in the Oligocene. In other words, it is clear that the two groups were converging back to a common ancestry. This may be discovered in the Bridger, but it seems more probable that these forms were immigrants. Another fact concerning the Uinta genus, which is important, is that the upper molars possessed the fifth or unpaired cusp which also occurred in the contemporary †oreodonts, as well as in the †anthracotheres and other Old World families.

SUBORDER TYLOPODA. CAMELS AND CAMEL-LIKE ANIMALS

Existing Tylopoda are all included in a single family, the Camelidæ, and by several authorities no other family, even of extinct forms, is admitted to the suborder. My own preference, however, is to refer the problematical little †hypertragulids to this group, as will be shown subsequently.

6. _Camelidæ. Camels and Llamas_

Under modern conditions, no mammals could seem more completely foreign to North America than those of the camel family, which, now restricted to two well-defined genera, inhabit central Asia and the colder parts of South America. Yet, as a matter of fact, this family passed through nearly the whole of its development in North America and did not emigrate to the other continents before the late Miocene or early Pliocene, and it is this North American origin of the family which explains its otherwise inexplicable distribution at the present time. To all appearances, the whole family had completely disappeared from this continent in the later Pleistocene, but in the middle and earlier portions of that epoch both true camels and large llama-like animals were very abundant on the Great Plains and in California, while they seem to have avoided the forested regions.

In order to appreciate the changes through which the camels and llamas have passed, it will be necessary to consider briefly the skeletal and dental structure which characterizes the modern genera. In the true camels (_Camelus_) the first and second upper incisors have been lost, but the third remains as a large, sharp-pointed tooth, as are also the upper canine and first premolar; thus there are three pointed, spike-like teeth in a row, with spaces between them, constituting with the lower canine a very effective lacerating apparatus. Behind the first premolar is a long gap, the second being suppressed; the third and fourth are grinding teeth, but unusually small. The molars are selenodont and high-crowned, though not extremely hypsodont. The lower incisors are large and shovel-shaped, the canine large and erect and there are but two lower premolars. The dental formula thus is: _i_ 1/3, _c_ 1/1, _p_ 3/2, _m_ 3/3.

The skull is long, with the facial region much and abruptly narrowed, which gives a triangular appearance to the head when seen from above; the orbit is completely encircled with bone and the sagittal and occipital crests are very prominent. The tympanic bullæ are large and filled with spongy bone. The condyle of the lower jaw is hemispherical and not, as it is in most ungulates, semicylindrical, and a curious, hook-like angulation is on the posterior border of the bone. The neck is very long, and the vertebræ have the exceptional peculiarity that the canal for the vertebral artery runs through the side of the neural arch, instead of perforating the transverse process, and thus is invisible externally; the odontoid process of the axis is spout-like. The legs and feet are very long; the humerus has a double bicipital groove and the fore-arm bones are coössified, and the ulna is so reduced that the radius carries the whole weight; in the lower hind leg the tibia supports the weight, and of the fibula only the lower end remains as the malleolar bone. There are but two digits in each foot, the third and fourth, the metapodials of which have coalesced to form a cannon-bone, which differs from that of the true ruminants, or Pecora, in the curious way in which the lower ends, separated by a Λ-shaped notch, diverge from each other, and by the fact that the keels of the lower articular surfaces are confined to the posterior side, not visible from the front. The ungual phalanges are small and nodular, and the hoofs, which carry no part of the weight, are hardly more than nails. Under the other phalanges is a broad pad of elastic tissue, upon which the weight rests, and the separation of the toes is very partial. The peculiar external appearance of the camels is largely due to structures which leave no trace in the skeleton, and especially to the great humps, one or two according to the species, which are accumulations of fat; the ears are short and rounded and the hair is not woolly, but almost straight.

The teeth and skeleton of the llamas (_Lama_) are closely similar to those of the camels, but the absence of humps, the long, pointed ears, the woolly hair and the much smaller size and lighter build give to the living animals a more marked difference of appearance from the camels than one would expect from a comparison of the skeletons alone. The dental formula is: _i_ 1/3, _c_ 1/1, _p_ 2/2, _m_ 3/3. The remaining upper incisor, the third, is recurved, as is also the canine, but the spike-shaped first premolar of the camels is absent and the other premolars are much smaller than in the latter. In the skull the brain-case is larger, and the sagittal and occipital crests are much less prominent. The skeleton differs hardly at all from that of the camels, except for its smaller size and more slender proportions. The toes are more distinctly separated, each having its own pad. Thus, among the existing representatives of the family are two very well-defined phyla, each characteristic of a different continent.

[Illustration: FIG. 208.—Guanaco (_Lama huanacus_).—By permission of the New York Zoölogical Society.]

The Blanco stage of the middle Pliocene, which has preserved but a meagre representation of the life of its time, has yielded a number of very large, llama-like species, not, however, ancestral to the modern species, for they had but one premolar in each jaw. From the lower Pliocene we have fuller information. In the Snake Creek stage the separation of the two modern phyla was complete, and there was a third one, now extinct, that of the browsing or “†giraffe-camels” (_†Alticamelus_) (see Fig. 127, p. 236), a term which must not be taken as implying any relationship with the giraffes, but merely a resemblance to them in proportions. These browsing camels were very large animals, but with relatively small heads and low-crowned teeth not suited for grazing; the neck was extremely long, made so by the great elongation of five of the vertebræ (second to sixth, inclusive), and the legs were also very long, fitting their possessors to browse upon trees. Much of the description of the appearance and habits of the Giraffe given by Flower and Lydekker would no doubt be applicable to these extinct camels. “To produce the extremely elongated neck the seven cervical vertebræ are proportionately long, which gives a somewhat stiff and awkward motion to the neck.... The Giraffe feeds almost exclusively on the foliage of trees ... for browsing on which its prehensile tongue and large free lips are specially adapted.”[8]

In teeth and skeleton the phyla of the true camels and of the llamas in the lower Pliocene did not differ very strongly from the living forms; the upper incisors were already reduced to one, but the premolars were not so small; the ulna and radius had coalesced and of the fibula only the lower end remained; the cannon-bones were completely formed, and that the pads of the feet had already been developed is shown by the phalanges, especially the irregular, nodular unguals.

The most ancient known camels of the Old World are found in the Pliocene of India, and the first llamas recorded in South America are also Pliocene. Since both camels and llamas existed together in North America, it may reasonably be asked why only one phylum migrated to Asia and only the other to South America. Why did not each continent receive migrants of both kinds? Without knowing more than we are ever likely to learn about the details of these migrations, it will not be possible to answer these questions, though plausible solutions of the problem suggest themselves. It is to be noted, in the first place, that a migration from the central portion of North America to Asia was by way of the far north and thus involved very different climatic conditions from those which must have been encountered in passing through the tropics to South America. It is perfectly possible that animals which lived together in temperate North America should have had very different powers of adaptation to heat and cold respectively, and the northern route may have been impassable to one and the southern route to the other. To this it might perhaps be objected that the llamas are cold-country animals, but this is true only of the existing species, for fossil forms are found abundantly in the Pleistocene of Ecuador, Brazil and Argentina. Another possibility is that both phyla did actually migrate to both continents and that only the camels succeeded in permanently establishing themselves in Asia and only the llamas in South America, though for this solution the fossils afford no evidence.

The camels of the upper Miocene did not differ sufficiently from those of the lower Pliocene to call for special notice other than to remark that the two phyla of the true camels and the llamas were hardly distinguishable and one genus (_†Procamelus_) may have been ancestral to both. In the middle Miocene the browsing camels (_†Alticamelus_) reached the acme of their importance and made no great progress subsequently. The generalized stock, from which the upper Miocene and lower Pliocene _†Procamelus_ descended, was represented by _†Protolabis_ and _†Miolabis_, smaller animals, which had a full set of upper incisors and premolars and the grinding teeth were not so high-crowned. In most of the species the metapodials had not fused to form cannon-bones and probably there were no pads on the feet, though _†Alticamelus_, the †Giraffe-Camel, had already developed both cannon-bones and pads.

[Illustration: FIG. 209.—Lower Miocene †giraffe-camel (_†Oxydactylus longipes_). Restored from a skeleton in the Carnegie Museum, Pittsburgh.]

In the lower Miocene the †giraffe-camels were represented by the genus _†Oxydactylus_, which was a considerably smaller animal than its successor _†Alticamelus_, of the middle Miocene and later formations, and had shorter neck and legs. The teeth, though brachyodont, were not very low-crowned. There was no cannon-bone, the two metapodials of each foot remaining separate. An especially noteworthy feature in this genus is to be observed in the character of the hoofs, which, as the ungual phalanges demonstrate, were narrow and pointed, like those of antelope and deer, and carried most of the weight. The member of the grazing series (_†Protomeryx_) was smaller in every way than its contemporary (_†Oxydactylus_) of the browsing line and had shorter neck and legs, though these were already long. The teeth were present in undiminished number, and the grinders, while not properly to be called hypsodont, showed a decided tendency to assume that character. The feet were in the same stage of development as in _†Oxydactylus_, that is to say, with two free digits and pointed, deer-like hoofs. We have thus the remarkable and most significant fact that, while the grazing and browsing camels of the lower Miocene were already distinctly separated, neither had yet attained to the type of foot-structure which _both_ of them afterwards independently acquired. This is a very instructive example of parallel evolution in closely related series.

[Illustration: FIG. 210.—Skeleton of _†Oxydactylus longipes_. Lower Miocene. (After Peterson.) For restoration, see Fig. 209.]

Of still another phylum of the camel family, the lower Miocene contains the only representatives yet discovered, the little “†gazelle-camels,” as they may be called. The single known genus (_†Stenomylus_, Fig. 131, p. 242) of this series was quite a small animal, much smaller than its contemporaries of the grazing or browsing series. _†Stenomylus_ was an extremely slender, cursorial creature and had a very exceptional feature in its dentition in the apparent presence of ten lower incisors, five on each side, the canine and first premolar having assumed the form and functions of the incisors; the molars were low-crowned. The head was rather small and rounded, the neck long and light, the limbs and feet elongate and excessively slender. The feet had two digits each, which were separate, not forming a cannon-bone, and the hoofs were narrow, pointed and deer-like. These delicate and graceful little animals had but a brief career, which seems to have reached its close in the lower Miocene. Perhaps their complete defencelessness made it impossible for them to maintain themselves against their enemies, despite their evident capacity for swift running.

The camels of the upper Oligocene (John Day) are still incompletely known, but appear all to have belonged to the series of grazers which led up to the modern genera. Future discovery may bring to light in the John Day earlier members of the †giraffe-camel series, of which a possible member is found in the uppermost substage of the White River, or perhaps both phyla united in the upper Oligocene, a question which remains to be determined. At all events, in the middle substage of the White River, or lower Oligocene, there is no evidence of more than a single phylum, from which the others were almost certainly derived, branching off from the main stem at different levels. First was given off the branch of the †giraffe-camels, then (or perhaps even earlier) that of the little †gazelle-camels, and, finally, the main stem bifurcated into the two phyla of the llamas and the true camels. The point of origin of the †gazelle-camels is still uncertain.

[Illustration: FIG. 211.—The White River camel (_†Poëbrotherium labiatum_). Restored from a skeleton in the museum of Princeton University.]

The typical White River genus (_†Poëbrotherium_) included a series of species which increased in size from the earlier to the later portions of the stage, but showed no such structural changes as to call for special notice. The larger of these species was somewhat taller than a sheep, but of much lighter proportions, with small, pointed head, long neck and body and long, very slender limbs and feet. The teeth were present in undiminished number, 44 in all; the lower incisors were small, simple, nearly erect and chisel-shaped, very different from the large, procumbent and shovel-like teeth of the modern genera, and the trenchant canines were much smaller than in the latter. The first premolar had an isolated position, the second and third were trenchant and much extended antero-posteriorly, quite as in many other groups of primitive artiodactyls. The molars, which were typically selenodont, were low-crowned in the upper jaw, but in the lower showed an incipient tendency to hypsodontism. The skull, by its shape and the characteristic narrowing of the face, immediately suggests the modern type, but differed in many details of structure, the most obvious of which were the incompletely closed orbits, the shallow and slender jaws, and the very large, hook-like process from the angle of the lower jaw, which, in greatly reduced form, is present in both of the Recent genera. The neck was relatively long, though by no means so long proportionately as it subsequently became, and the vertebræ had already acquired the peculiarity found in all the succeeding camels, of the exceptional position of the canal for the vertebral artery, save in the sixth vertebra, where it pierced the transverse process, as in mammals generally; the odontoid process of the axis was neither spout-like nor peg-like, but of intermediate form, convex below and flat above. The body was long and light, and the ribs were much more slender than in the Recent genera. The fore and hind limbs, which were of nearly equal length, were very slender; the humerus had a single bicipital groove; the fore-arm bones were fully coössified and in the lower leg only the two ends of the fibula remained. The feet were already in the stage of development which persisted through the lower Miocene in all of the phyla, with two separate digits and nodular remnants of two others, and deer-like hoofs.

It would be of interest to compare this little White River camel with its contemporary genus of horses, _†Mesohippus_, and to observe in how many respects they have followed a parallel course, and how nearly _†Poëbrotherium_ occupied the same position with reference to the modern camels and llamas as _†Mesohippus_ did to the Recent horses; but such a comparison would involve too many technicalities to be profitably undertaken here. Suffice it to say that in many details there was a genuine parallelism in the progress of these two widely separated families from a polydactyl ancestry towards an extreme of digital reduction, ending in the horses in the single-toed and in the camels in the two-toed foot. The members of the two series kept nearly equal pace in their slow progress, with the camels a little in advance, since they were the first to attain the modern state of development in the height of the teeth and the structure of the feet, though eventually the horses surpassed them in both respects.

In the upper Eocene (Uinta stage) there were at least two kinds of camels, the time-relations of which to each other are not known, that is, whether they were contemporary or successive. The best-known genus, _†Protylopus_, may perhaps not be in the direct line of camel descent, but it so nearly represents the proper ancestral stage that, for all practical purposes, it will serve nearly as well. It was a much smaller animal than the smallest of the White River species, and was hardly larger than a “jack-rabbit.” The teeth of each jaw were in continuous series and the canines were but slightly longer than the incisors; the premolars had less antero-posterior extension than in _†Poëbrotherium_, and all the molars above and below were very low-crowned. The skull was almost a miniature copy of that of _†Poëbrotherium_, but more primitive in a number of details, the most important of which was that the tympanic bullæ were much smaller and hollow, not filled with spongy bone. The neck, concerning which it would be very desirable to have information, is almost the only part of the skeleton that is not known. The fore limb was considerably shorter than the hind, making the back slope downward from the rump to the shoulders; in the fore-arm the two bones were entirely separate and in the lower leg the fibula, though very slender, was still complete. In the manus there were four functional digits, the laterals not very much smaller than the median pair; but in the pes the lateral metatarsals were reduced to mere bony threads, to which small phalanges, in full complement, were attached, making tiny dew-claws.

With _†Protylopus_ ends the genealogy of the camels so far as it can be definitively traced, but in the middle of the Bridger stage is found a genus, _†Homacodon_ (family †Dichobunidæ), which is a probable member of the series. However, until the connecting link can be found in the upper Bridger, this conclusion cannot be demonstrated and _†Homacodon_ itself is incompletely known. It was a very small animal, even less in size than _†Protylopus_, and had not yet acquired the selenodont molars. These teeth were quadritubercular, _i.e._ with four principal cusps arranged, in the upper molars, in a square, and with a minute cuspule between each transverse pair, while the lower molars were narrower and had only the four principal cusps. The cusps were not conical, as they are in the pigs, but angular and pyramidal, the first step toward the assumption of the selenodont form. The skull was not specifically cameline in appearance, but rather indifferent, as though almost any kind of an artiodactyl might have been derived from it. The feet were decidedly more primitive than those of the Uinta genus, having four functional digits each, perhaps five in the manus. While it cannot be positively stated that _†Homacodon_ was the actual ancestor of _†Protylopus_, it nearly represents what we should expect that ancestor to be.

[Illustration: FIG. 212.—Diagram to illustrate the development of the skull and molar teeth in the camel tribe, in ascending geological order. _A_, _†Protylopus petersoni_, Uinta Eocene. _B_, _†Poëbrotherium wilsoni_, White River. (After Wortman.) _C_, _†Procamelus gracilis_, upper Miocene. (After Cope.) _D_, _Lama huanacus_, the modern Guanaco.]

In the lower Eocene (Wasatch stage) lived a tiny creature, _†Trigonolestes_ (family †Trigonolestidæ), smaller even than _†Homacodon_ of the Bridger, and one of the most ancient and primitive of known artiodactyls, but, unfortunately, still represented only by very imperfect specimens, so that much which it would be highly desirable to learn must await the finding of better material. The upper molars were triangular and tritubercular, _i.e._ with three principal cusps arranged in a triangle, and are hardly to be distinguished from those of other early mammalian orders. From the teeth alone the artiodactyl nature of the animal would not have been suspected, and, in fact, they were, when first discovered, referred to primitive monkeys. The feet probably had five toes each, but this is not certain, and the femur had the third trochanter, the only known artiodactyl of which this is true. As this little Wasatch genus is so imperfectly known, it would be premature to claim it as the starting point of the camel family, and yet it may very well have been so. Better material of this genus and the links of the chain which belong in the upper Bridger and the Wind River respectively must be recovered before this earliest portion of the family history can be written in more than tentative fashion.

[Illustration: FIG. 213.—Right manus of camels. _A_, _†Protylopus_, Uinta. _B_, _†Poëbrotherium_, White River. _C_, _†Procamelus_, upper Miocene. (After Cope.) _D_, Recent Guanaco.]

[Illustration: FIG. 214.—Right pes of camels. _A_, _†Protylopus_. _B_, _†Poëbrotherium_. _C_, _†Procamelus_. (After Cope.) _D_, Guanaco.]

The mode of evolution displayed by the camels does not differ in any significant respect from that seen in the horses. There was the same increase in bodily stature and in the relative lengths of the limbs and feet, the same kind of diminution in the number of digits from the original five, the same reduction of the ulna and its coalescence with the radius and the loss of the fibula save for its two ends. There was also a similar development of the high-crowned, or hypsodont, grinding teeth, from the low-crowned, or brachyodont, type. In still another respect there was a similarity in the mode of development of the two families, namely, in the way in which the several phyla of each originated. For the earlier portion of their history there was in each but a single distinguishable series, though it is very possible that fuller knowledge and more complete material would enable us to distinguish more than one. This monophyletic condition continued through the Eocene and most of the Oligocene, but in the upper portion of the latter and, more markedly in the lower Miocene, the two families branched out, each in its own fashion.

Of course, there were differences in the development of the camels and horses, some conditioned by the fundamental distinction between artiodactyl and perissodactyl, such as the didactyl foot as the possible minimum and the formation of cannon-bones in the camels. Other differences are characteristic of the latter family, such as the great elongation of the neck and the peculiar structure of its vertebræ, the formation of pads on the feet and concomitant reduction of the hoofs. In a general way, the two families kept quite an even pace in their advance from the more primitive to the more specialized condition and, though the camels were the first to acquire certain modifications, the horses ultimately surpassed them.

Even more close was the parallelism in evolution between the camels and the true ruminants (suborder Pecora), and this case is of particular importance as clearly demonstrating the development, in two independent but related lines, of similar structures not derived from a common ancestry. This comparison must naturally await the description of the Pecora.

7. _†Hypertragulidæ. †Hypertragulids_

This was a very peculiar family, of exclusively North American distribution and of doubtful systematic position, the known history of which extended from the upper Eocene into the lowest Miocene and then abruptly terminated. None of its members attained to considerable size, the largest hardly surpassing a sheep, and some were extremely small. In view of its comparatively brief career, this family was surprisingly ramified, and no less than four phyla may be distinguished within its limits.

[Illustration: FIG. 215.—_†Syndyoceras cooki_, lower Miocene. Restored from a skeleton in the museum of the University of Nebraska.]

One of the phyla which persisted into the lower Miocene was there represented by a most fantastic creature (_†Syndyoceras_) with four horn-like outgrowths from the skull, one pair arising from the anterior part of the face and curving outward away from each other, and the hinder pair, which were placed over the eyes, curved toward each other at the tips and were shaped much like a cow’s horns in miniature. The shape of these bony protuberances makes it unlikely that they were sheathed in horn and probably they were merely covered with skin like the horns of the giraffes. This description applies only to the skull of the male; that of the female is not yet known, but there is good reason to believe that in that sex the horns were much smaller or wanting, as in nearly all existing deer. The skull was long, narrow and low; the orbits were small, completely enclosed in bone and unusually prominent; the nasal bones were exceedingly short, as though indicating the existence of a proboscis, but this can hardly have been the case, for the nasal opening was divided into anterior and posterior portions by the bony bridge which united the bases of the forward pair of horns. In no other known mammal does such a division of the nasal opening occur. The upper incisors had all disappeared, but there was a small upper canine tusk and another formed by the first lower premolar, while the real lower canine had gone over to the incisor series. This exceptional arrangement is a point of resemblance to the †oreodonts (see p. 372). The grinding teeth were brachyodont. The fore limb is not known, but the hind limb has been completely recovered; it was stout and not very long in proportion to the length of the head. The fibula was completely reduced, only the ends remaining, and the pes was didactyl, the two metatarsals uniting in a cannon-bone; the hoofs were like those of deer and antelopes.

[Illustration: FIG. 216.—_†Protoceras celer_, upper White River; males on the right and left, female in the middle. Restored from skeletons in the American Museum and Princeton University.]

No representative of this series has yet been found in the upper Oligocene; and it is not yet possible to say whether their absence from the John Day beds, as in several other cases already referred to, was due to an actual geographical difference in contemporary faunas, or whether it is merely one of the accidents of preservation and collecting. In the upper White River, however, was another most curious animal (_†Protoceras_), a forerunner, if not a direct ancestor, of _†Syndyoceras_. The exact relationship between the two forms can hardly be determined, until the genera, one or more, which once connected them shall have been recovered, though it is obvious that they belonged to the same series. _†Protoceras_ was a smaller animal and, if anything, an even more bizarre-looking object, for the anterior protuberances were broad, prominent and everted plates of bone, not even suggesting horns in their form, and the posterior pair were short and club-shaped; in the female neither pair was more than indicated. The dentition was very similar to that of _†Syndyoceras_, except that the upper tusk was considerably larger and scimitar-shaped; the female had no tusks. In the fore-arm the two bones were just beginning to coalesce, but in the lower leg the fibula was completely reduced. The manus had four complete and functional digits, the laterals not very much shorter and thinner than the median pair; but the pes was already didactyl, though the metatarsals were separate, not fused into a cannon-bone; two long and pointed splints were the vestigial remnants of the second and fifth digits.

[Illustration: FIG. 217.—_†Protoceras celer_, skull of male. (After Osborn and Wortman.)]

It is not yet possible to trace this phylum below the level of the uppermost White River beds, yet that will very probably be accomplished by future exploration.

The second phylum of the family was represented in the lowest Miocene by _†Hypertragulus_, a genus of much smaller animals than those of the preceding series, which went back to White River times without essential change, and was abundant in the John Day stage. Despite this fact, the structure of the genus is still incompletely known and much remains to be learned, but enough has already been ascertained to justify the association of this phylum with the _†Protoceras-†Syndyoceras_ series in one family as reasonable. The number of upper incisors in _†Hypertragulus_ has not been ascertained, but the canines were enlarged and tusk-like, the lower one not having gone over to the incisors, as it had in the preceding group. The skull had much resemblance to that of the contemporary camels, the sudden narrowing of the facial region giving it a very llama-like appearance; the orbit was open and on the face in front of it was a conspicuous vacuity. The ulna and radius were coössified and there were four digits in the manus, two in the pes, but no cannon-bone was formed.

[Illustration: FIG. 218.—Skull of _†Leptomeryx evansi_, White River. (After Matthew.)]

The third phylum, that of _†Leptomeryx_, had about the same range in time as the preceding one, though it has not yet been found in the John Day, and the genus is assuredly known only from the White River beds, in which it is not uncommon. _†Leptomeryx_ comprised a number of species, all very small animals, and none larger than a jack-rabbit. (See Fig. 277, p. 563.) In size, proportions and appearance, these dainty little creatures must have been very like the existing chevrotains or “mouse-deer” of Asia and the Malay islands, and by many writers they have been classed in the same suborder, the Tragulina. The upper incisors had been suppressed and the upper canine reduced to very small size, while the lower canine had become functionally one of the incisors. The skull had a very long and slender facial region, but had a less llama-like appearance than in _†Hypertragulus_. The neck was short and the fore limbs much shorter than the hind, so that the back sloped downward from the rump to the shoulders, as in the chevrotains. There was a remarkable, indeed quite unparalleled, difference between the fore and hind limbs and feet, the hinder extremity being not only much longer, but also much more specialized, while the anterior one retained in very large degree its primitive characteristics. Thus, in the fore-arm the ulna was complete and separate from the radius, but in the lower leg the fibula was reduced to its minimum. In the manus there were four entire and functional digits, in the pes only two, which were joined in a cannon-bone.

Finally, there was a fourth phylum, that of _†Hypisodus_, which was confined to the White River stage and is still incompletely known. This was a tiny creature, much smaller than any of the preceding ones, and is the only known White River ungulate with fully hypsodont grinding teeth. Another very exceptional peculiarity of its dentition was that in the lower jaw it had ten incisor-like teeth; not only the canine, but the first premolar as well, had assumed the character of the incisors. This same peculiarity is found in the lower Miocene †gazelle-camel, _†Stenomylus_ (see p. 394), but in no other mammal.

A considerable assemblage of genera belonging to this family occurs in the upper Eocene, but the material yet obtained is too fragmentary to permit the assignment of these forms to the different phyla, though it is very probable that among them are to be found ancestors of all the White River and subsequent genera.

While there is little difference of opinion as to the propriety of including in the family †Hypertragulidæ the four phyla described in the foregoing pages, the systematic position and the relationships of that family as a whole are matters of debate and likely long to remain so. Dr. Matthew refers the entire group to the suborder Tragulina and regards _†Leptomeryx_ as being closely related to the direct ancestry of the American deer, a view which is accepted by Professor Osborn, but in which I am unable to concur. My own belief is that the family was an early offshoot from the cameline stock and therefore referable to the Tylopoda, in which suborder they are here included. It would be out of place to enter upon a discussion of this perplexing problem, which can hardly receive a definitive solution until the artiodactyls of the Uinta stage are thoroughly understood. As in so many other series, the key of the mystery lies hidden in the Uinta fauna, which is still so inadequately known.

SUBORDER PECORA. TRUE RUMINANTS

This is the most advanced, specialized and diversified group of the artiodactyls, though the ruminating habit is shared by both Tylopoda and Tragulina. In this multitude of forms, giraffes, deer, antelopes, sheep, goats, oxen, buffaloes, bisons, etc., it is difficult to find a clue to a natural arrangement or classification. As a whole, the suborder is a well-defined group, and many structural characters, not all of which is it needful to enumerate here, are common to all of its members. The upper incisors are invariably absent, and, save in a few of the deer, the upper canine also, while the lower canine has become incisiform; the premolars are always three in number in each jaw and the molar-pattern is selenodont throughout. The odontoid process of the axis is spout-shaped. Except in a few deer, the Pecora all have bony outgrowths of the skull in the form of antlers or horns, at least in the males, many females being hornless. The ulna is coössified with the radius and the fibula is lost, except the lower end, which is a separate malleolar bone. There is always, in both fore and hind feet, a cannon-bone, the lower ends of which are parallel, not divergent, as they are in the Tylopoda, and each articular surface is encircled all around by a prominent median keel, as in the horses, which in the other suborders, as in mammals generally, is confined to the posterior side and not visible from the front. (_Cf._ Figs. 220 and 214, p. 401.) In no existing member of the Pecora are there complete lateral digits, and in several modern genera they have been completely suppressed; but in most there is, behind the functional pair of digits, a pair of “dew-claws,” the bones of which are more or less completely reduced, often to mere nodules. The stomach, which in the Tylopoda and Tragulina is three-chambered, is in the Pecora divided into four distinct parts.

[Illustration: FIG. 219.—Left manus of Patagonian Deer (_Hippocamelus bisulcus_). _S._, scaphoid. _L._, lunar. _Py._, pyramidal. _Td._, _M._, coössified trapezoid and magnum. _Un._, unciform. _Mc. II_ and _V_, rudimentary second and fifth metacarpals. _Mc. III_ and _IV_, cannon-bone. _Ph. 1_, _2_, first and second phalanges. _Ung._, ungual phalanx.]

[Illustration: FIG. 220.—Left pes of Patagonian Deer. _Cal._, calcaneum. _As._, astragalus. _N._, _Cb._, coössified navicular and cuboid. _Mt. III_, _IV_, cannon-bone. Other letters as in Fig. 219.]

As already intimated, the subdivision of the Pecora into smaller groups is far from easy. “The great difficulty which all zoölogists have felt in subdividing them into natural minor groups arises from the fact that the changes in different organs (feet, skull, frontal appendages, teeth, cutaneous glands, etc.) have proceeded with such apparent irregularity and absence of correlation that the different modifications of these parts are most variously combined in different members of the group.”[9] Two main sections of the suborder are, however, sufficiently well defined, (1) the Cervicornia and (2) the Cavicornia.

SECTION CERVICORNIA. DEER AND GIRAFFES

This section includes two families, the giraffes and the deer. Inasmuch as the former have not now and never did have any representatives in the western hemisphere, for the purposes of this book the section becomes identical with the deer family.

8. _Cervidæ. Deer_

In most of the deer now existing the male has antlers. The antler is a bony outgrowth from the frontal bone of the skull and is annually shed and replaced, increasing, as a rule, in size and in the number of branches with each renewal. During the period of growth the antler is richly supplied with blood-vessels and covered with skin and is then said to be “in the velvet,” which dries and peels off when growth is complete; after the rutting season a layer of bone at the base of the antler is resorbed, loosening the antler, which is then shed. There is, however, a permanent, cylindrical process, of greater or less length, from each frontal, the “_pedicle_,” from which the antler is annually reproduced, and around the base of the antler and shed with it is a roughened ring, the “_burr_.” Among the different genera of deer there is great variety in the form and size of the antler, from a single spike to the immense and complicated appendages of the Wapiti (_Cervus canadensis_). As a rule, the “_beam_” or main stem of the antler and its branches or “_tines_” are cylindrical and tapering; but in some cases, as in the Moose (_Alce_) and the Fallow Deer (_Dama_), the antler is very broad and flat and is then said to be “_palmated_.” Except in the Reindeer and Caribou (_Rangifer_) the female is without antlers.

In the skeleton there is little difference between the deer and the Cavicornia, but there are some differences in the teeth. In the males of those deer which have no antlers, such as the Musk-Deer (_Moschus moschiferus_) and the Chinese Water-Deer (_Hydropotes inermis_), as well as in certain forms with very small antlers, like the muntjacs of Asia (_Cervulus_ and _Elaphodus_), the upper canine is a long, thin, recurved and sabre-like tusk, a very effective weapon. Speaking of the Indian Muntjac or “Barking Deer” (_Cervulus muntjac_), Flower and Lydekker say, “When attacked by dogs the males use their sharp canine teeth with great vigour, inflicting upon their opponents deep and even dangerous wounds.” In other forms of deer the upper canines are small or absent. The grinding teeth are brachyodont, but in the existing genera they have higher crowns than in the Tertiary progenitors of the family, and in the Axis and Hog Deer of India (_Axis axis_ and _A. porcinus_) the molars are quite hypsodont.

As was shown in Chapter V, the existing deer of North America are of two kinds: (1) the northern, which are plainly of Old World origin and so closely similar to Old World species that many naturalists deny the necessity of making distinct species for the American forms. The best known of these are the Wapiti (_Cervus canadensis_), the Caribou (_Rangifer caribou_) and the Moose (_Alce americanus_). (2) The southern deer, of which the common Virginia Deer (_Odocoileus virginianus_) is a familiar example, though overlapping in their range that of the northern genera, are peculiar to the Americas, and, though not exactly autochthonous, they must have had a long American ancestry. In the Pleistocene we find the same genera and mostly the same species, their distribution over the continent shifting in accordance with the many climatic changes of that epoch. There was, however, at least one Pleistocene genus (_†Cervalces_) different from any now living and different from any known in the eastern hemisphere. The most complete specimen of this animal is a skeleton in the museum of Princeton University, found beneath a bog in northern New Jersey, though other bones, collected in Kentucky and elsewhere, are very probably referable to it. _†Cervalces_ was very nearly related to the Moose, the neck, body, limbs and feet being almost identical in the two genera, but the skull and antlers were notably different; the nasal bones were not nearly so much shortened as in the Moose, indicating that the proboscis-like snout was not so large or inflated as in the latter. The antlers were quite unique; though in general like those of the Moose, they were much less palmated and they had, in addition, a great trumpet-like plate of bone on the lower side of each antler (see Fig. 117, p. 209), such as occurs in no other known member of the family. Although _†Cervalces_ has not been found in the Old World, it was almost certainly an immigrant from eastern Asia.

The Moose, Caribou and Wapiti were unquestionably immigrants and came in not earlier than the Pleistocene. Nothing is known in the Pliocene or more ancient Tertiary epochs of North America which could be twisted into forms ancestral to these typically Old World genera. With the southern deer (_Odocoileus_, etc.) the matter stands differently, for these have a probable American ancestry extending back to the lower Miocene and possibly much farther. On the other hand, it is not altogether certain that these may not have been Pliocene immigrants, for their genealogy is still in an extremely fragmentary and unsatisfactory condition. The North American genus, _Odocoileus_, extended back to the Pliocene with very little change. The annoying, unrecorded gap of the upper Pliocene and the meagre representation of the middle Pliocene mammals given by the Blanco leave us without information regarding the deer of that time. In the lower Pliocene and through the whole Miocene we meet with frequent remains of a genus (_†Blastomeryx_) which was quite probably the ancestor of the American types of deer. It was considerably smaller than any of the existing North American species and had no antlers, but possessed the sabre-like, upper canine tusks, which characterize the muntjacs and hornless deer of Asia. The limb-bones had already attained nearly their present state of development, as regards the reduction of ulna and radius, formation of cannon-bones, etc. _†Blastomeryx_ probably entered North America in the lower Miocene, but, as was mentioned previously (p. 409), Dr. Matthew and Professor Osborn regard the genus as autochthonous and descended from the †Hypertragulidæ.

[Illustration: FIG. 221.—Lower Miocene †hornless deer (_†Blastomeryx advena_). Restored from a skeleton in the American Museum of Natural History.]

In the middle Miocene _†Blastomeryx_ gave rise to an aberrant genus (_†Merycodus_) which has been made the type of a distinct family (†Merycodontidæ, see table, p. 362), but this is perhaps unnecessary. _†Merycodus_ had deer-like antlers, but completely hypsodont teeth such as no known member of the Cervidæ possesses. The middle Miocene species (_†M. osborni_) was a little creature, not more than eighteen or twenty inches high at the shoulder, and had a branched antler of three tines, which was considerably longer than the skull, while in the species of the upper Miocene (_†M. furcatus_) the antler was shorter and simply forked. From the number of specimens of these animals found in which the burr is incomplete or absent, it may be inferred that the antler was not always deciduous. The legs were long and very slender, and apparently there was no trace of the lateral digits, even in the fore foot. These peculiar hypsodont deer persisted even in the older Pleistocene.

[Illustration: FIG. 222.—Miocene †deer-antelopes (_†Merycodus osborni_, middle Miocene, and _†M. furcatus_, upper Miocene). Restored from specimens in the American Museum.]

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.

11. _Bovidæ. Sheep, Bisons, Oxen, etc._

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.

On the other hand, it is equally obvious that the camels and llamas have been separated from the Pecora at least since the middle Eocene, and, consequently, the many points of agreement between the two suborders, other than those shared with all artiodactyls, are not due to inheritance from a common ancestry, but have been independently acquired in the two series. It will be instructive to note some of the more important of these independent similarities: (1) the selenodont and more or less hypsodont character of the grinding teeth; (2) the spout-shaped odontoid process of the axis; (3) the great reduction of the ulna and its coössification with the radius; (4) the loss of the fibula, except for its lower end, which persists as a separate malleolar bone; (5) the formation of cannon-bones by the fusion of the third and fourth metapodials; (6) the development of a complex, many-chambered stomach. Other points of likeness might be cited, but those already given will suffice to show how very important this parallel mode of evolution often proves itself to be.