Chapter IX: History of the Artiodactyla (1)

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

_†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.

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.

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.