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.