HISTORY OF THE †AMBLYPODA AND †CONDYLARTHRA
These are two orders of hoofed animals which long ago vanished from the
earth and no member of either is known to have survived later than the
Eocene epoch; both were of great antiquity, dating back to the Paleocene,
perhaps even to the Cretaceous. The last of the †Amblypoda are found in
the lowest Uinta or highest Bridger, but they were relatively abundant in
all the more ancient beds. The following table gives the more important
American forms:
Order †AMBLYPODA. †Short-Footed Ungulates
Suborder †TALIGRADA
I. †PERIPTYCHIDÆ.
_†Periptychus_, Paleoc.
II. †PANTOLAMBDIDÆ.
† _Pantolambda_, Paleoc.
Suborder †PANTODONTA
III. †CORYPHODONTIDÆ.
_†Coryphodon_, Wasatch and Wind River.
Suborder †DINOCERATA
IV. †UINTATHERIIDÆ.
_Bathyopsis_, Wind River. _†Elachoceras_, Bridger.
_†Uintatherium_, do. _†Eobasileus_, do.
As is shown in the table, the suborder †Taligrada is entirely Paleocene
in distribution, the †Pantodonta are lower Eocene and the †Dinocerata
chiefly middle Eocene, though persisting into the upper. The †Dinocerata
were the most striking and characteristic of Bridger mammals, and two
or three phyla of them may be distinguished, though for our purposes
this is hardly necessary, for these strange and bizarre creatures
were all very much alike. From the commonest and best-known genus
(_†Uintatherium_) they are called †uintatheres. They were large and
ponderous animals, the veritable giants of their time, far exceeding any
of their contemporaries. In appearance they were among the most fantastic
of the many curious beasts which the fossils have revealed.
The skull carried three pairs of bony protuberances, or horn-like
outgrowths; one pair on the nasal bones suggest by their shape and
character that they formed the support of dermal horns like those of
the paired-horn rhinoceroses (_†Diceratherium_) of the Oligocene and
lower Miocene. (See p. 239.) The second pair, which were moderately
high and thick prominences, almost cylindrical in shape and tapering
but slightly to their bluntly rounded ends, were chiefly outgrowths of
the maxillaries, or upper jaw-bones. From their shape, it is likely
that these were not sheathed in horn, but were merely covered with
skin, as were also the third pair, which arose from the parietals.
These were massive, club-shaped prominences, eight or ten inches high
and broadening to the free ends, a shape which makes it impossible to
suppose that these were true horn-cores covered with horny sheaths. A
high crest of bone, representing the occipital crest, enclosed the top
and back of the cranium, connecting the posterior pair of “horns” and
dying away in front of them. The top of the cranium had thus a deep,
basin-like character, such as is to be found in no mammal outside of this
suborder and was one of the most peculiar features of this extraordinary
skull. The brain-cavity was absurdly small, the growth of the brain not
having kept pace with that of the body; the cavity is hidden away in
the postero-inferior portion of the skull, the immense thickness of the
cranial walls being somewhat lightened by the formation of sinuses, but
these were much less extensive and pervasive than in other very large,
horned or tusk-bearing mammals, such as elephants, rhinoceroses, etc.
Probably, as in the case of the †titanotheres and †entelodonts, this
deficiency of brain-development was at least one of the factors which led
to the early extinction of the group. The premaxillaries were slender
and rod-like bones, which did not meet in the middle line and carried no
teeth. The long and massive nasal bones and the position of the nasal
opening show that these animals cannot have had a proboscis of any kind.
The lower jaw was remarkable for the great bony flange which, in the
males, descended on each side from the lower border, near the anterior
end, and served to protect the great canine tusks from fracture.
[Illustration: FIG. 230.—Skull of †uintathere (_†Uintatherium alticeps_),
lower jaw supplied from another species. Princeton University Museum. For
restoration, see Fig. 231, p. 447.]
The female skull differed in two respects from that of the male: (1) the
horn-like protuberances were much more slender and less prominent; (2) as
the upper canine did not form a tusk, the lower jaw had no flanges. The
skull of the artiodactyl _†Protoceras_ (p. 406) was remarkably similar to
that of the †uintatheres.
The dental formula was: _i_ 0/3, _c_ 1/1, _p_ 3/3, _m_ 3/3, × 2 = 34.
The upper incisors were completely lost and the lower ones had the very
unusual peculiarity of being bilobate, or having the crown separated
into two well-defined cusps. The upper canines in the males were very
large, relatively thin, recurved and sabre-like tusks, with acute points
and sharp edges, which must have been terrible weapons, though it is
difficult to see how they were used; probably the mouth was widely
opened, so as to clear the points of the tusks, and the animal then
struck with them, as a snake does with its fangs. The lower canine was
very small and was included in the incisor series, the shape and function
of which it had assumed. Thus, the †uintatheres, with their toothless
premaxillaries and, to all appearances, eight lower incisors, formed a
curious parallel to the true ruminants (Pecora), and, as in the latter,
they must have had a firm elastic pad on the premaxillaries, against
which the lower incisors could effectively bite, when cropping the soft
plants which formed the diet of these great beasts. The grinding teeth
were low-crowned and surprisingly small in comparison with the size of
the skull. The premolars and molars were nearly alike and had two or more
transverse crests.
Aside from the altogether exceptional character of the skull, the
skeleton was quite strikingly elephantine in appearance, so much so, in
fact, that these animals have repeatedly been referred to the Proboscidea
and some writers are still of the opinion that the two orders were
related. There is, however, no sufficient ground for this view; the
undeniable likenesses are much more probably to be ascribed to the
operation of convergent development.
[Illustration: FIG. 231.—One of the elephantine †amblypods
(_†Uintatherium alticeps_) of the Bridger stage. Male in foreground,
female behind. Restored from specimens in the museums of Yale and
Princeton universities.]
The neck was of moderate length, sufficiently long to enable the
animal to reach the ground with the lips, a necessity in the absence
of a proboscis. The body was very long and, as is shown by the length
and curvature of the ribs and the great breadth of the hip-bones,
extremely bulky. The limbs were very massive, and the long bones had
lost the marrow-cavities, being filled with spongy bone, as in the
elephants, †titanotheres and most other very heavy mammals. The bones
of the fore-arm and lower leg were separate. The hip- and thigh-bones
and shin-bones were remarkably elephantine in character and, if found
isolated, might readily be referred to some unknown proboscidean, but the
bones of the fore limb were quite different from those of the elephants.
The feet likewise had a very proboscidean appearance, notwithstanding
important and significant deviations in structure; they had the same
shortness and massiveness and a similar reduction in the size of the
hoofs, and the presence of all five digits added to the resemblance.
Undoubtedly, the feet had the same columnar shape and arrangement of
elastic pads. The living animal must have had an appearance quite similar
to that of a rather small elephant, not exceeding six or seven feet in
height at the shoulders and therefore not surpassing the largest modern
rhinoceroses, the broad-lipped species of Africa (_Opsiceros simus_).
Of course, the head must be excepted from the comparison, as that was
totally unlike the head of any existing creature; with its long and
narrow shape, its fantastic protuberances and its lack of a proboscis,
it had no suggestion of likeness to any proboscidean. Whether the great
body was naked, or clothed with hair, it is of course impossible to
determine with confidence, but, all things considered, it seems unlikely
that the hair should have been completely lost in any terrestrial mammal
at so early a period. As we have seen in the preceding chapters, hairy
elephants and rhinoceroses continued into and through the Pleistocene,
not only in the cold regions of the north, as is shown by the hair of
the American †Mastodon. In the tropics conditions were different, and in
that uniformly warm climate the loss of hair by the very large mammals
probably took place long before the Pleistocene. At all events, it is
a significant fact that no hairless land mammals are now known in any
region which has severe winters. It is true that the middle Eocene
climate over most of North America was warm-temperate or subtropical,
and the †uintatheres may, in consequence, have been hairless, but there
is no evidence of this.
[Illustration: FIG. 232.—Skull of _†Elachoceras parvum_ (lower jaw
restored). Princeton University Museum.]
Within the limits of the †uintathere family, considerable modification
and change may be traced, which, as in the case of the Proboscidea,
principally affected the skull and the general stature. It is hardly
worth while to deal separately with the two or more phyla which may
be distinguished, for the differences between them are relatively
unimportant. In the uppermost part of the Bridger stage almost the latest
representatives of the family are found and the genus (_†Eobasileus_)
was of the largest size. These animals had remarkably long and narrow
heads and very large, shovel-shaped nasal protuberances; in the males the
upper canine tusks were very long and curved back nearly in a semicircle.
In the middle portion of the stage the species of _†Uintatherium_ were
somewhat smaller and had shorter, wider and higher heads, the tusks,
though well developed, were not quite so long, nor so strongly recurved;
in some species they were nearly straight, with “hastate” or spear-head
point. In the same horizon is found a third genus (_†Elachoceras_)
which was probably a survival persisting from the lower Bridger, in
which none of these animals and little of anything else has yet been
found. _†Elachoceras_ was hardly half as large as the common species of
_†Uintatherium_ and its skull might be described as a preliminary sketch
for that of the latter; the nasal horns were extremely small, or, more
probably, entirely absent; the median pair were mere low knobs, hardly
an inch in height, and the posterior pair were simply thickenings of the
crest which enclosed the top of the cranium on three sides, scarcely
rising above it. This crest itself was much less prominent than in
_†Uintatherium_ and the basin-like top of the skull, in consequence, very
much shallower. The upper incisors and the first premolar had already
been lost and the upper canine enlarged into a sabre-like tusk, which,
however, was relatively smaller than in the succeeding genera. The
grinding teeth were quite the same as in the latter. Unfortunately, the
skull of _†Elachoceras_ is the only part of the animal which is known,
but, so far as that is concerned, it is precisely what we should expect
the forerunner of _†Uintatherium_ to be; an ancestor made to order could
hardly be more diagrammatic. It might, of course, be objected that no
such relation as that of ancestor and descendant could obtain between
these two genera, because they were contemporaries, but the case is like
that of the ancestral elephants described in the preceding chapter.
_†Mœritherium_ and _†Palæomastodon_ are found together in the Egyptian
Oligocene, the former surviving for a considerable time after it had
given rise to the latter, and in the upper Eocene only _†Mœritherium_
occurs. Many similar instances might be given, just as grandfathers often
live long with their grandchildren.
In the Wind River stage, or upper division of the lower Eocene, lived the
still incompletely known _†Bathyopsis_, of which, however, sufficient
material has been obtained to show that it was much less specialized than
any of the Bridger genera. This genus comprised animals much smaller
than its successor, _†Elachoceras_ of the middle Eocene, being smaller
than a tapir; it stood in much the same relation to _†Elachoceras_ as
the latter did to _†Uintatherium_. In the American Museum of Natural
History is a highly interesting skull of _†Bathyopsis_, which will
shortly be described by Professor Osborn. The premaxillaries have not
been preserved, and it is therefore impossible to say whether the upper
incisors had already been suppressed or not, and though the upper canine
has not been found, there can be no reasonable doubt that it was a tusk.
The lower canine had not yet gone over to the incisor series, but was
a thin though large tusk. There was one more lower premolar, four in
all, than _†Uintatherium_ possessed, and all the premolars were somewhat
smaller and simpler than the molars. The small skull had a broad and
somewhat concave cranial roof, with slightly raised enclosing crest,
and the horn-like protuberances of the posterior and median pairs were
present in an incipient stage. Whether those of the nasal pair were also
indicated is not known, but probably they were not. The lower jaw was of
very peculiar shape; the flange of the inferior border was not so well
defined as in _†Uintatherium_, but had no hinder margin and rose very
gradually backward.
The series of genera in descending order, _†Eobasileus_, _†Uintatherium_,
_†Elachoceras_ and _†Bathyopsis_, immediately impresses the observer
as being a natural phylogenetic series of successive ancestors and
descendants. Unfortunately, only the skull is known in the two last
named, but there is no ground for supposing that the discovery of the
skeletons would require any alteration in the series as we now have
it. No member of this series has yet been found in the Wasatch, but
there can be no doubt that it was represented in that stage, for a
recent expedition from the American Museum has collected teeth of a
_†Bathyopsis_-like form in still older beds.
SUBORDER †PANTODONTA
During the older part of the lower Eocene the †uintatheres must have
been a rare and unimportant element of the fauna, at least in those
parts of the continent whose history we know. Their place was taken by
another suborder, the †Pantodonta, which was not ancestral to them, but
collaterally related and descended from a common ancestry. The largest
and most dominating of Wasatch mammals was the genus _†Coryphodon_, which
also occurred in the lower Eocene of Europe, and the species of which
ranged in stature from a tapir to an ox, though of much heavier form than
the latter. The latest surviving species lived in the Wind River stage as
a contemporary of _†Bathyopsis_, but then the suborder gave way to the
†uintatheres.
In _†Coryphodon_ (see Fig. 142, p. 279) the number of teeth was
unreduced, a fact which is recorded in the name of the suborder, the
dental formula typical of all the primitive ungulates being applicable
to the genus. This formula was: _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 3/3, ×
2 = 44. The upper incisors were rather small, but functional, and the
canines of both jaws were formidable tusks, though not rivalling in
size the great sabres of the †uintatheres; the premolars had a simpler
structure than the molars, which resembled those of the †uintatheres in
a general way, but not closely. The skull differed greatly from that of
the †uintatheres in having no horn-like protuberances, and was relatively
large and heavy, the cranium having a broad, flat roof and no sagittal
crest, and the lower jaw had no descending flange from the inferior
border; in every way this skull was more normal and less bizarre-looking.
The neck was proportionately longer than in the †uintatheres, the body
long and the tail of medium length; the trunk-vertebræ had surprisingly
small and weak spines, perhaps an indication of aquatic habits. The limbs
were quite short and very heavy, and the bones, in comparison with those
of the †uintatheres, were less proboscidean and more perissodactyl in
character. For example, the femur retained the third trochanter and the
long bones had marrow-cavities. The feet, on the contrary, were very like
those of the †uintatheres, being extremely short and five-toed and with
reduced, nodular hoof-bones; even in the details of the wrist and ankle
joints there was no important difference between the two groups.
SUBORDER †TALIGRADA
None of the ungulate series considered in the foregoing chapters can be
traced back to a time earlier than the Wasatch, and many of them not
so far, but in the case of the †Amblypoda the line may be carried down
through the Paleocene. In the upper stage of that epoch (Torrejon) the
order was represented by _†Pantolambda_ (Fig. 143, p. 285), a member of
the third suborder, †Taligrada. The best-known species of the genus was
an animal with head and body somewhat smaller than those of a sheep and
much shorter legs. The teeth were present in unreduced number, 44 in
all; the canines were tusk-like, but very much smaller proportionately
than those of _†Coryphodon_; the premolars were smaller and simpler than
the molars, which closely represent the common starting point, whence
the curious tooth-patterns found in the subsequent genera of the various
families were derived. The skull was long and narrow and had a prominent
sagittal crest; the neck was of ordinary length, about equal to that
of the head; the body was long and the tail very long, much as in the
great cats. The hip-bones were narrow and slender and not bent outward,
having no such breadth as in _†Coryphodon_. The limbs were short and
relatively heavy, and the various bones were of such primitive character
that, if found isolated and not in association with teeth or foot-bones,
one would hardly venture to consider them as belonging to any hoofed
animal; the humerus had a very prominent deltoid crest and an epicondylar
foramen, and the femur had the third trochanter. The five-toed feet
were very short, and the digits were arranged in a spreading manner and
were relatively much more slender than in _†Coryphodon_. Each digit
terminated in a flat, pointed, well-developed hoof; evidently there was
no elastic pad to bear the weight, such as recurs in nearly all very
heavy ungulates. The gait of the animal was probably semi-plantigrade,
the hoofs being the principal points of support.
While _†Pantolambda_ was an undoubted ungulate and a member of the
†Amblypoda, there were many structural features in its skeleton which
point to a relationship with the primitive flesh-eaters. In the lower
stage of the Paleocene, the Puerco, the genus _†Periptychus_ would seem
to be the most ancient known member of the order, but it is still very
imperfectly understood.
* * * * *
In the mode of evolution of the †Amblypoda, so far as that is recorded by
the fossils, there is much to recall the development of the Proboscidea,
though the story began and ended at far earlier dates and may be traced
back to a much more primitive stage.
(1) There was a rapid increase of stature, especially of bulk, in the
†coryphodonts, but decidedly more gradual in the †uintatheres, which
eventually attained a far larger size.
(2) The upper incisors were suppressed and the canines grew into
formidable tusks, at first straight, then the superior one, enlarging
still farther, acquired a curved, scimitar-like shape, while the inferior
one dwindled and became functionally one of the incisors.
(3) The grinding teeth remained low-crowned throughout, but acquired a
more complex pattern, and the premolars became almost like the molars.
(4) The skull underwent a most remarkable transformation. Beginning with
a form that might have belonged to almost any of the ancient mammals,
hoofed or clawed, having very prominent sagittal and occipital crests,
long cranium and short face, it became in _†Coryphodon_ flat-roofed,
with moderately elongated face, while in the †uintatheres the top of
the cranium gradually took on a deeply concave basin-shape and, with
equal gradualness, three pairs of horn-like protuberances; the lower jaw
developed a great bony flange for the protection of the upper tusks.
These peculiarities grew more and more exaggerated and were most
striking in the terminal genus of the series, _†Eobasileus_.
(4) Unfortunately, nothing is yet known of the skeleton of _†Bathyopsis_
and _†Elachoceras_, so that it is not practicable to follow out all the
stages of skeletal modification, though the general course of development
is sufficiently plain. The neck did not change greatly, except to become
very strong and heavy and to grow shorter proportionately as the skull
was lengthened. The body remained long throughout the series, but gained
greatly in bulk, as the stature of the animal increased.
(5) The limb-bones lost their primitive character, such as the
epicondylar foramen of the humerus and the third trochanter of the
femur, and then, with the great increase of the weight to be supported,
the marrow-cavities were filled with spongy bone and the hip-bones
increased enormously in width; the femur lost its cylindrical shape
and was flattened antero-posteriorly, which gave it a very elephantine
appearance. None of the limb-bones was suppressed or greatly reduced in
size, nor was there any coössification between them.
(6) The feet early gained their definitive character; at no time was
there any loss of digits, but the originally divided toes were, in the
genera of the Wasatch and subsequent stages, united into the columnar
foot, and the hoofs were reduced from their primitively pointed shape to
nodular form.
As in the Proboscidea, therefore, there was comparatively little change
in the skeleton after the massive and bulky proportions had been
acquired, but great and continual modification of the skull. At the time
when the †Amblypoda finally disappeared, no ungulate had acquired the
hypsodont dentition. Had the group survived till the middle Miocene, a
time when the spread of grassy plains so profoundly affected the feeding
habits of many herbivorous mammals, the high-crowned teeth might have
been developed in them also, and this, in turn, would have produced other
changes in the skull, making closer the parallel with the Proboscidea.
In conclusion, a few words may be said concerning the geographical
distribution of the †Amblypoda. In the Paleocene the only known
representatives of the order were those of North America, but the
†coryphodonts of the lower Eocene migrated to the Old World; indeed, the
genus _†Coryphodon_ was first described and named from English specimens,
but there were no such abundance and variety of these animals in Europe
as there were in the western United States. The †uintatheres were
strictly North American in distribution and no member of the suborder
has ever been found outside of this continent. Animals referred to the
†Amblypoda by some authorities have been obtained in the Oligocene
and Miocene of South America, but the assignment has been made upon
insufficient evidence. (See p. 508.)
ORDER †CONDYLARTHRA
The †Condylarthra were a group of exceedingly primitive ungulates, which
served to connect the hoofed and clawed mammals in quite an intimate
manner. So few indeed were the distinctively ungulate characters which
they had acquired, that it is still premature to make any positive
statements regarding their geographical distribution, because unusually
well-preserved specimens are required to make sure of their presence
in any particular region. Concerning North America there is no room
for question, and there is hardly any doubt that they existed in the
Paleocene of Europe. The South American remains which have been referred
to this order may very well prove eventually to belong to it properly,
but until both feet and skulls have been obtained in unequivocal
association, the reference can be only tentative. In North America they
ranged through the Paleocene and lower Eocene, but are not known from any
subsequent formation, and even in the Wind River only a few stragglers
survived.
The principal American families and genera are as follows:
I. †MENISCOTHERIIDÆ.
_†Meniscotherium_, Wasatch and Wind River.
II. †PHENACODONTIDÆ.
_†Protogonodon_, Puerco. _†Euprotogonia_, Torrejon.
_†Phenacodus_, Wasatch and Wind River.
1. _†Phenacodontidæ_
The typical Wasatch genus _†Phenacodus_, which is very fully known from
nearly complete skeletons, included species which varied in size from a
fox to a small sheep; the same genus occurred in the Wind River, but not
later. _†Phenacodus_ had the unreduced dental formula: _i_ 3/3, _c_ 1/1,
_p_ 4/4, _m_ 3/3, × 2 = 44.
[Illustration: FIG. 233.—Skeleton of the Wasatch †condylarth,
_†Phenacodus primævus_. American Museum. For restoration, see Fig. 141,
p. 278.]
The incisors were small and simple, the canines tusk-like, but of no
very great size, the premolars smaller and simpler than the molars. The
latter were of the quadrituberculate pattern, of four simple, conical
cusps arranged in two pairs, a pattern which is common to the earlier
and less specialized members of many ungulate groups. The skull was
long, narrow and low, with long and well-defined sagittal crest. As
in primitive skulls generally, the cranial region was long and the
face short, the eyes being very far forward; this does not imply large
brain-capacity, indeed, the brain was very small, but merely that the
portion of the skull behind the eyes was relatively long. The jaws were
short and shallow, in accordance with the small and low-crowned teeth
which they carried. The neck was of medium length, but the body was
elongate and the tail was very long and stout. The hip-bones were narrow
and slender, as in primitive ungulates generally. The limbs were short
and stout and retained many very primitive characteristics. The humerus
had a prominent deltoid crest and an epicondylar foramen; the fore-arm
bones were separate and the ulna quite unreduced, being almost as stout
as the radius. The femur had the third trochanter and the leg-bones were
distinct, though the fibula was slender. The feet, which were short, had
five digits each, but the third toe was enlarged, while the first and
fifth were shortened, as though preparing to disappear and thus give rise
to a three-toed perissodactyl foot. The ankle-bone (astragalus) had a
rounded, convex lower end, fitting into the navicular, so that it might
readily be taken for that of a clawed mammal.
2. _†Meniscotheriidæ_
A second family of Condylarthra was represented in the lower Eocene by
the genus _†Meniscotherium_ and was in some respects considerably more
advanced than the †phenacodonts. These were small animals, in which the
molars had acquired a crescentic pattern, recalling that seen in the
early horses and in the †titanotheres and †chalicotheres, and other
perissodactyl families. In the upper molars the two external cusps
had been so extended as to form a continuous outer wall, each of the
cusps having a concave external face and the two uniting in a prominent
median ridge. The lower molars had two crescents, one behind the other,
as in several families of both perissodactyls and artiodactyls. The
body and tail were long, the limbs relatively longer and lighter than
those of _†Phenacodus_ and the five-toed feet were so like those of
the modern conies, or klipdasses, of Africa and Asia Minor, that by
some investigators the family has been referred to the same order, the
Hyracoidea, but the suggestion is not a probable one. It is much more
likely that these problematical little †meniscotheres were merely a
short-lived branch of the †Condylarthra.
[Illustration: FIG. 234.—Lower Eocene †condylarth, _†Meniscotherium
terræ-rubræ_. Restored from a skeleton in the American Museum.]
The †condylarths were quite abundantly represented in the Paleocene,
where the genus _†Euprotogonia_ was the forerunner of the Wasatch
_†Phenacodus_, but had an even more primitive type of dentition. The
upper molars were not quadritubercular, but tritubercular, the three
cusps arranged in a triangle, the two outer ones forming the base and
the single inner one the apex. This type of upper molar was, or is
still, common to the primitive and unspecialized members of a great
many mammalian orders, marsupials, insectivores, rodents, carnivores,
lemurs, artiodactyls, etc., and there is strong reason to believe that
the tritubercular molar was the common starting point for almost all
types of mammalian dentition. However that may be, _†Euprotogonia_ is of
great interest as materially helping to close the gap between the clawed
and the hoofed mammals, belonging, as it did, to the latter and yet
retaining in dentition, limbs and feet so many characteristics of the
former.
†Condylarthra were probably present in the lowest Paleocene (Puerco
stage), but the material so far obtained is so fragmentary that there can
be no certainty on this point.
It is not at all probable that any of the North American †Condylarthra
should be regarded as ancestral to any of the more advanced ungulate
groups; on the contrary, they would appear to have come to an end in
the Wind River, leaving no descendants behind them. It is further true,
as was mentioned above, that the presence of †Condylarthra in other
continents, while very probable, cannot be positively asserted, because
the evidence is incomplete. Yet it would be a great mistake to assume,
for this reason, that these most primitive of ungulates were devoid of
evolutionary importance and interest. As is so often the case, where,
in the absence of the direct ancestry, the collateral relations afford
very valuable information as to the course of descent and modification,
the †Condylarthra throw useful light upon the origin of the ungulate
groups. It is extremely probable that the †condylarths, or some very
similar series of primitive hoofed mammals, had a very wide and perhaps
cosmopolitan range at the end of the Cretaceous and beginning of the
Tertiary period, and that, in the still unidentified region, where the
artiodactyls and perissodactyls arose, it was from a condylarthrous
ancestry. Possibly, all the other ungulate orders may yet be traced
back to the same stock, but it is rather more likely that the ungulates
include several series of quite independent origin. At all events, it is
quite certain that the clawed mammals long antedated the hoofed types and
that the latter arose, either once or at several separate times, from the
former. The †Condylarthra show how one, at least, of these transitions
was effected, and thus, in principle, how all were accomplished.