HISTORY OF THE †LITOPTERNA AND †ASTRAPOTHERIA
Besides the four well-defined groups which make up the †Toxodontia (or
†Notoungulata) there are two other extinct orders of indigenous South
American ungulates, which remain to be considered. These did not have
the exceptional development of the auditory region of the skull which
characterized the †Toxodontia. The best known and most important genera
of the †Litopterna are listed in the following table:
†LITOPTERNA. †Litopterns
I. †MACRAUCHENIDÆ.
_†Macrauchenia_, Plioc. and Pleist.
_†Scalibrinitherium_, Paraná. _†Theosodon_, Santa
Cruz. _†Cramauchenia_, Patagonian. _†Protheosodon_,
Deseado.
II. PROTEROTHERIIDÆ.
_†Epitherium_, Monte Hermoso. _†Diadiaphorus_,
Santa Cruz and Paraná. _†Proterotherium_, do.
_†Thoatherium_, Santa Cruz. _†Deuterotherium_,
Deseado. † _Prothoatherium_, do.
III. DIDOLODIDÆ.
_†Didolodus_, Casa Mayor. _†Lambdaconus_, do.
_†Notoprogonia_, do. _†Proectocion_, do., etc., etc.
Only one of the families of this suborder survived into the Pampean
stage, where it was represented by a single genus, _†Macrauchenia_. Like
all the other large Pampean mammals of distinctly South American type,
this was a grotesque creature, from the modern point of view. The genus
was first discovered by Darwin, who says of it: “At Port St. Julian, in
some red mud capping the gravel on the 90-foot plain, I found half the
skeleton of the Macrauchenia Patachonica, a remarkable quadruped, full as
large as a camel. It belongs to the same division of the Pachydermata
with the rhinoceros, tapir, and palæotherium; but in the structure of
the bones of its long neck it shows a clear relation to the camel, or
rather to the guanaco and llama.”[12] The views upon classification and
relationship here expressed have been superseded, but the passage is an
important one in the history of scientific opinion.
_†Macrauchenia_ (Fig. 120, p. 216), as Darwin says, was as large as
a camel; it had an unreduced dentition of 44 teeth and in each jaw
the teeth were arranged in continuous series and were quite decidedly
hypsodont. Both in the upper and the lower jaws the incisors formed
a nearly straight transverse row and have a “mark,” or enamel pit,
like that seen in the horses; the canines were but little larger than
the incisors and did not form tusks. The premolars were smaller and
simpler than the molars. The upper molars had two concave and crescentic
external cusps, connected by a median ridge, as in several families of
perissodactyls; two transverse crests and several accessory spurs and
enamel-pockets gave to the grinding surface, when somewhat worn, the
appearance of considerable complexity. The lower molars had the two
crescents, one behind the other, which recurred in almost all the South
American types of ungulates; the vertical pillar which so generally in
these types arose in the inner concavity of the posterior crescent was
wanting in the permanent teeth of _†Macrauchenia_, but present in the
milk-premolars.
No part of this remarkable animal was more curious than the skull, which
was quite small in proportion to the rest of the skeleton. It was long,
narrow and low, sloping and tapering forward to a blunt point at the end
of the muzzle, though there was a slight broadening here to accommodate
the transverse row of incisors. The sagittal crest was replaced by a
short, narrow and flat area; the cranium was shortened and the face
elongated, the orbits, which were completely encircled in bone, having
been shifted behind the line of the teeth, as in the modern horses.
The nasal bones were reduced to a minimum, a mere vestige of their
original length, the anterior nasal opening being directly over the
posterior, making the nasal passage vertical. Such an arrangement is an
almost positive proof that in life the animal had a flexible proboscis,
a conclusion which is confirmed by the presence, on the top of the
head and behind the nasal opening, of deep pits for the attachment of
the proboscis-muscles. A very curious feature of this skull was that
the bones of the upper jaw, the maxillaries and premaxillaries of the
opposite sides, united in the median line, making a long, solid, bony
rostrum in front of the nasal opening, a character not found in other
land mammals.
The neck was almost as long as in a camel and its vertebræ agreed with
those of the latter in the very exceptional character of having the canal
for the vertebral artery passing longitudinally through the neural arch,
instead of perforating the transverse process. As Darwin says in the
passage quoted above, “it shows a clear relation to the ... guanaco and
llama,” but this is founded on the postulate that such a likeness must,
of necessity, imply relationship. As was shown in the chapters on the
Artiodactyla and Perissodactyla, it is the general rule among long-necked
ungulates that the odontoid process of the axis assumes a spout-like
shape, but _†Macrauchenia_ was an exception and had an odontoid which
retained its primitive and peg-like shape; it was, however, relatively
very short and in cross-section was no longer circular, but oval. This
may be regarded as a step toward the assumption of the spout-like form,
but the extinction of the family put an end to further changes in that
direction.
The body was rather short and the limbs very long, giving the animal
a stilted appearance, while the feet were relatively short. The
proportionate lengths of the different limb-segments was unusual; the
upper arm was short, the fore-arm very long, the thigh long and the
lower leg quite short. The humerus was very heavy; the ulna and radius,
which were firmly coössified, formed a very long compound bone, which
was broad transversely and thin antero-posteriorly. The long femur had
only a small and inconspicuous third trochanter and the shaft was broad
and thin, being flattened, or “compressed” antero-posteriorly. The
tibia and fibula were united at both ends; the former was very heavy at
the upper end, but diminished downward in width and thickness, and the
fibula articulated with the calcaneum, as in the artiodactyls. The feet
were tridactyl and had mesaxonic symmetry; that is to say, the median
digit, or third of the original five, was symmetrical in itself and was
bisected by the middle line of the foot, while the lateral toes (second
and fourth), each of which was asymmetrical, formed a symmetrical pair.
It is this perissodactyl character of the foot to which Darwin refers
when he says that _†Macrauchenia_ “belongs to the same division of the
Pachydermata with the rhinoceros, tapir and palæotherium.” On the other
hand, the very significant structure of the ankle-joint was radically
different from that of the Perissodactyla; not only did the calcaneum
have a special facet for articulation with the fibula, but the lower end
of the astragalus was a convex “head,” resting only on the navicular, as
in the †Toxodontia, †Condylarthra, Hyracoidea and other very primitive
groups of hoofed animals and in clawed mammals generally. Such a
combination of characters is not known in any of the perissodactyls and
precludes the reference of the †Litopterna to that order, though such
a reference is strongly maintained by several authorities. The ungual
phalanges were small and appear to suggest the presence of pads on the
feet.
The appearance of _†Macrauchenia_ in life must have been sufficiently
strange. The small head with its proboscis and the long neck and legs
should probably be regarded as indicative of browsing habits, though
the hypsodont teeth show that grazing was at least an occasional mode
of feeding. The long limbs and short feet gave to the extremities an
appearance unlike that of any existing hoofed animal. The form and
size of the ears and the character of the hairy coat are, of course,
conjectural.
In the later Pliocene the family was represented by forms which differed
so little from the Pampean _†Macrauchenia_ as to call for no particular
notice, but in the presumably lower Pliocene of the Paraná stage,
occurred several genera, all unfortunately but imperfectly known, which
are of interest as being less specialized than _†Macrauchenia_ and as
showing the way in which some of the peculiarities of the latter were
acquired. In _†Scalibrinitherium_, which may be taken as an example of
these genera, the teeth were brachyodont; the upper molars were rather
less complex than those of _†Macrauchenia_, while the lower molars
had the pillar in the concavity of the posterior crescent, which the
Pampean genus retained only in the milk-teeth. As we have repeatedly
found, the milk-dentition is often conservative and retains primitive
or archaic features which have been lost in the permanent teeth, and
_†Macrauchenia_ is another illustration of the same principle. In the
skull of _†Scalibrinitherium_ the nasal bones, though very short, had not
suffered such extreme abbreviation as in the succeeding genus, the nasal
opening was farther forward and the maxillaries united in the superior
median line for only a short distance, while the premaxillaries were
fused together for their whole length. The orbit had not been shifted
entirely behind the teeth, but was above the third upper molar.
[Illustration: FIG. 244.—Santa Cruz †macrauchenid (_†Theosodon
garrettorum_) and predaceous marsupial (_†Borhyæna tuberata_). Restored
by C. Knight from skeletons in the museum of Princeton University.]
Next in the ascending series, to use the genealogist’s term, came
the genus _†Theosodon_ of the Santa Cruz, of which almost all the
skeletal parts are known and thus make possible a full comparison with
_†Macrauchenia_, which assuredly was its direct descendant. In view
of the great lapse of time involved, the differences between the two
genera were less than might have been expected, though the more ancient
animal was in all respects the more primitive. _†Theosodon_ was, in the
first place, considerably smaller, not much exceeding a llama in size;
the teeth had lower crowns than even those of _†Scalibrinitherium_ and
the incisors were arranged in line with the grinding teeth, not in
a transverse row, but curving inward slightly, so that those of the
opposite sides nearly met in front. The incisors, canine and first
premolar were simple, sharply pointed, conical teeth, which gave an
almost reptilian expression to the anterior part of the skull. The upper
molars were on the same fundamental plan as those of _†Macrauchenia_,
but in a less advanced stage of development, the transverse crests being
incomplete and the internal cusps had a certain degree of separateness
from the crests and from each other. It is evident that the upper
molars were derived from the quadritubercular type. The lower molars
had the vertical pillar in the concavity of the posterior crescent very
prominently developed.
The resemblance of the skull to that of _†Macrauchenia_ is obvious at
the first glance, but it was less specialized and departed less from the
ordinary ungulate type. The cranium was longer and the face shorter, the
orbit, which was incompletely closed behind, extending over the second
molar. There was a sagittal crest, the length of which differed much in
the various species; the nasal bones were already very short, though
decidedly longer than in the subsequent genus _†Scalibrinitherium_, and
the anterior nasal opening was extended forward as a long, narrow slit,
because the maxillaries did not come into contact with each other in the
superior median line, and the premaxillaries touched each other, but were
not coössified. The nasal canal, though very short, was horizontal, not
vertical. The skulls of the three genera thus displayed three successive
stages in the backward shifting of the orbit and of the anterior nasal
opening, in the shortening of the nasal bones and in the formation of a
solid rostrum by the fusion of the upper jaw-bones. No doubt also the
living animals exhibited a corresponding gradation in the development of
the proboscis.
[Illustration: FIG. 245.—Development of the skull in the †Macrauchenidæ,
side views. _A_, _†Theosodon_, Santa Cruz. _B_, _†Scalibrinitherium_,
Paraná. (After Ameghino.) _C_, _†Macrauchenia_, Pampean. (After
Burmeister.) _n._, nasal bones.]
[Illustration: FIG. 246.—Development of the skull in the †Macrauchenidæ.
_A_, _†Theosodon_. _B_, _†Scalibrinitherium_. (After Ameghino.) _C_,
_†Macrauchenia_. (After Burmeister.)]
The neck of _†Theosodon_ was even longer proportionately than in
_†Macrauchenia_ and the transference of the canal for the vertebral
artery from the transverse processes to the neural arch had already taken
place, except in the first, sixth and seventh vertebræ, and was thus
less complete than in the Pampean genus, in which all the vertebræ of
the neck, save the seventh, had the canal in its exceptional position.
The odontoid process of the axis was less modified than in the latter,
being relatively longer and more conical. The body was rather short, and
the spines of the trunk-vertebræ were proportionally higher and more
prominent. No caudal vertebræ have been found, but, from the shape of
the sacrum, it is evident that the tail was short.
The limbs were long, but more slender and less elongate than in
_†Macrauchenia_, in which the growth of the neck did not keep pace with
that of the limbs, the lengthening of the proboscis probably compensating
for this. The shoulder-blade had two conspicuous metacromia, very much as
in the contemporary †toxodont, _†Nesodon_, but shorter and more widely
separated. The humerus was short and quite slender and the fore-arm
bones, which were much longer, did not coössify. The femur had a more
slender and rounded shaft than in _†Macrauchenia_ and a much larger
third trochanter; the leg-bones were also separate from each other.
The tridactyl feet were so like those of the Pampean genus, that no
particular account of them is necessary, and the proportions of the limb
segments were similar in both genera, short upper arm and lower leg, very
long fore-arm and thigh, and short feet.
[Illustration: FIG. 247. Left manus of _†Theosodon_. _S._, scaphoid.
_L._, lunar. _Py._, pyramidal. _Tm._, trapezium. _Td._, trapezoid. _M._,
magnum. _U._, unciform. _V._, rudimentary fifth metacarpal.]
The appearance of the living animal, as shown in the restoration, was
no doubt somewhat like that of _†Macrauchenia_, but less bizarre. That
there must have been some sort of a proboscis or prehensile upper lip,
is indicated by the greatly shortened nasal bones, but this may not
have been longer than in the existing Moose or Saiga Antelope. The long
neck, short body and tail and long limbs suggest an animal not unlike a
Guanaco, but larger and heavier. The hair may or may not have had the
woolly character given to it in the drawing; upon such a point there can
be no certainty.
In the older formations preceding the Santa Cruz, the †macrauchenids are
known only from fragmentary material, though something of their history
may be made out even from these fragments. _†Protheosodon_, of the
Deseado stage, was considerably smaller than the Santa Cruz genus and had
more primitive upper molars, in that the internal cusps and intermediate
cuspules were isolated and conical, not forming transverse crests. Still
smaller were the several genera (_†Lambdaconus_, etc.) related to the
†macrauchenids found in the Casa Mayor Eocene, which have been referred,
perhaps correctly, to the †Condylarthra. In these the formation of the
external wall of the almost bunodont upper molars was in progress, by the
fore-and-aft extension and transverse thinning of the external cusps;
the internal pair of cusps and the cuspules were separate and conical.
With much confidence, it may be inferred that in these little animals
the skull was normal, the nasal bones were long and that the feet were
five-toed, but demonstration is lacking.
* * * * *
The second family of the †Litopterna, the †Proterotheriidæ, were
remarkable for their many deceptive resemblances to the horses. Even
though those who contend that the †Litopterna should be included in the
Perissodactyla should prove to be in the right, there can be no doubt
that the †proterotheres were not closely related to the horses, but
formed a most striking illustration of the independent acquisition of
similar characters through parallel or convergent development. The family
was not represented in the Pleistocene, having died out before that
epoch, and the latest known members of it lived in the upper Pliocene
of Monte Hermoso. In the still older Paraná formation more numerous
and varied forms occurred, but only from the Santa Cruz have materials
been obtained of sufficient completeness to furnish a full account of
the structure of these extraordinary animals. Not that this remarkable
character was due to grotesque proportions; on the contrary, they looked
far more like the ordinary ungulates of the northern hemisphere than
did any of their South American contemporaries; it is precisely this
resemblance that is so notable.
In Santa Cruz times the family was represented by a large number of
species, which have been grouped in four or five genera, which differed
sufficiently to require generic separation, yet were closely similar.
In all of them the dental formula was: _i_ 1/2, _c_ 0/1, _p_ 4/4, _m_
3/3, × 2 = 36. Except in one genus (_†Thoatherium_) a pair of small
tusks was formed by the enlargement of the second upper and third lower
incisors, as in the †toxodonts, but the first upper and lower and the
third upper incisors, which were retained in the †toxodonts, were lost
in this family, as was also the upper canine, and the lower canine was
very small, of no functional use. The teeth were brachyodont and, except
the small tusks, displayed no tendency at any time toward the acquisition
of high crowns. The premolars were less complex than the molars, though
the last one approximated the molar-pattern. The upper molars had two
crescentic outer cusps, meeting in a vertical ridge and together forming
the outer wall; the transverse crests were imperfect, especially the
hinder one which was often merely the intermediate cuspule, and did not
fuse with the external wall. The lower molars had the two crescents, one
behind the other, which recur in the †macrauchenids, all the suborders
of the †Toxodontia, except the †Pyrotheria, and other South American
ungulates, but the pillar in the posterior crescent, which was so
characteristic of the groups named, was reduced to very small proportions
and sometimes suppressed altogether. It should be noted, however, that
this was the loss of an element which was formerly present.
The skull had a long cranium and rather short face, with long, high
sagittal crest. The neck was short, the odontoid process of the axis
peg-shaped, and the canal for the vertebral artery was in its normal
position. The body was rather short, like that of a deer or antelope; the
number of trunk-vertebræ is not definitely known in any of the genera,
but was very probably 19 or 20, and the tail must have been short.
[Illustration: FIG. 248.—Skull of _†Diadiaphorus_, Santa Cruz. American
Museum.]
The limbs were slender and of moderate length; there was no
coössification between the bones of the fore-arm or the lower leg. The
feet were three-toed, except in one genus (_†Thoatherium_) in which they
were single-toed, and nearly or quite the whole weight was carried upon
the median digit, the laterals being mere dew-claws. The shape of the
hoofs and the whole appearance of the foot were surprisingly like those
of the three-toed horses, but there were certain structural differences
of such great importance as, in my judgment, to forbid the reference of
these animals, not merely to the horses, but even to the perissodactyls.
In studying the †Litopterna, one is continually surprised to note the
persistence of archaic and primitive characters in association with a
high degree of specialization.
[Illustration: FIG. 249.—Three-toed †proterothere (_†Diadiaphorus
majusculus_), Santa Cruz. Restored by C. Knight, from skeletons in the
American Museum and Princeton University.]
The largest Santa Cruz representatives of the family were the species
of _†Diadiaphorus_, animals considerably taller than a sheep and of
heavier build. Their appearance was not unlike that of a short-necked,
hornless antelope, but with the feet of the three-toed horses! These feet
were, however, merely superficially like those of the horses, differing
in points of fundamental significance. In the horses, the reduction of
the digits was accompanied by a readjustment of the carpal and tarsal
articulations, so that, in proportion as the median toe was enlarged
and the laterals reduced, the weight was shifted more entirely upon
the former. This is the method of digital reduction which Kowalevsky
called “adaptive” and is exemplified in all existing artiodactyls and
perissodactyls and by none more perfectly than by the monodactyl horses.
In “inadaptive reduction,” the method followed by _†Diadiaphorus_ and
the other genera of this family, there was no readjustment, or a very
imperfect one, of the articulations, the lateral digits, however small
and rudimentary, retaining the connections which they had when they were
of full size and function. This distinction may seem to be unimportant,
but its significance is shown by the fact that not a single ungulate with
inadaptively reduced feet has survived to the present time.
[Illustration: FIG. 250.—Left pes of _†Diadiaphorus_, from specimens in
Princeton University and the American Museum. _Cal._, calcaneum. _As._,
astragalus. _N._, navicular. _Cn. 3_, external cuneiform. _Cb._, cuboid.]
In still another respect the feet of _†Diadiaphorus_ deviated markedly
from those of the horses, viz. in the great proportionate length of
the phalanges, especially of the first one, and the shortness of the
metapodials, the three phalanges of the median digit together exceeding
in length the metacarpal or metatarsal, while in the horses this
proportion is reversed. The skull of this genus was short, deep and with
an anterior taper; it had a long sagittal crest, but a brain-chamber of
good capacity, considering its geological date. The nasals were quite
short, though the degree of shortening was not such as to suggest the
existence of a proboscis. In general appearance the skull recalls that of
one of the larger †oreodonts (p. 372) of the North American Oligocene.
To the genus _†Proterotherium_, the type of the family, belonged a
great number of Santa Cruz species, for at that time the genus was in
a state of most vigorous development and the species were so variable
that satisfactory discrimination of them is exceedingly difficult.
They were all much smaller and slighter animals than the species of
_†Diadiaphorus_, but did not differ from them in any important structural
character. The skull in this genus closely resembled that of the one
last named, save for its smaller size and lighter and more slender
proportions; the nasal bones were considerably longer and the occiput was
somewhat wider.
[Illustration: FIG. 251.—Skull of _†Thoatherium_, Santa Cruz. Princeton
University Museum.]
A more isolated position was held by the genus _†Thoatherium_, which
was very clearly demarcated from all of the other genera of the family.
Its species were the smallest of the commoner Santa Cruz members of the
order and were of very light and graceful form. The dental formula was
the same as in the other genera, but there were no tusks; the single
upper and two lower incisors were of nearly the same size and simple,
chisel-like form. The upper molars had the same elements as in the
preceding genera, but somewhat differently connected, the two internal
cusps and the anterior intermediate cuspule being united into a nearly
longitudinal ridge. The skull was light, slender and pointed; the nasals
were shortened, though less than in _†Diadiaphorus_; the sagittal crest
was shorter than in the latter and the occiput was far narrower. The neck
was short, the body of moderate length and the tail short. The limbs
and especially the feet were proportionately more elongate and slender
than in any other known genus of the family, giving quite a stilted
appearance to the skeleton. The fore-arm bones were not coössified, but
the ulna was much more reduced than in any of the other genera of the
family, and the same is true of the fibula, which, though very slender,
showed no tendency to unite with the tibia. The limb-bones, especially
the femur, had a decided resemblance to those of _†Mesohippus_, the lower
Oligocene tridactyl horse of North America, with the smaller species of
which, _†M. bairdi_, _†Thoatherium_ agreed well in size. Most remarkable
of all were the feet, _which were more strictly monodactyl than those
of any other known mammal_. The single functional digit, the third, had
on each side of its upper end a very small, scale-like nodule of bone,
the last vestiges of the lateral digits, corresponding to the immensely
larger splints of the horse. Despite the unrivalled completeness of
digital reduction which _†Thoatherium_ displayed, the mode of reduction
was inadaptive and the rudimentary metapodials retained the same carpal
and tarsal connections that they originally had in the pentadactyl manus,
a very great difference from the horses. The ankle-joint also was of
the same primitive character as in the other †Litopterna. The feet were
relatively longer and more slender than in the other †proterotheres and
the metapodial of the single functional digit longer in proportion to the
phalanges.
[Illustration: FIG. 252.—Single-toed †proterothere (_†Thoatherium
minusculum_), Santa Cruz. Restored by C. Knight from a skeleton in the
museum of Princeton University.]
The appearance of the living animal, aside from the character of the
hair, colour-pattern, etc., may be closely inferred from the skeleton.
It was a much smaller and more graceful animal than its contemporary and
relative _†Diadiaphorus_, as light and agile as a gazelle. The head had
some resemblance to that of a small horse, but the neck was much shorter
than in the horses; the body also was shorter than in the latter, and
the proportions of the trunk and limbs were quite as in the smaller
antelopes. But these likenesses to horses and antelopes were, it must
again be emphasized, superficial; the fundamental characteristics of
structure were more primitive than in the most ancient known artiodactyls
and perissodactyls.
[Illustration: FIG. 253.—Left pes of _†Thoatherium_. Princeton University
Museum. Letters as in Fig. 250.]
With the aid of the fragmentary material which alone represents the
†proterotheres in the formations preceding and following the Santa
Cruz in time, it is not practicable to trace the development of the
various phyla in a satisfactory manner. Two of the Santa Cruz genera,
_†Diadiaphorus_ and _†Proterotherium_, continued into the lower Pliocene
(Paraná), and two additional ones have been named, but little is known
about them. The latest known member of the family so far discovered is
a genus (_†Epitherium_) from the upper Pliocene of Monte Hermoso, a
tridactyl form like _†Diadiaphorus_. It is a noteworthy fact that the
most advanced and specialized genus of the entire family ended with
the Santa Cruz, while the less differentiated types survived till a
considerably later period. Possibly, it was the incoming of the highly
efficient Carnivora from North America that led to the extermination of
the last †proterotheres.
Turning backward from the Santa Cruz, the family may be traced without
any question to the Deseado stage of the Oligocene, though nothing but
teeth has yet been obtained, while in the Eocene it would appear to have
become merged in the same group of small, †Condylarthra-like animals with
quadritubercular molars, as those which are regarded as the probable
ancestors of the †macrauchenids. However likely this conclusion may seem
to be, its confirmation must await the discovery of much more complete
specimens than are now available.
ORDER †ASTRAPOTHERIA. †ASTRAPOTHERES
In the Santa Cruz another group of peculiar South American ungulates,
the †Astrapotheria, made its last recorded appearance. Though not at all
uncommon in that formation, no complete or even partial skeleton has
yet been found, but merely the skull and a few bones of the limbs and
feet. For this reason there is much doubt as to the systematic position
and relationships of these animals, which were among the most curious
of the many strange mammals which made up the Santa Cruz fauna. They
were mentioned in connection with the †Amblypoda (p. 456) as possible
representatives of that order in South America, but, as will be seen
later, this is an improbable conclusion, and the group appears to have
been indigenous in the southern continent, in which, at all events, it
had a very long history. It has not been found in any formation later
than the Santa Cruz, unless the Friasian fauna, which contains it, should
be removed from that stage, of which it apparently forms the latest
division.
I. †ASTRAPOTHERIIDÆ.
_†Astrapotherium_, Santa Cruz and Patagonian.
_†Astrapothericulus_, Patagonian.
_†Parastrapotherium_, Deseado. _†Astraponotus_,
Astraponotus Beds. _†Albertogaudrya_, Casa Mayor.
II. †TRIGONOSTYLOPIDÆ.
_†Trigonostylops_, Casa Mayor. _†Edvardocopeia_,
Astraponotus Beds.
The genus _†Astrapotherium_, which was the only well-defined
representative of its family and order in the Santa Cruz stage, contained
several species, some of them the largest animals of their time, as
well as the most grotesque in appearance. The dentition differed in
some important respects from that of all the other South American
ungulates, the formula being: _i_ 0/3, _c_ 1/1, _p_ 2/1, _m_ 3/3, × 2 =
28. The upper incisors had completely disappeared, but the lower ones
were large and, what was an exceptional character, they were partially
divided into two lobes, somewhat as in the Eocene †uintatheres of North
America (p. 446). The canines were very large and formidable tusks, which
grew throughout life and apparently formed no root; the upper tusk was
nearly straight and was obliquely truncated by the strongly curved and
sharp-pointed lower tusk. This arrangement was very unusual among South
American hoofed mammals, many of which had no tusks at all; and in those
which possessed them, such as the †toxodonts (p. 468), they were mostly
incisors. Only in the †astrapotheres and †homalodotheres were there
canine tusks, and in the latter group they were small and of limited
growth. All the teeth, except the canines, were brachyodont and, though
rather high-crowned, formed roots before coming into use. The premolars
were small and greatly reduced in number (2/1), and in pattern were
simpler than the molars. The upper molars were constructed on essentially
the same plan as in the †Toxodonta; indeed, the first specimen of this
genus collected was referred to a large species of _†Nesodon_ by Owen.
On the other hand, the resemblance to the rhinoceros teeth is very
decided, and has led several writers to postulate a relationship between
the †astrapotheres and the rhinoceroses. The lower molars were of the
bicrescentic pattern so frequently met with already; these teeth were
very narrow in proportion to their length and strongly suggest those of
_†Metamynodon_, the supposedly aquatic rhinoceros of the North American
Oligocene (p. 346). It may be confidently inferred that so small a
number of premolars was due to reduction from a full series, and this is
confirmed by the milk-dentition, in which the premolars were 4/3.
[Illustration: FIG. 254.—Head of _†Astrapotherium magnum_. Santa Cruz.
Restored from a skull in the museum of Princeton University.]
The skull was extremely peculiar, more so than in any other of the
contemporary genera of hoofed animals. The toothless premaxillaries were
quite small, but thick, and must have supported an elastic pad, against
which the lower incisors could effectively bite in cropping herbage. The
nasal bones were extremely short and there must have been a proboscis or
greatly inflated snout, probably the former; the immense development of
sinuses in the frontal bones elevated the whole forehead into a great,
dome-like convexity, a feature which is not equalled in any other known
mammal. The orbits were open behind and the brain chamber was small,
so that the sagittal and occipital crests were very high and strong,
to afford sufficient surface for the attachment of the great temporal
muscles. The horizontal portion of the lower jaw was shallow vertically,
but very thick and massive, and the symphyseal region was broad and
depressed.
Unfortunately, the skeleton is still very incompletely known. Of the
vertebræ, only the atlas and axis have been recovered, and these
resembled those of the Santa Cruz †toxodont _†Nesodon_, on a larger
scale. The scapula had a very thick spine, without the projections which
were found in most of the Santa Cruz ungulates. The limb-bones were long
and comparatively slender, and the processes for muscular attachment
were singularly small and weak; the bones of the fore-arm and lower leg
did not coössify and were proportionately elongate, the tibia being but
little shorter than the femur. The latter had the flattened shaft which
recurs in nearly all of the very heavy ungulates, but retained a remnant
of the third trochanter. If the feet found isolated in the Santa Cruz and
Deseado stages have been correctly referred to this order, then the genus
was five-toed and the feet were broad, short and heavy, quite elephantine
in appearance, especially the fore foot. The ankle-joint was very
peculiar and the calcaneum had no articulation with the fibula, which it
had in all the other indigenous South American ungulates.
Incomplete as the material is, it is yet possible to form some general
conception of this extraordinary animal when in life. The head was short,
broad and deep, rounded and very probably furnished with a proboscis; the
neck was of moderate length, so that the mouth could not reach the ground
without a straddling of the fore legs. The body was no doubt long, the
limbs long and rather slender, giving the animal a stilted appearance,
the feet very short, broad and columnar. Several species of the genus
are known, which differed much in size, the largest (_†A. giganteum_)
probably exceeding any modern rhinoceros in height and length, and the
smallest (_†A. nanum_) not much larger than a Wild Boar.
_†Astrapothericulus_, of the Patagonian stage, was smaller than the
average species of the Santa Cruz genus, and had teeth of the same
number, but the canines were not capable of indefinite growth, and the
lower molars had the pillar in the posterior crescent so characteristic
of the South American hoofed animals. In the Deseado stage, on the
contrary, the †astrapotheres were of larger size, and in the commonest
genus, _†Parastrapotherium_, the grinding teeth had lower crowns and the
premolars were more numerous, at least 3/2. In the still more ancient
_†Astraponotus_, which gives its name to the upper Eocene (or lower
Oligocene) of Patagonia, the premolars were present in full series.
In the Casa Mayor the order was abundantly represented by still more
primitive genera, which assuredly had an undiminished number of teeth,
though this has not been proved. One of these genera, _†Albertogaudrya_,
was the largest animal of its time and the highly probable ancestor of
the series leading to the Santa Cruz _†Astrapotherium_.
The second family of the order, the †Trigonostylopidæ, did not survive
beyond the Eocene and is so imperfectly known that any account of it
would be to small profit.
As stated above, the †Astrapotheria were an isolated group and their
relationships are problematical and are likely to remain so pending the
discovery of much more complete specimens of the various genera which
made up the series. I am inclined to the opinion, however, that all of
the indigenous groups of South American ungulates, which inhabited that
continent before the great immigration from the north, were derivatives
of the same stock and more nearly related to one another than to any of
the orders which lived in other regions.
* * * * *
In looking over the labyrinth of ungulate history, as recorded by the
fossils, certain facts stand out clearly, while others are still very
obscure. It is like trying to trace the plan of vast and complicated
ruins, which here are deeply buried in their own débris, there are fully
exposed and in another place are swept away so completely that hardly a
trace remains. But the problem is far more complex than any which can
be presented by buildings, for the factor of repeated migrations from
continent to continent comes in to obscure the evidence. Had each of the
great land areas received its original stock of early mammals and then
been shut off from communication with any other, many of the difficulties
would be removed, but the story would lose half its interest.
Within the limits of the family, giving to that group the broad and
elastic definition which has hitherto been employed, we have repeatedly
found it feasible to construct a phylogenetic series which very nearly
represents the steps of structural modification as they occurred in
time. Much less frequently is it possible to trace allied families to
their common starting point, and, so far as the hoofed animals are
concerned, in no case have we yet succeeded in doing this for the
separate orders. The obstacle lies in the fact that the ordinal groups
were already distinct, when they made their first appearance in the
known and accessible records, and the hypothetical ancestors common to
them all, or to any two of them, are to be sought in regions of which we
know little or nothing. Nevertheless, certain legitimate inferences may
be drawn from the available evidence. It remains to be proved whether
the assemblage of hoofed mammals, as a whole, was of single or multiple
origin. Have all ungulates been derived from a common stock, or did they
arise independently from several groups of clawed mammals? While the
records cannot be followed back to the point, or points, of origin of
the various orders, yet it is a noteworthy fact that, between several of
them, the differences grow less marked as the more ancient members are
reached, as though they were converging to a common term; others again
show little such approximation, and the most probable conclusion from
the evidence now at hand is that the ungulate assemblage is composed of
several independent series.
One such series is that of the Hyracoidea and Proboscidea, to which
Dr. Schlosser has given the name “Subungulata,” and has pointed out
its relationship to the †Condylarthra, which, however, is not a close
one and may be illusory. Another apparently natural group is that of
the peculiarly South American forms, the †Toxodontia, with its four
suborders, the †Litopterna and the †Astrapotheria, which all appear
to be traceable to closely allied families in the Eocene, whose teeth
strongly suggest derivation from the †Condylarthra; but the material does
not permit any positive statements. The Artiodactyla and Perissodactyla
have so many similarities that they have always been regarded as closely
related groups, but the distinction between them was almost as sharply
drawn in their most ancient known members as it is to-day, and there was
no distinct tendency to converge back into a common stem. Their mutual
relationships are thus obscure, but the Perissodactyla, at least, seem to
be derivable from a †condylarthrous ancestry.
The †Condylarthra, as a whole, were by far the most primitive of the
ungulates, which they connected with the clawed mammals. None of the
genera yet discovered can be regarded as ancestral to any of the higher
orders, but it is entirely possible that in the upper Cretaceous period
the †Condylarthra were spread over all the continents, except Australia,
and that from them the other ungulate orders arose in different regions.
At all events, the †Condylarthra show how the transition from clawed to
hoofed types may have occurred and perhaps actually did so, but it would
be premature to affirm this.