One of the genera (_†Protomeryx_), which did not much exceed a sheep
in size, probably represented the main stock, which led to the camels
and llamas of to-day. A second (_†Stenomylus_) was a still smaller
animal, with remarkably long and slender legs, and might be called a
“gazelle-camel,” while a third (_†Oxydactylus_, _see_ Fig. 209, p. 392),
which was larger and apparently the beginning of the †giraffe-camels,
was noteworthy for its long neck. All of these lower Miocene camels
had deer-like hoofs, the characteristic pad or cushion which gives
such an exceptional appearance to the feet of modern llamas and camels
not being fully developed till a later period. A very important new
element in the North American fauna was the appearance of the first deer
(_†Blastomeryx_), which came in the latter part of the Arikaree stage
and were the forerunners of a renewed immigration from the Old World,
which had been broken off during the upper Oligocene. This, however, is
a disputed point; Professor Osborn and Dr. Matthew believe that these
animals were truly indigenous and derived from a long line of American
ancestry. The same genus continued through the middle Miocene, as we have
already seen, and therefore no further description of it is called for.
The limits of the _South American_ Miocene are very doubtful. The Paraná
formation, here regarded as lower Pliocene, may prove to be more properly
referable to the upper Miocene. No other upper Miocene is known.
[Illustration: FIG. 131.—The †gazelle-camel (_†Stenomylus hitchcocki_)
of the lower Miocene. Restored from skeletons in the Carnegie Museum,
Pittsburgh.]
To the earlier, probably middle, Miocene may be referred the wonderful
Santa Cruz fauna of Patagonia. It is extremely difficult to convey to
the reader any adequate conception of this great assemblage of mammals,
because most of them belonged to orders which have altogether vanished
from the earth and are only remotely like the forms with which we are
familiar in the northern hemisphere. To one who knows only these northern
animals, it seems like entering another world when he begins the study of
the Santa Cruz fossils. If any North American mammals had then entered
South America, which is not probable, they had not extended their range
as far as Patagonia. Marvellously rich and varied as the Santa Cruz fauna
was, it did not contain everything that we should expect to find in it;
several recent families of undoubtedly indigenous South American origin
have left no ancestors in the early Miocene formations. For this, there
are several obvious reasons. In part, these gaps in the history are
merely due to the accidents of collecting and some of them will almost
certainly be filled by future exploration. Other absentees will probably
never be found, because the Santa Cruz beds are known only in the very
far south, and the Miocene climate of the region, though much milder and
more genial than the present one, must have been unsuitable for many
tropical animals. Again, the Patagonia of that time appears to have been
a country of open plains, with few trees, and hence arboreal forms were
rare.
[Illustration: FIG. 132.—Diagram to illustrate the comparative sizes of
the Santa Cruz mammals, a modern pointer dog, within the rectangle, to
give the scale. 1. _†Cladosictis lustratus_, predaceous marsupial. 2.
_†Protypotherium australe_, †typothere. 3. _†Eocardia excavata_, rodent.
4. _†Stegotherium tesselatum_, armadillo. 5. _†Propalæohoplophorus
australis_, †glyptodont. 6. _†Hapalops longiceps_, †ground-sloth. 7.
_†Thoatherium minusculum_, †litoptern. 8. _†Astrapotherium magnum_,
†astrapothere. 9. _†Prothylacynus patagonicus_, predaceous marsupial.
10. _†Theosodon garrettorum_, †litoptern. 11. _†Nesodon imbricatus_,
†toxodont.]
While great numbers of large, flightless birds, some of them of enormous
size, were entombed in the volcanic ash and dust which were spread
over such wide areas and to such great depths, the extreme scarcity of
reptiles is surprising; a few remains of lizards have been found, but no
snakes, crocodiles, or tortoises, and we have no information as to the
plant-life of the region at that time. The mammals were almost all of
small or moderate size; only one or two species were really large.
One very striking and characteristic feature of the Santa Cruz fauna is
the great abundance of marsupials which it contained and which resembled
more or less those of modern Australia. There were no true Carnivora and
their places were taken by a variety of carnivorous marsupials, some
of which (_e.g._ _†Prothylacynus_) were as large as wolves and were
closely similar to the so-called Tasmanian Wolf (_Thylacynus_). Another
genus (_†Borhyæna_) had a short, bullet head, not unlike a small Puma
in appearance and, besides, there were many smaller beasts of prey, in
size like badgers and minks. Opossums were common and there were many
very small herbivorous marsupials, which resembled, though perhaps but
superficially, the Australian phalangers. At the present day South
America contains no Insectivora, but in the Santa Cruz there was one
family (†Necrolestidæ) of this order which bore considerable resemblance
to the “golden moles” of South Africa. An extraordinary variety of
rodents inhabited Patagonia in Santa Cruz times, all of them belonging
to the Hystricomorpha, or porcupine suborder, and all referable to
existing South American families. There were none of the northern forms
of rodents, neither rats, mice, squirrels, marmots, hares, nor rabbits,
but a very numerous assembly of tree-porcupines, cavies, chinchillas,
coypus and the like. The genera, though closely allied to existing ones,
are all extinct, and the animals were very generally smaller than their
modern descendants. A few small monkeys of unmistakably Neotropical type
have been found, but like other arboreal and forest-living animals, they
are very rare among the fossils.
The Edentata were more abundant and diversified than at any other time
in South American history of which the record is preserved. Two of the
modern subdivisions of this order have not been certainly identified
in the Santa Cruz collections, the arboreal sloths and the anteaters,
and though they may be found there at any time, it will only be as
stragglers from the warmer forested regions to the north, where these
forms had doubtless long been present. Unfortunately, however, nothing
is directly known concerning the life of those regions in Miocene
times. On the other hand, three groups of edentates, two of them now
extinct, were very copiously represented in the Santa Cruz formation,
the armadillos, †glyptodonts and †ground-sloths. Of the many armadillos,
some quite large, others very small, only a few can be regarded as
directly ancestral to those now in existence; the truly ancestral
forms were probably then living in the forests of Brazil and northern
Argentina, in the same areas as the ancestral tree-sloths and anteaters.
In comparison with the giants of the Pliocene and Pleistocene, the Santa
Cruz †glyptodonts were all small, the carapace rarely exceeding two
feet in length, and, what it is particularly interesting to note, they
departed much less widely from the armadillo type than did their gigantic
successors. The †ground-sloths were present in actually bewildering
variety and they also were very small as compared with the huge animals
of the Pleistocene, none of them exceeding the Black Bear in height or
length, though proportionally much more massive, and many were no bigger
than foxes. They had small heads, long bodies, heavy tails and short,
thick legs; their teeth show that they were plant-feeders, but their feet
were armed with long, sharp and formidable claws. Among this great host
of Santa Cruz †ground-sloths may readily be noted the probable ancestors
of the gigantic creatures which were such characteristic elements of the
Pliocene and Pleistocene faunas.
There was an extraordinarily rich and varied assemblage of hoofed
animals, all utterly different from those of the northern hemisphere and
belonging to groups which have never been found outside of South and
Central America. Of these groups there were five, which by different
writers are variously regarded as orders or suborders, a matter of very
secondary importance. Individually, the commonest of the hoofed mammals
were the †Toxodonta, which ranged in size from a sheep to a tapir,
heavily built and clumsy creatures, with absurdly small, three-toed feet;
in some of the species there was a small median horn on the forehead. As
with the †glyptodonts and †ground-sloths, the contrast in size between
the Santa Cruz ancestors and the Pleistocene descendants was very
striking. A very numerous and varied group was that of the †Typotheria,
all small animals, some no larger than rabbits, others the size of
small foxes. It requires a decided effort to think of these †typotheres
as being really hoofed animals at all, as their whole appearance must
have been much more like that of rodents, yet their structure clearly
demonstrates their near relationship to the †toxodonts. Still a third
group of the same series, the †Entelonychia, is of great interest, for,
as in the †chalicotheres of the northern hemisphere, the hoofs had been
transformed into claws and their five-toed feet had a truly grotesque
appearance, not diminished by the long and powerful limbs and relatively
small head.
This is the third example of that paradoxical creature, a “hoofed animal”
with claws instead of hoofs, and in each of the three instances, there
is every reason to believe, the transformation proceeded independently.
Among the perissodactyls the †chalicotheres (p. 238) underwent this
change; in North America the †Agriochœridæ, a family of artiodactyls,
had a very similar history, while in South America the †Entelonychia
arose from the same stock as the †toxodonts, with which they were nearly
allied. They were among the largest animals of Santa Cruz times and
ranged in size from an ox to a rhinoceros.
There was a fourth group, the †Astrapotheria, concerning which our
knowledge is tantalizingly incomplete, some species of which were the
largest of known Santa Cruz mammals, while others were much smaller.
They had short, domed heads, with a considerable proboscis, and were
armed with formidable tusks, which were the enlarged canine teeth, the
only known instance of large canine tusks among the indigenous South
American hoofed animals. The limbs were long and not very massive, the
feet short, five-toed and somewhat elephantine in appearance. These
bizarre animals would seem to have held a rather isolated position among
the South American ungulates, and though they may be traced back to the
most ancient mammal-bearing beds of that continent, their relationships
are still obscure; much more complete material must be obtained before
this problem can be definitely solved. Both the †Astrapotheria and the
†Entelonychia died out shortly after the end of the Santa Cruz.
From many points of view the most interesting members of the Santa
Cruz fauna were the †Litopterna, an order which also went back to the
earliest South American Tertiary. In the Miocene and Pliocene the order
was represented by two very distinct families, the †Macrauchenidæ and
†Proterotheriidæ, which were superficially very unlike. In the Santa Cruz
beds is found a genus (_†Theosodon_) which was apparently the direct
ancestor of the Pampean _†Macrauchenia_. The Miocene genus was a much
smaller animal and had hardly more than an incipient proboscis, but
otherwise was very like its Pampean successor; it was somewhat larger and
heavier than a Llama and probably bore some resemblance to that animal
in appearance. The long, narrow head, with its prehensile upper lip,
must have had an almost reptilian likeness from the numerous uniform and
sharp-pointed teeth with which the front of the jaws was supplied; the
neck was elongate, the body short and rather slender and the legs long,
ending in three nearly equal toes.
The †proterotheres, on the other hand, were almost the only Santa Cruz
ungulates which had nothing _outré_ or grotesque about them to the eye of
one habituated to the faunas of the northern hemisphere. They were small,
graceful animals, very like the Miocene horses of the north in their
proportions, though having much shorter necks and shorter, heavier heads.
In some genera of this family (_e.g._ _†Diadiaphorus_, _†Proterotherium_)
the feet were three-toed and most surprisingly horse-like in shape, but
one genus (_†Thoatherium_) was absolutely single-toed, more completely
monodactyl than any horse. The horse-likenesses ran all through the
skeleton and are so numerous and so striking that several writers have
not hesitated to incorporate the †Litopterna with the Perissodactyla,
but this I believe to be an error. If the †proterotheres were not
perissodactyls, as I am convinced they were not, they afford one of the
most remarkable examples of convergent evolution among mammals yet made
known.
3. _Oligocene_
_North America._—The John Day formation of eastern Oregon represents
the upper Oligocene and has yielded a very extensive series of mammals,
though with some obvious gaps that remain to be filled by future work.
The land-connection with the Old World which had existed in the lower
Oligocene and was restored in the lower, or at latest in the middle,
Miocene, was interrupted in John Day times, and so the mammals assumed a
purely indigenous character.
No opossums or other marsupials have been found, and nothing is known
of the Insectivora. Of the Carnivora, there were but three families,
and one of these, the mustelines, was represented but scantily by a
few small species. Cats of the †sabre-tooth subfamily were common and
one species was quite large, almost equalling the Jaguar in length;
but most of the species were small, much smaller than the Pleistocene
members of the group. True cats are not definitely known to have been
present, but there were two genera (_†Nimravus_ and _†Archælurus_)
which have been called the “false †sabre-tooths,” which may prove to be
referable to that series. The dogs, on the other hand, were remarkably
numerous and diversified, more so than ever before or since; none of
them was very large, the largest but little exceeding the Timber Wolf
in size, and some were extremely small; but the number of distinct
genera and species and the differences among them are quite remarkable.
Both long and short-faced forms and early stages of the “†bear-dogs,”
and “†hyena-dogs,” and ancestral forms of the wolves and dholes may
be distinguished, a truly wonderful assemblage. The rodents also were
numerous and varied, including ancient and extinct genera of the
beavers, squirrels, mice, pocket-gophers and hares and the earliest
distinguishable ancestors of the sewellels (Aplodontiidæ).
The remainder of the known John Day fauna was composed of artiodactyls
and perissodactyls. The latter had suffered serious losses as compared
with the preceding or White River stage. Up to and through White River
times the perissodactyls had held their own in actual diversity, though
the rise of the artiodactyls had put an end to the dominant position
which they had maintained in the Eocene. With the John Day the actual
decline may be said to have begun. The rhinoceroses were represented
chiefly by the †diceratheres, with a transverse pair of horns, some
species of which were much larger than those of the lower Miocene.
Hornless rhinoceroses have not yet been certainly found, though there is
every reason to believe that they then existed, as they unquestionably
did both before and after. Tapirs occurred but rarely and the horses were
individually abundant, though in no great diversity; they were smaller
and lighter than the horses of the lower Miocene. Enough has been found
to demonstrate the presence of the clawed †chalicotheres, but not to show
how they differed from their immediate successors.
In the number of individuals, species, genera and families, the
artiodactyls of the John Day much exceeded the perissodactyls. The
peccaries were numerous, but smaller and more primitive than those of
the succeeding age, as were also the †giant pigs, or †entelodonts,
but the latter were very large. The peculiarly North American family
of the †oreodonts was very numerously represented, and one genus
(_†Promerycochœrus_), comprising animals not unlike the Wild Boar in size
and shape, was the probable beginning of the series of proboscis-bearing
†oreodonts, which led to such grotesque forms in the middle and upper
Miocene. A family closely allied to the †oreodonts, and by many
writers included in the latter, is the very remarkable group of the
†Agriochœridæ, which was distinguished by the long, stout and cat-like
tail and by the possession of claws instead of hoofs. The family is not
known to have existed later than the John Day and no trace of it has been
found in the succeeding formations. The camels seem to be all comprised
in a single genus (_†Protomeryx_) which was the same as that found in the
lower Miocene. A very small and dainty little creature (_†Hypertragulus_)
belonged to another family, the relationships of which are not clear.
To the middle and lower Oligocene is referred the great White River
formation of South Dakota, Nebraska, Wyoming, etc., which is divisible
into three clearly marked substages. The White River contains the
best-known fauna of all of the North American Tertiaries, for collecting
in these beds has been carried on for more than sixty years, and a
greater number of complete and nearly complete skeletons has been secured
than from any of the other formations. It is plainly evident that a
land-connection existed with the Old World, which was interrupted in the
John Day, as is shown by the intermigration of characteristic forms; but
some barrier, presumably climatic, prevented any complete interchange of
mammals, and very many genera and even families remained confined to one
continent or the other.
The aspect of the White River fauna changes in accordance with the
direction from which it is approached. If one comes to the study of it
from the Eocene, it displays a very modern aspect, given by the almost
complete disappearance of the archaic groups of mammals and by the great
multiplication of genera and species belonging to the progressive orders.
These genera, it is true, are all extinct, but many of them stood in an
ancestral relationship to modern forms. On the other hand, if approached
from the Miocene side, the White River mammals seem to be very ancient
and primitive and very different from anything that now lives. We speak
of horses and rhinoceroses, dogs and cats, in this fauna, but those
terms can be employed only in a very wide and elastic sense to designate
animals more or less distantly allied to those of the present day.
[Illustration: FIG. 133.—1. _†Archæotherium._ 2. Ancestral camel
(_†Poëbrotherium_). 3. _†Merycoidodon._ 4. _†Agriochœrus._ 5. Ancestral
horse (_†Mesohippus_). 6. _†Hoplophoneus._ 7. _†Bothriodon._ 8.
_†Hyænodon._ 9. †Cursorial rhinoceros (_†Hyracodon_). 10. _†Protoceras._
11. Hornless rhinoceros (_†Cænopus_).]
Several species of opossums, some of them very small, were the only
marsupials in North America then, as they are now. There was quite a
variety of Insectivora; some were survivals of a family that was abundant
in the Eocene, others, like the hedgehogs, moles and shrews, were
probably immigrants. Here we find the last of a group (order or suborder)
of ancient and primitive flesh-eaters, the †Creodonta, that had played a
great rôle in the Eocene and Paleocene of North America and Europe. In
White River times but a single family (†Hyænodontidæ), with two genera,
remained of the Eocene host. One of these genera (_†Hemipsalodon_), a
very large beast of prey, which was almost identical with the Old World
genus _†Pterodon_, was confined to the lower substage of the White River
beds in the Northwest Territory of Canada; the other, _†Hyænodon_, which
was also an Old World form, was represented abundantly in the United
States by many species. In size, these species ranged from a small fox
to a large wolf, but they all had disproportionately large heads, and
small, weak feet, with blunt claws, so that they must have been very
curious-looking creatures and were probably carrion-feeders rather than
active catchers of prey. The White River members of the family were
migrants from the eastern hemisphere, for, though small and primitive
representatives of it occurred in the North American Eocene, as well as
in the corresponding formations of Europe, the family appears to have
died out in America and to have been renewed by the Oligocene migration.
[Illustration: FIG. 134.—White River †titanothere (_†Titanotherium
robustum_) reduced to the same scale as Fig. 133.]
Coincident with this decline of the †creodonts and, no doubt, causally
connected with it, was the rise of the true Carnivora, which for the
first time were numerous and were divisible into three distinct families.
Small and primitive representatives of the wolves (_†Daphœnus_) and
possibly also of the foxes (_†Cynodictis_) were quite common, and there
were a few species of the musteline family, evidently immigrants and the
most ancient yet found in America. There were several species of the
†sabre-tooth cats (_†Dinictis_ and _†Hoplophoneus_) all of which, except
in the uppermost substage, were quite small, few of them exceeding the
Canada Lynx in size. A much larger animal (_†Eusmilus_, also European)
appeared in the latter part of the stage. None of the true cats, or
feline subfamily, has been obtained. Nothing is yet known of the time
and place of origin of the †sabre-tooth series, for they appeared at
approximately the same date in Europe and America, and in neither
continent have any possible ancestors been found in preceding formations.
The problem is like that of the Proboscidea (_see_ p. 234), but Egypt
has given no help in the case of the †sabre-tooths, and, by a process
of elimination, we reach the conclusion that these strange creatures
probably arose somewhere in Asia and sent out migrants eastward and
westward.
The Rodentia were fairly abundant and present a strange mixture
of ancient and comparatively modern types. One very common genus
(_†Ischyromys_), which was the last remnant of a family almost limited
to the North American Eocene, was associated with the earliest American
mice, arboreal and ground squirrels, beavers and rabbits; some, if not
all, of these were immigrants.
The hoofed mammals were present in fairly bewildering variety, but were
restricted to the two orders of the Perissodactyla and Artiodactyla. The
Perissodactyla, while they no longer had the relatively dominant position
which they held in the middle Eocene (_see_ p. 270), had suffered no
actual loss; and no less than seven families of them, or six by another
scheme of classification, had members in the North America of White
River times, a very notable difference from the present order of things,
when there are but three families in the entire world, none of which
enters North America. The Eocene family of the †titanotheres became
extinct at the end of the lower substage of the White River, but in that
substage there was a marvellous abundance of these huge beasts, some of
which were of almost elephantine stature and bulk. The pair of great
bony, horn-like protuberances on the nose varied much in size and form
in the different species, short to very long, triangular, cylindrical,
flattened and shovel-shaped, and gave these ungainly creatures somewhat
the appearance of strange and very large rhinoceroses. The †titanotheres
were a typically North American family, but sent migrants to the Old
World, at least two species reaching southeastern Europe. Rhinoceroses
too were extremely numerous and diversified throughout the stage and are
very plainly divisible into three strongly contrasted series, which are
sometimes regarded as three subdivisions of the same family and sometimes
put into two separate families. One of these series, the †hyracodonts
(_†Hyracodon_), was composed of small, long-necked and long-legged,
slender and lightly built, cursorial animals, but with short, heavy
heads, which gave them a somewhat clumsy look; having neither horns nor
tusks, they were entirely defenceless and depended for their safety
upon speed alone. The second series, or †amynodonts (_†Metamynodon_),
formed the very antithesis of the first,—large, heavy, short-necked, and
short-legged and probably amphibious in manner of life, they were armed
with formidable tusks; and their skulls were so curiously modified as
to bear a distinct resemblance to the skull of a huge carnivore. The
†amynodonts migrated to the Old World and occur in the Oligocene of
France, but the †hyracodonts would seem never to have left North America.
The third series, that of the true rhinoceroses, comprised several genera
at different levels in the White River beds (_†Trigonias_, _†Cænopus_,
etc.); they were of uncertain origin and it has not yet been determined
whether they were immigrants or of native stock. Many species have been
found, varying much in size, up to that of a modern tapir, and not unlike
one in proportions, for they were of lighter build and had relatively
longer legs than any existing rhinoceros. The species of the lower
and middle substages were all hornless, but in the uppermost substage
we find skulls with a pair of nasal horns in an incipient stage of
development. This was the beginning of the †paired-horned rhinoceroses
(_†Diceratherium_) which so flourished in the John Day and the lower
Miocene.
[Illustration: FIG. 135.—†Hornless rhinoceros (_†Cænopus tridactylus_)
of the White River stage. Restored from a skeleton in the American
Museum.]
Of the horses there was no great variety and all the species so far
discovered are included in a single genus (_†Mesohippus_), though there
was a decided increment in the size of the successive species from the
earlier to the later portion of the stage. Looked at superficially, it
seems absurd to call these little creatures “horses” at all and the term
can be justified only as implying that they were ancestral members of the
family. The largest of the White River species hardly exceeded a sheep in
size and all of them had comparatively short necks, long and slender legs
and three-toed feet. The low-crowned grinding teeth show that they were
browsers, not grazers. The abundant Eocene family of the †Lophiodontidæ
made its last appearance in the White River, where it was scantily
represented by slender, long-legged animals (_†Colodon_), with feet
singularly like those of the contemporary horses, except that there were
four toes in the front foot. Tapirs (_†Protapirus_) were very much less
common than rhinoceroses or horses and were hardly half as large as the
existing species of the family and of relatively far more slender form;
the development of the proboscis had already begun. Lastly, the presence
of the clawed †chalicotheres has been reported from the lower Oligocene
of Canada, but the material is too fragmentary for generic reference.
Though the number of artiodactyl families yet identified among the White
River fossils is no larger than that of the perissodactyl families,
the artiodactyls greatly preponderated in individual abundance. The
peccaries, which were fairly common, resembled those of the John Day,
but were considerably smaller. Of the camels, there were two series,
one of which (_†Eotylopus_), lately described by Dr. Matthew, is of yet
unknown significance, while the other (_†Poëbrotherium_) was apparently
the ancestor common to all the subsequent phyla of camels and llamas.
This extremely interesting genus had species which ranged in size from a
gazelle to a sheep, had two toes in each foot, a moderately elongate neck
and teeth which were beginning to assume the high-crowned character.
From this it may be inferred that those animals were, partly at least,
of grazing habit, which was rare among White River ungulates, most
of which fed upon leaves and soft and succulent plants. An extinct
family, the †Hypertragulidæ, were a greatly diversified group of dainty
little creatures, one of which (_†Hypisodus_) was no larger than a
rabbit and had high-crowned teeth. The other genera (_†Leptomeryx_,
_†Hypertragulus_) must have resembled in form and proportions the tiny
little chevrotains or “mouse-deer” of the East Indian islands. Late
in the age arose a larger form of this family, nearly equalling the
Musk-Deer in size, the extraordinary genus _†Protoceras_, which was,
especially the males, a grotesque object. The males had a pair of upper
canine tusks and two pairs of prominent long protuberances on the skull.
This, or some similar form, must have been the ancestor of the still more
bizarre _†Syndyoceras_ of the lower Miocene.
The †oreodonts were by far the commonest of White River mammals, and
evidently they roamed the woods and plains in great herds. There
were several species, larger and smaller, of the abundant genus
(_†Merycoidodon_) but the largest did not surpass a modern peccary in
size and was somewhat like that animal in appearance, but had a shorter
head and much longer tail. In the upper substage appeared a very peculiar
genus of this family (_†Leptauchenia_), animals with short, deep, almost
monkey-like heads, and presumably aquatic in habits. The _†agriochœrids_
were very much less common; they may be described roughly as †oreodonts
with very long, cat-like tails and clawed feet.
[Illustration: FIG. 136.—_†Merycoidodon culbertsoni_, the most abundant
of White River †oreodonts. Restored from a skeleton in the American
Museum of Natural History.]
All of the foregoing artiodactyl families were exclusively North
American in Oligocene distribution; even the camels did not reach
Asia till the Pliocene, and the other families never invaded the Old
World at all. There were, however, two additional families, which also
occurred in the eastern hemisphere, whence one of them, and possibly
the other, was derived. The unquestionably Old World family, that of
the †anthracotheres, was represented in the White River by two genera
(_†Bothriodon_ and _†Anthracotherium_), which were short-legged,
long-snouted, swine-like animals, which have no near relations in the
modern world. The other family, the †giant pigs, which we have already
met with in the lower Miocene and upper Oligocene, is of doubtful origin,
and nothing has yet been found in the preceding formations of either
North America or Europe which can be regarded as ancestral to them. The
White River genus (_†Archæotherium_) was very like the John Day and
Arikaree genera, but most of the species were much smaller and some were
not so large as a domestic pig. In the uppermost beds, however, are found
huge species, which rivalled those of the subsequent formations. That
these strange animals were rooters and diggers and therefore pig-like in
habits is indicated by the manner in which the teeth are worn.
[Illustration: FIG. 137.—†Giant pig (_†Archæotherium ingens_) from
the lower White River stage. Restored from a skeleton in the museum of
Princeton University.]
_South America._—The older continental Tertiary formations of South
America cannot be correlated with those of North America or Europe,
because they have nothing in common. Difficult as it is to give a
correct and adequate conception of the Tertiary mammalian life of the
northern hemisphere to one who has not made a study of it, it is far more
difficult in the case of South America. The stock of adjectives, such
as “peculiar,” “bizarre,” “grotesque” and the like, already overworked
in dealing with northern forms, is quite hopelessly inadequate where
everything is strange. In addition to this, we are seriously handicapped
in treating of the Oligocene and Eocene of South America by very
incomplete knowledge. Many fossils have been collected and named, but
the great majority of these are known only from teeth; a few skulls and
limb-bones have been described, but no skeletons, and therefore much is
very uncertain regarding these faunas.
The Deseado formation (Pyrotherium Beds) has been variously referred by
different writers from the upper Cretaceous to the lower Miocene, but
its most probable correlation is with the Oligocene. Though most of the
mammalian groups are the same as those of the Santa Cruz, the proportions
of the various orders in the two faunas are very different, but, to some
extent, the difference is probably illusory and due to the conditions
of fossilization, for, as a rule, the small mammals are much less
frequent and well preserved in the older beds. As in the Santa Cruz, the
marsupials were the only predaceous mammals, and some of them attained
gigantic size; but no such variety of these beasts of prey has been found
in these beds as occurred in the middle Miocene. In addition, there
were numerous small herbivorous marsupials. One of the most striking
differences from the Santa Cruz fauna was in the very much smaller number
of Edentata, which, instead of being extremely common, are quite rare
among the fossils. No doubt there was a real and substantial difference
in this respect, but it was probably not so great as it seems, and
the same three suborders are found in both formations. One of the few
†ground-sloths that have been obtained was very large (_†Octodontherium
crassidens_), a much larger animal than any species of the suborder that
is known from the Santa Cruz. The †glyptodonts were also rare, and only
two genera and species have been described from very scanty remains.
Armadillos, on the other hand, were much more common, and no less than
eleven genera have been named, three of which occurred also in the Santa
Cruz. Among these was the remarkable genus _†Peltephilus_, in which
the anterior two pairs of plates of the head shield were modified into
horn-like spines.
Equally striking was the remarkable diminution of the Rodentia, as
compared with those of the Santa Cruz, though, of course, this is an
inaccurate mode of stating the truth, occasioned by the fact that we are
following the history in reverse order. It would be preferable to say
that the rodents underwent a remarkable expansion in the Santa Cruz.
These rodents of the Deseado stage are the most ancient yet discovered
in South America and represent only two families, both belonging to
the Hystricomorpha, or porcupine group. If, as Dr. Schlosser and
other European palæontologists maintain, the Hystricomorpha were all
derived from a family of the European Eocene, this would necessitate a
land-connection between South America and the Old World independent of
North America, for the latter continent had no hystricomorph rodents
until the connection between the two Americas was established.
The great bulk of the Deseado fauna is made up, so far as individual
abundance is concerned, of hoofed animals belonging to the typically
South American groups. The †Toxodonta were represented partly by
genera which were the direct ancestors of the common Santa Cruz
genera (_†Pronesodon_, _†Proadinotherium_), and, more numerously, by
a very peculiar family, the †Notohippidæ, which had highly complex,
cement-covered grinding teeth. Still a third family of this suborder,
the †Leontiniidæ, was highly characteristic of the Deseado fauna and
is not known from the Santa Cruz. These were large animals, with a
small horn on the tip of the nose and low-crowned, comparatively simple
grinding teeth. Even more abundant were the †Typotheria, small forms
which were ancestral to the Santa Cruz genera, larger ones which died out
without leaving successors and one quite large animal (_†Eutrachytherus_)
which seems to have been the ancestor of the Pliocene and Pleistocene
_†Typotherium_. This series is not known to have been represented in
the Santa Cruz and may have withdrawn from Patagonia at the end of the
Deseado stage.
[Illustration: FIG. 138.—Horned †toxodont (_†Leontinia gaudryi_), Deseado
stage. Restored from a skull in the Ameghino collection.]
The †Entelonychia, those strange toxodont-like animals with claws instead
of hoofs, were much more numerous and varied than they were afterward in
the Santa Cruz, when they were on the verge of extinction, and included
both very small and very large species. The †Pyrotheria, a suborder which
is not met with in the Santa Cruz or later formations, likewise included
some very large forms. The typical genus, _†Pyrotherium_, included large,
relatively short-legged and very massive animals; the upper incisors
formed two pairs of short, downwardly directed tusks, and in the lower
jaw was a single pair of horizontally directed tusks; the grinding teeth
were low-crowned and had each two simple, transverse crests. These
grinding teeth and the lower tusks so resemble those of the ancestral
Proboscidea in the Oligocene of Egypt, that the †pyrotheres have actually
been regarded as the beginnings of the †mastodons and elephants, but this
is undoubtedly an error. The †Astrapotheria, another group which became
extinct at or soon after the end of the Santa Cruz, were relatively
abundant in the Deseado and counted some very large species. Finally,
the †Litopterna were represented by the same two families as continued
through the Pliocene and one of them far into the Pleistocene. The
horse-like †proterotheres were present, but not enough of them has been
obtained to show whether or not they were in a notably less advanced
stage of development than those of the Santa Cruz. The †macrauchenids
were quite similar to those of the latter formation, though considerably
smaller. In addition, there were a few genera, survivals from earlier
times, which were not referable to either of these families.
The large number of genera, especially among the †toxodonts and
†typotheres, which had high-crowned, cement-covered teeth, may be taken
as an indication that grazing habits had already begun to be prevalent.
Of this wonderful assemblage of hoofed animals, divisible into six
separate groups, whether of ordinal or subordinal rank, not a trace
remains to-day. Not only are all the species, genera and families
extinct, but the suborders and orders also. Further, this was a very
strictly autochthonous fauna, so far as the hoofed animals were
concerned, and no member of any of the six groups has ever been found
outside of the Neotropical region.
4. _Eocene_
_North America._—In the western interior of North America the Oligocene
followed so gradually upon the Eocene, that there is great difficulty
in demarcating them and much difference of opinion and practice obtains
as to where the boundary line should be drawn. Not to depart too widely
from the scheme used by Professor Osborn, the Uinta stage is here
treated as uppermost Eocene, though this is a debatable procedure. For
several reasons, the extraordinarily interesting and significant Uinta
fauna is far less completely known than that of the preceding Bridger
and succeeding White River stages. For one thing, it has been much less
thoroughly explored, and it may be confidently expected that future
exploration will greatly enlarge our knowledge.
The smaller mammals of the Uinta are particularly ill-known. No
Insectivora have yet been found, though this gap will assuredly be
filled; rodents are scanty in the collections and include only two
families, one the †ischyromyids, which were still common in the
White River, the other of doubtful position, but not improbably to
be considered as the beginning of the pocket-gophers (Geomyidæ). The
archaic flesh-eaters, or †Creodonta, were represented by two families,
one comprising smaller animals with somewhat cat-like, shearing
teeth (†Oxyænidæ), the other, very large beasts with crushing teeth
(†Mesonychidæ), neither of which continued into the White River. As
compared with the middle and lower Eocene, the †creodonts had greatly
diminished and, to replace them, the true Carnivora were beginning to
come in. As yet, however, only small and very primitive dog-like forms
are known and no trace of †sabre-tooths or mustelines has been found.
Indeed, it is very doubtful whether members of these families ever will
be found in the Uinta, for their presence in the succeeding White River
was probably due to immigration.
The Perissodactyla were the preponderant type of hoofed animals, and
ancestral forms of most of the White River genera have already been
identified. The †titanotheres (_†Diplacodon_, _†Protitanotherium_)
were much smaller and lighter than those of the lower White River and
had much shorter horns. The †hyracodonts, the lightly built, cursorial
rhinoceroses, were represented by a genus (_†Triplopus_) which was
smaller and more slender than the White River form (_†Hyracodon_) and its
teeth were of distinctly more primitive character. The heavy, massive
and presumably aquatic †amynodonts (_†Amynodon_) were likewise smaller
and less specialized than their descendants of the Oligocene. No member
of the true rhinoceros series has yet been identified in the Uinta,
but there is some reason to think that they were nevertheless present.
Tapirs are distinctly indicated by certain fossils, but they are still
too incompletely known to make possible any statement as to their degree
of development. The horses (_†Epihippus_), like the other families
mentioned, were much smaller and distinctly more primitive than their
successors in the Oligocene.
The Artiodactyla were, for the first time in the history of North
America, as numerous and as varied as the perissodactyls and, with the
exception of the peccaries and †anthracotheres, representatives of
all the White River families are known. The finding of the peccaries
is merely a question of further exploration, but the †anthracotheres
were migrants from the Old World, and there is no likelihood that they
will be discovered in the Uinta at any future time. Fairly large,
pig-like animals, probably referable to the †giant-pigs or †entelodonts,
occurred, but nothing has yet been found which can be considered as
the direct ancestor of the White River genus. As was true of the
perissodactyls, the Uinta artiodactyls were nearly all much smaller and
more primitive than their Oligocene descendants and the differences are
most interesting from the evolutionary point of view. The ancestral
camel (_†Protylopus_) was a little creature no bigger than a fox-terrier,
though the †hypertragulids (_†Leptotragulus_) were as large as
_†Leptomeryx_ and _†Hypertragulus_ of the White River. The most ancient
known members of the †oreodonts (_†Protoreodon_) and the †agriochœrids
(_†Protagriochœrus_) are found in the Uinta.
The middle Eocene fauna, Bridger stage, though it passed upward very
gradually into that of the Uinta, was yet, on the whole, very different
from the latter. It was exclusively indigenous and so radically distinct
from the mammals of corresponding date in Europe as to preclude the
possibility of a land-bridge with that continent. In the lower Eocene,
as will be shown in a subsequent page, the communication between the two
continents was broadly open and the faunas of the two continents were
much more closely similar than they have ever been since. It is really
remarkable to see with what comparative rapidity the two regions, when
severed, developed different mammals under the operation of divergent
evolution. Had the separation continued throughout the Tertiary and
Quaternary periods, North America would now have been as peculiar
zoölogically as South America is, a result which has been prevented by
the repeated renewal of the connection.
The characteristic features of the Bridger mammalian fauna were chiefly
due to the great expansion and diversification of certain families,
which began their career at an earlier stage, and to the disappearance
of many archaic groups which had marked the more ancient faunas. Other
archaic groups, however, survived and even flourished in the Bridger,
and of these it is particularly difficult to convey a correct notion
to the reader, because they were so utterly unlike anything that now
lives. One of these orders, the †Tæniodontia, which had so many points
of resemblance to the †ground-sloths that several writers have not
hesitated to include them in the Edentata, survived only into the older
Bridger, but the equally problematical †Tillodontia then reached their
culmination, though they were not very numerous. Though not at all
related to that group, the †tillodonts looked like huge rodents, with
their chisel-like incisor teeth. There was a remarkable assemblage of
Insectivora, more numerous and varied than in any subsequent formation,
no less than six families being known. One of these somewhat doubtfully
represented the moles and two others modern Asiatic groups. The very
unexpected discovery of an armadillo in the Bridger has been reported,
but the propriety of referring this animal to the armadillos, or even
to the edentates, has not yet been proved, and it would therefore be
premature to discuss its significance. The only marsupials were opossums.
So far as our information extends, there were no true Carnivora in the
Bridger, all the beasts of prey of the time belonging to the archaic
†Creodonta, which then reached their maximum development in numbers
and diversity. One family (†Oxyænidæ) included large and powerful
flesh-eaters, with cat-like dentition and short, rounded, lion-like
heads, long bodies and tails and short, heavy limbs, giving them the
proportions of otters. Another (the †Hyænodontidæ) comprised small,
long-headed, fox-like and weasel-like animals, which doubtless preyed
upon small mammals and birds. A third family (†Mesonychidæ) was made
up of moderate-sized, long-jawed creatures, which must have resembled,
rather remotely, short-legged and long-tailed wolves and hyenas. Their
habits and mode of life are somewhat problematical, for their grinding
teeth were blunt, not adapted to the shearing of flesh, and their claws
were broad, almost hoof-like. Such creatures could hardly have subsisted
by the pursuit of living prey and were probably carrion-feeders and more
or less omnivorous. The †Miacidæ, a family which connected the †creodonts
and true carnivores and might almost equally well be placed in either
group, were externally much like the small †hyænodonts, but were more
efficiently equipped for the capture and devouring of prey.
[Illustration: FIG. 139.—A mesonychid †creodont (_†Dromocyon velox_)
of the Bridger stage. Restored from a skeleton in the Museum of Yale
University.]
Of the archaic and extinct orders of hoofed animals, the only one which
persisted from earlier times into the Bridger and greatly flourished
there was the †Amblypoda, one family of which (†Uintatheriidæ) was
preëminently characteristic of middle Eocene life, becoming very rare
and then dying out in the upper Eocene. The †uintatheres of the Bridger
underwent considerable modification in size and appearance within the
limits of the stage, the larger and stranger species appearing toward
the end of the time. Most of these great creatures may fairly be called
gigantic, for they equalled the largest modern rhinoceroses and smaller
elephants in size. The body, limbs and feet were so elephantine in
character that they were once believed to be ancestral Proboscidea, but
the teeth and the fantastic skull were so radically different that this
belief was long ago abandoned. The upper canine teeth were converted, in
the males, into formidable spear-like or scimitar-like tusks, protected
by great flange-shaped expansions of the lower jaw; bony knobs on the
end of the nose probably supported a pair of dermal horns like those of a
rhinoceros and, in addition, a pair of high, cylindrical, horn-like, bony
protuberances arose above the eyes and another, more massive pair, near
the back of the head. It would be difficult to imagine more extraordinary
creatures than the †uintatheres, which were the largest land-mammals of
their time. The family was entirely confined to North America, no trace
of them having been found in any other continent.
While the backward and archaic orders, most of which have left no
descendants in the modern world, had thus a stately representation in
Bridger times, they were outnumbered in genera, species and individuals
by the progressive orders, which are still in more or less flourishing
existence. The Primates, whether lemurs or monkeys, were numerous,
and this, so far as is definitely known, was their last appearance in
extra-tropical North America. They may at any time be found in the
Uinta, but there is small probability that they will ever turn up in
the White River or later formations. The many rodents all belonged to
the †ischyromyids, an extinct family which, there is much reason to
believe, was ancestral to many families of the squirrel-like suborder of
Sciuromorpha. Most of them were species of a single genus (_†Paramys_)
and varied in size from a mouse to a beaver, or even larger.
The Perissodactyla may be said, in one sense, to have reached their
culmination in the Bridger; not that many of them, such as the horses
and rhinoceroses, did not advance far beyond their state of development
in the Eocene, but at no subsequent time did the order as a whole
possess such dominating importance. There were five or six families
of perissodactyls in the Bridger, and their remains are much the most
abundant fossils found there. Individually, the commonest perissodactyls
of the time were the †titanotheres, of which there were several genera
and many species, differing chiefly in size and proportions, though the
largest hardly exceeded a modern tapir in stature and was not dissimilar
in appearance. These Bridger †titanotheres were considerably smaller
than those of the Uinta and therefore very much more so than the White
River forms; it was not till the latter stage that the family lived up
to its name of “titanic beasts.” By far the commonest of the genera in
the middle and lower Bridger was _†Palæosyops_, which was hornless, while
in the upper part of the beds are found genera (_e.g._ _†Manteoceras_
and _†Dolichorhinus_) in which the horns were just beginning to appear.
Another extinct family, the †Lophiodontidæ, which was very abundant in
the European Eocene, formed a very subordinate element in this fauna and
included a number of small tapiroid genera (_e.g._ _†Helaletes_).
[Illustration: FIG. 140.—Some characteristic mammals of the Bridger
Eocene reduced to a uniform scale, with a pointer dog, in frame,
for comparison. 1. Primitive rhinoceros (_†Hyrachyus eximius_). 2.
†Tritemnodon agilis. 3. _†Patriofelis ferox_, and 4, _†Dromocyon
velox_, †creodonts. 5. Primitive rodent (_†Paramys delicatior_). 6.
_†Uintatherium alticeps._ 7. †Titanothere (_†Mesatirhinus superior_).]
The horses (_†Orohippus_) were very small and primitive creatures, no
bigger than a fox, with four toes in the front foot and three in the
hind. So completely different in appearance and proportions were these
little animals from any of the modern horses, that it requires an effort
of the imagination to think of them as belonging to the same family, and
it is only by employing the family to designate a _genetic series_ that
such a classification can be justified. The †hyracodonts, or cursorial
rhinoceroses, were very abundantly represented by a number of small and
medium-sized animals (_†Hyrachyus_) which had less specialized teeth,
shorter neck and limbs than their upper Eocene and Oligocene successors,
and four toes in the front foot; one genus (_†Colonoceras_) had a
pair of nasal horns, but would seem to have died out without leaving
descendants. In the upper part of the beds is found the Uinta genus
_†Triplopus_, with three-toed fore foot; and in the same division occurs
another Uinta genus, _†Amynodon_, the most ancient known species of the
supposedly aquatic rhinoceroses. True rhinoceroses, that is animals which
were directly ancestral to the modern members of the family, have not
been identified and may not have been present in North America; that is
still an open question. Tapirs, all of them quite small, were relatively
common, but are still very incompletely known. The earliest known members
of the clawed †chalicotheres were of Bridger date.
It is worth remarking that, except a single genus in the upper and later
portion of the stage (_†Triplopus_), all of the Bridger perissodactyls
had four toes in the front foot and three in the hind, while in the White
River beds above the lowest substage the number three in both fore and
hind feet was almost equally universal.
One of the most radical and striking differences between the Uinta and
Bridger faunas was the rarity of Artiodactyla in the latter, which is
in almost equally strong contrast with their abundance in the middle
Eocene of Europe. Most significant of these rare Bridger artiodactyls
were the little creatures (_†Homacodon_), hardly so large as a domestic
cat, which may fairly be regarded as a very early stage, if not the
actual beginning, of the great camel family, which was destined to
play so conspicuous a part in the life of America, North and South.
Small pig-like animals (_†Helohyus_) which were no doubt ancestral
to the peccaries, were fairly common and there were, in addition,
relatively large animals (_†Achænodon_) allied, but not ancestral, to the
†giant-pigs of the Oligocene; some of these were considerably larger than
a full-grown Wild Boar (_Sus scrofa_).
Among all the many hoofed mammals of the Uinta and Bridger there was not
a single one that had the high-crowned, persistently growing teeth of
the grazers; all of them must have had browsing habits and have fed upon
such soft vegetable tissue as did not rapidly abrade the teeth. The same
statement applies, _à fortiori_, to the stages antecedent to the Bridger
and therefore to the entire Eocene and Paleocene. From these facts it may
be inferred that the grasses had not yet taken possession of wide areas.
Concerning the Bridger fauna, Professor Osborn, who has done so much to
elucidate it, says: “On the whole, it is a very imposing, diversified and
well-balanced fauna, with an equal distribution of arboreal, cursorial,
aquatic, fossorial, carnivorous and herbivorous types.”
The lower Eocene is divisible into two stages, in descending order, the
Wind River and Wasatch, both extensively exposed in central Wyoming. As
would be expected from its stratigraphical position, the Wind River
fauna was largely transitional between that of the Bridger above and that
of the Wasatch below. Unfortunately, the fossils are far less numerous
than those of the Bridger and not so well preserved, and therefore give
us a less adequate conception of the life of that time. The archaic,
non-progressive orders were strongly represented, but already the
progressive groups were in a numerical majority of species; most of
these archaic orders may be most advantageously described in connection
with the Wasatch. Opossums were almost certainly present, though the
available specimens are too fragmentary for assured determination.
The †tillodonts, †tæniodonts and insectivores differed little from
the Wasatch representatives of these orders, except that the Bridger
†tæniodont, _†Stylinodon_, which had rootless, persistently growing
teeth, was associated with the Wasatch genus _†Calamodon_. On the other
hand, the primitive flesh-eaters, or †creodonts, which were referable to
Wasatch families, were less numerous and varied and formed a mixture of
Bridger and Wasatch genera. The †Oxyænidæ, the family with cat-like teeth
and head, had both the smaller Wasatch genus _†Oxyæna_ and the very large
Bridge _†Patriofelis_. Of the blunt-toothed †Mesonychidæ, one very large
animal (_†Pachyæna_) survived from the Wasatch. The small forms of the
family †Hyænodontidæ were common, and there were numerous species of the
progressive family †Miacidæ.
Among the hoofed animals there were two of the antique orders which
became extinct before the end of the Eocene, indeed, one of these groups,
the †Condylarthra, made its last appearance in the Wind River. This
extremely primitive group, which, in a sense, connected the hoofed with
the clawed mammals, will be described under the more ancient faunas.
The other order, the †Amblypoda, was represented by two very different
families, one of which, the †uintatheres, was so flourishing in the
Bridger, where it formed the most characteristic and by far the most
striking element of the fauna. The Wind River genus (_†Bathyopsis_)
was a very much smaller animal than any of the Bridger forms and its
horn-like protuberances were in an incipient state, while in various
other respects it was decidedly more primitive than its successors. The
second family was represented by the genus _†Coryphodon_, which did
not survive into the Bridger, but was especially characteristic of the
Wasatch fauna, with which it will be described.
Turning now to the progressive orders, we note that the rodents, lemurs
and monkeys were very similar to those of the Bridger and belonged to
the same families, but were decidedly less numerous. This difference,
however, may be rather apparent than real and due to the much more
favourable conditions for the preservation of small mammals in the middle
Eocene. Among the Perissodactyla, the horses were intermediate in size
and structure between those of the Bridger and those of the Wasatch, but
were decidedly nearer to the latter. The †lophiodonts, so far as known,
were represented by a single genus (_†Heptodon_) which also occurred in
the Wasatch. The modest beginnings of the †titanotheres, the family which
became so very conspicuous in the middle and upper Eocene and lowest
Oligocene, may be noted in the Wind River fauna, in which there were
two genera. One of these (_†Eotitanops_), the very probable ancestor
of all the subsequent genera, was quite small, about two-thirds the
size of a modern tapir, while the other (_†Lambdotherium_) was a much
smaller, lighter and more slender animal and apparently belonged to an
abortive, short-lived phylum. Then, too, the first of the †hyracodonts,
or cursorial rhinoceroses, made their appearance here in the genus
_†Hyrachyus_, which was afterward so common in the Bridger.
No Artiodactyla have yet been found in the Wind River, though there can
be little doubt that they then inhabited North America, as they did both
before and afterward.
The Wind River fauna was of so much less peculiar and isolated character
than that of the Bridger as to suggest a connection with the eastern
hemisphere, a suggestion which is strengthened by the unheralded
appearance of the †titanotheres and †hyracodonts, of which no forerunners
have been found in the Wasatch.
The lowest and most ancient of the Eocene faunas is that of the Wasatch
formation, which is extensively developed in central and southern
Wyoming, Utah and New Mexico. The fauna of this stage is plainly
divisible into two groups: (1) those types which were the descendants of
American Paleocene mammals and were therefore indigenous, and (2) the
immigrants from other continents. The indigenous mammals, which almost
all belonged to orders now extinct, few of which survived later than the
Eocene, must have given a very bizarre appearance to the assemblage,
especially as they were more numerous, varied and, for the most part,
larger and more conspicuous than the newcomers. Marsupials have not yet
been found, but the occurrence of opossums in the Bridger and probably
in the Wind River gives some reason to believe that they were in North
America during Wasatch times also. The †Tæniodontia, which bore a certain
resemblance to South American edentates, had one pair of incisor teeth
above and below enlarged and chisel-shaped, somewhat like those of
rodents. The †Tillodontia were much smaller than those of the Bridger,
and their incisors were only beginning to take on the chisel-like form.
Insectivora were quite abundant, and three, or perhaps four, families
were represented in the Wasatch; some of these resembled the modern
aquatic insectivores of the west African rivers and others were more like
European hedgehogs.
The flesh-eaters all belonged to the †Creodonta, and, though rather less
diversified than those of the Bridger, were yet relatively abundant.
In size, they ranged from little creatures not larger than a weasel up
to truly enormous beasts, and differed, no doubt, largely in habits
and manner of life. For the most part, the families were the same as
those of the Bridger †creodonts, but the genera all were different. The
†oxyænids (_†Oxyæna_) were much smaller and lighter than the large and
massive representatives found in the middle Eocene, and their teeth were
not so cat-like. Another group of predaceous animals (_†Palæonictis_)
which also inhabited Europe, but did not survive the lower Eocene
in either continent, had short, broad and very cat-like heads. The
†mesonychids were far larger than those of the Bridger, a departure from
the ordinary rule, and the several species of the common Wasatch genus
(_†Pachyæna_) had grotesquely large heads. A family (†Arctocyonidæ),
of very extensive geographical range and great antiquity, had its last
representatives here in a very curious animal (_†Anacodon_) which had the
flat-crowned, tuberculated grinding teeth of the bears and the enlarged,
scimitar-like upper canines of the †sabre-tooth cats. Such a combination
seems utterly incongruous and no one would have ventured to predict
it. The progressive family of †creodonts (†Miacidæ) was already quite
numerously represented, but only by small forms, which must have preyed
upon small mammals, birds and lizards.
Two archaic orders of hoofed mammals were fairly numerous. One, the
†Condylarthra, comprised quite small, five-toed animals, with long
tails and short feet and extremely primitive in structure. A genus
(_†Phenacodus_) of this order was long regarded as being ancestral to
most of the higher orders of ungulates, but this belief has proved to
be untenable. More numerous were the †Amblypoda, one genus of which
(_†Coryphodon_), though persisting into the Wind River, was especially
characteristic of the Wasatch. The †coryphodonts were the largest of
lower Eocene mammals, and some of the species equalled a tapir or
small rhinoceros in length and height, but had heavier limbs; as the
skeleton conclusively shows, these must have been heavy, clumsy and
exceptionally ugly brutes, with formidable tusks, large head, but
relatively more slender body, short and massive limbs and elephantine
feet. In appearance, these strange beasts were not altogether unlike
the Hippopotamus and were perhaps more or less amphibious in habits.
The other family of †Amblypoda, the †uintatheres, have not yet been
registered from the Wasatch, but they will undoubtedly be found there, as
they were unquestionably present at that time.
[Illustration: FIG. 141.—_†Phenacodus primævus_, the best known Wasatch
representative of the †Condylarthra. Restored from a skeleton in the
American Museum of Natural History.]
[Illustration: FIG. 142.—The commonest of Wasatch ungulates, the
†amblypod, _†Coryphodon testis_. Restored from a skeleton in the American
Museum of Natural History.]
All of the preceding groups were of the archaic, non-progressive type
and have long been extinct. With the sole exception of one †creodont
family (†Miacidæ) and perhaps some of the insectivores, they have no
descendants or representatives in the modern world. All of them appear to
have been indigenous and derived from North American ancestors, though
it is possible that a few were immigrants. We now turn to the orders
which were more significant of the future, because they had within them
the potency of a far higher development. These progressive groups were
all immigrants, coming to North America from some region which cannot
yet be positively identified, but most probably was Asia. From the same
region and at a corresponding period of time Europe received many of the
same forms, and so many genera were at that time common to the latter
continent and North America that a broad and easy way of intermigration
must have been open.
One of these immigrant orders, the Rodentia, the most ancient known
members of which were these species from the North American Wasatch,
was represented by the same family (†Ischyromyidæ) and some of the same
genera (_†Paramys_, _†Sciuravus_) as throve also in the Bridger stage.
There were two orders of hoofed mammals, which were newcomers to the
western world, Perissodactyla and Artiodactyla. Of the former was a genus
(_†Eohippus_) of the most ancient American horses. These most interesting
little animals, no larger than small foxes and domestic cats, would
hardly be called horses, were it not for the long series of gradual and
successive modifications which led from _†Eohippus_ up to the modern
horses. The graceful little creatures had a short neck, curved back, and
relatively short, slender limbs, with four functional toes in the front
foot and three in the hind; and, though they differed from existing
horses in almost every detail of teeth and skeleton, there was something
unmistakably equine about them. From the abundance of their remains it
may be inferred that herds of them swarmed in the forests and glades of
Wasatch times. The second perissodactyl family, the †Lophiodontidæ, which
comprised considerably larger animals, never attained to importance in
America, but flourished and became greatly diversified in Europe. What
are believed to be the most ancient tapirs yet discovered (_†Systemodon_)
were individually very common in the Wasatch. This tapir was no larger
than a Coyote, had no proboscis and was so little like a tapir in outward
appearance that an observer might well be pardoned for overlooking the
relationship; even the skeleton is of so indifferent a character that the
reference of this genus to the tapirs cannot be positively made.
Of equal significance for the future was the arrival of the Artiodactyla,
of which there were members of three families in the Wasatch, though
individually they were much less common than the horses. These were
geologically the oldest known artiodactyls, Europe having yielded none
of this date, and are still too imperfectly known to justify any very
positive statements about them. One genus, however (_†Trigonolestes_),
tiny little creatures, like rabbits in size, would seem to represent the
beginnings of the great ruminant tribe, now so very important a factor in
the life of the world. A second genus (_†Eohyus_), considerably larger,
is very doubtfully referable to the pigs; while a third (_†Parahyus_),
still larger, was the first in the short-faced series of the
†entelodonts, which persisted in ever increasing size through the whole
Eocene, but could hardly have been ancestral to the true †entelodonts,
or †giant-pigs, of the Oligocene, the place and time of whose origin are
unknown.
Another immigrant order of great interest, since we ourselves belong
to it, the Primates, made its first appearance in North America in the
Wasatch, but was not destined to long life or great importance in this
continent, where it did not survive the Eocene. Several different kinds
of small, lemur-like and monkey-like creatures dwelt in the tree-tops
of the Wasatch forests. One genus (_†Anaptomorphus_) had a remarkable
likeness to the modern Tarsier (_Tarsius spectrum_) of the Malay
peninsula and islands.
_South America._—The Eocene of South America, referred by some writers to
the upper Cretaceous, is very incompletely and unsatisfactorily known.
The Casa Mayor formation (or Notostylops Beds), which has yielded a
great variety of mammals, for the most part very fragmentary, probably
contains not one but several successive faunas which have not yet been
fully discriminated, and that of the next succeeding Astraponotus Beds is
still but a scanty list. This list, however, includes the most ancient
†glyptodonts yet discovered and the most ancient †astrapotheres in the
narrow sense of the term. The Astraponotus Beds may be either Eocene or
Oligocene in date.
Taking the Casa Mayor faunas as a whole, they were a very numerous and
diversified assemblage of small mammals, without a single large one
among them. There were no monkeys or rodents; otherwise, the orders
were in almost all cases the same as those which made up the Santa Cruz
fauna. The marsupials were represented by the opossums and by several
of the carnivorous kinds, the only beasts of prey that South America
had until the migrations from the north brought in the true Carnivora,
late in the Miocene or very early in the Pliocene. There were also
numerous small marsupials of peculiar type, of which the last living
survivor is _Cænolestes_, of Ecuador. Throughout the stage, armadillos
were present in considerable variety, but are known only from the bony
plates of the carapace, and therefore little can be determined as to
their relationships to the modern families. Only a single and very
problematical genus of the †ground-sloths, which afterwards throve so
mightily in the Miocene and Pliocene, has been obtained and that in the
later portion of the stage.
The orders of hoofed mammals were represented by many small animals,
most of which are known only from the teeth, which show these Casa Mayor
genera to have been far more primitive and less specialized than their
descendants in the Deseado and Santa Cruz stages. All of them had the
low-crowned grinding teeth of the browsers, and no grazers were then in
existence, so far as is known. No †toxodonts, in the more restricted
sense of that term, have been found, but the two allied suborders of
the †Typotheria and †Entelonychia were numerously represented. Of the
former there were two families and of the latter three, which is more
than in the Deseado or Santa Cruz formations. One of the families of the
†Entelonychia (†Notostylopidæ) consisted of very small, rodent-like
animals, with a pair of chisel-shaped incisors in upper and lower jaw,
and a second family (†Homalodontotheriidæ) contained genera which would
seem to have been directly ancestral to those of the Santa Cruz, but
were very much smaller than their successors. The very large and massive
†Pyrotheria of the Deseado stage were represented by small animals,
in which the grinding teeth had two pairs of conical tubercles, not
yet united into transverse crests. Two families of the †astrapotheres,
in the broad sense, were far smaller than their Oligocene and Miocene
descendants. To the †Litopterna are referred a number of genera, in which
the grinding teeth were tuberculated and had very imperfectly developed
crests, so as strongly to suggest the teeth of the †Condylarthra.
However, until something is ascertained regarding the skeleton,
especially the feet, of these animals, their relationships will remain
more or less doubtful.
It will be observed that these Casa Mayor faunas not only were made up
exclusively of small animals, but also that they already were typically
and characteristically South American and bore the stamp which remained
essentially the same until the successive waves of migration from the
north so greatly modified the composition of the Neotropical fauna. The
absence of rodents and monkeys and the comparative unimportance of the
Edentata gave a somewhat different character to these ancient faunas from
those of the Santa Cruz and later formations.
5. _Paleocene_
_North America._—A very important discovery is one lately made by
American Museum parties of a formation intermediate between the Wasatch
and Torrejon. The interesting fauna of these beds has not yet been
described, but it may be remarked that it contained none of the immigrant
orders.
The vegetation of the Paleocene was already very modern in character,
and nearly all of the common forest-trees were represented by species
which differed but slightly from those of the present. The grasses were
already in existence, but, there is good reason to believe, they had not
attained to much importance and did not cover the plains and open spaces
as they did in the Miocene and still continue to do. As the grasses
afford the principal food-supply of so many grazing animals, the matter
of their abundance and extension is a very significant one in the history
of mammalian development, and, as we have already learned, eventually led
to widespread and profound modifications of structure, especially of the
teeth. While there is thus nothing very strange about the plant-world of
Paleocene times, the higher animal life was almost totally different from
that of modern times and made up a most curious and bizarre assemblage,
from which nearly all the familiar Recent types were absent. The reptiles
had been greatly impoverished by the world-wide and, as yet, unexplained
destruction which overtook them at the end of the Mesozoic era, but it is
possible that in both North and South America a few of the huge Dinosaurs
survived the decimation of the class. Very characteristic of the
Paleocene in North America and Europe were large, lizard-like reptiles,
allied to the New Zealand Tuatara, while crocodiles and tortoises
abounded; snakes were present, but do not appear to have been very common.
It is the mammals which were the strangest element of Paleocene life,
and our imaginary observer would find no creature that he had ever seen
before. The difference from modern mammalian life was not merely one of
species, genera or even families, but of orders, for only one, or at most
two, of the orders now living were then to be found in North America, and
both of these (marsupials and insectivores) were primitive and archaic
groups, which seem like belated survivals in the modern world. There
were no rodents, or true carnivores, no lemurs, monkeys, artiodactyls,
perissodactyls or proboscideans.
In the _Torrejon_, or upper Paleocene, there were many herbivorous
marsupials, with very complex grinding teeth and chisel-like incisors,
but no carnivorous or insectivorous members of the order have been found.
Insectivora were present. Of the †creodonts, or primitive flesh-eaters,
there were no less than five families; the bear-like †Arctocyonidæ, which
died out in the Wasatch, were quite numerous, and the problematical
†Mesonychidæ were much smaller and more primitive mammals than those of
the Eocene. Passing over two families which did not survive the Torrejon,
we may note the first of the †Miacidæ, the progressive family which led
eventually to the true Carnivora. The hoofed animals all belonged to
the archaic †Condylarthra and †Amblypoda; of the former there were many
genera and species referable to three families, one of which contained
the forerunners of the Wasatch _†Phenacodus_. The genus _†Pantolambda_ of
the Amblypoda may well have been ancestral to both the †coryphodonts and
the †uintatheres of the Eocene.
[Illustration: FIG. 143.—The Torrejon forerunner (_†Pantolambda
bathmodon_) of _†Coryphodon_. Restored from a skeleton in the American
Museum of Natural History.]
The _Puerco_ fauna was much like that of the Torrejon, but even less
advanced and diversified. The herbivorous marsupials were more abundant,
and some of them (_†Polymastodon_) larger than those of the Torrejon;
Insectivora may have been present, but this is doubtful. The †creodonts,
so far as they have been discovered, were less numerous, varied and
specialized than those of the Torrejon and included but one of the
families which passed over into the Eocene. The †Condylarthra were much
less common and the †Amblypoda but doubtfully represented, but the
edentate-like †Tæniodontia were conspicuous.
[Illustration: FIG. 144.—Head of an †allotherian marsupial
(_†Polymastodon taöensis_) from the Puerco stage. Restored from a skull
in the American Museum of Natural History.]
Not only were the Paleocene faunas radically different from the mammals
of our time, but they could not have been ancestral to the latter, being
hardly more than an advanced and diversified Mesozoic assemblage. It is
true that some of its elements, such as the †Condylarthra, †Amblypoda
and †Creodonta, developed greatly and played an important part in the
life of the Eocene, but of these only a few †creodonts continued into the
Oligocene and all became extinct without leaving any descendants behind
them. Another curious fact concerning the Paleocene mammalian faunas is
that they were made up entirely of small and very small animals; not a
single mammal as large as a sheep has yet been found in these beds, and
the same is true of Europe.
That a land-connection with the Old World existed during the Paleocene
epoch, is indicated by the similarity of the faunas of North America and
Europe.