THE SUCCESSIVE MAMMALIAN FAUNAS OF NORTH AND SOUTH AMERICA
The natural method of telling a story is to begin at the beginning and
go on to the end, but to deal in that manner with the many different
assemblages of mammals which have in turn inhabited the western
hemisphere has the great drawback of beginning with a time when
everything was utterly strange to the modern eye. Could the reader be
carried back to the far distant days of the Paleocene epoch, he would
find himself in a completely unfamiliar world; and there is therefore a
real practical advantage in reversing the story and starting with the end
and thus proceeding gradually from the more to the less familiar. The
foregoing chapter gave a sketch of the more striking and characteristic
mammals which inhabit the Americas to-day, and we may now take a step
backward to the epoch immediately preceding our own, the Pleistocene.
As was shown in Chapter V, the Pleistocene was a time of many and great
climatic vicissitudes, periods of cold, when the northern part of the
continent was buried under great ice-sheets, alternating with far milder
periods, when the climate was much as at present, or even warmer. These
climatic changes necessitated many changes in the distribution of
animals and plants, increasing cold driving them southward, while the
return of more genial conditions permitted the northward migration of
southern forms. The effects of these changes of climate are still plainly
visible in the geographical arrangement of living beings in the northern
continents and many anomalies of distribution, otherwise inexplicable,
are thus made clear. Attention was long ago directed to the fact that
the tops of high mountains support a flora and fauna which, on the
lowlands, will be found only hundreds, or even thousands, of miles to the
northward. The plants which grow on the summits of the White Mountains
of New Hampshire recur in Labrador, but not in the intervening area; the
vegetation and animals of the high Alps are those of the Arctic regions,
and many similar instances might be cited. Hooker and Darwin were the
first to find a highly probable explanation of this curious phenomenon by
referring it to the climatic changes of the Pleistocene epoch. During the
last period of cold and glaciation, the northern plants and animals were
driven far to the south and occupied the lowlands along the ice-front and
well beyond it; when milder conditions gradually returned, the northern
forms not only retreated northward, but also ascended the mountains,
as the latter were freed from ice, and thus became cut off as isolated
colonies. The general explanation of “discontinuous distribution” (see
p. 138) is thus always the same, viz., that the intervening regions were
once occupied by the forms now so widely separated, which, for one reason
or another, have vanished from the connecting areas.
I. QUATERNARY FAUNAS
_North America._—The Quaternary faunas of North America are extremely
difficult to correlate and place in chronological order, because, for
the most part, they are found in locally restricted areas, such as
tar-pools, bogs, caverns and similar places. Professor Osborn has,
however, succeeded in making an admirable arrangement, which, though it
will doubtless be corrected and expanded by future research, represents a
most important advance. Of the general problem he says: “The study of the
mammals of the Quaternary has by no means progressed so far in America as
in Europe; it will be many years before the faunistic succession can be
worked out with such chronologic accuracy and precision as has at last
been attained by European geologists and palæontologists.” According to
Osborn’s arrangement, there are three principal successive Pleistocene
faunas, two of which appear to have coincided with interglacial stages,
and the third with the last reëstablishment of glacial conditions on a
grand scale. Regarding the details of these faunas, there still remains
much uncertainty, and consequently there will be no attempt made here to
do more than discriminate between the general Pleistocene assemblage, on
the one hand, and that of the last cold period, on the other. It must be
emphasized that we are as yet unable to assert that all of the animals
listed together were actually living at the same time.
It is probable that the Pleistocene fossils already obtained give us a
fairly adequate conception of the larger and more conspicuous mammals of
the time, but no doubt represent very incompletely the small and fragile
forms. With all its gaps, however, the record is very impressive; “the
early and mid-Pleistocene life of North America is the grandest and most
varied assemblage of the entire Cenozoic Period [_i.e._ era] of our
continent” (Osborn). There is the further advantage that the fossils
have been gathered over a very great area, extending from ocean to ocean
and from Alaska to Central America. Thus, their wide geographical range
represents nearly all parts of the continent and gives us information
concerning the mammals of the great forests, as well as of the great
plains.
Those divisions of the early and middle Pleistocene which enjoyed milder
climatic conditions had an assemblage of mammals which, from one point
of view, seems very modern, for most of the genera, and even many of the
species, which now inhabit North America, date back to that time. From
the geographical standpoint, however, this is a very strange fauna, for
it contains so many animals now utterly foreign to North America, to find
near relatives of which we should have to go to Asia or South America.
Some of these animals which now seem so exotic, such as the llamas,
camels and horses, were yet truly indigenous and were derived from a long
line of ancestors which dwelt in this continent, but are now scattered
abroad and extinct in their original home, while others were migrants
that for some unknown reason failed to maintain themselves. Others again
are everywhere extinct.
[Illustration: FIG. 113.—Some of the more characteristic Pleistocene
mammals, reduced to a uniform scale, with a pointer dog (in the frame)
to show relative sizes.—1. †Columbian Elephant (_Elephas †columbi_).
2. Giant †Ground-Sloth (_†Megalonyx jeffersoni_). 3. †Stag-Moose
(_†Cervalces scotti_). 4. †American Mastodon (_†Mastodon americanus_).
5. †Giant Beaver (_†Castoroides ohioensis_). 6. †Texas Horse (_Equus
†scotti_). 7. †Sabre-tooth Tiger (_†Smilodon californicus_).]
Most surprising, perhaps, in a North American landscape, is the presence
of the Proboscidea, of which two very distinct kinds, the †mastodons and
the true elephants, are found together. Over nearly the whole of the
United States and southern Canada, and even with sporadic occurrence in
Alaska, ranged the †American Mastodon (_†Mastodon americanus_) which was
rare in the plains, but very abundant in the forested regions, where it
persisted till a very late period and was probably known to the early
Indians. This animal, while nearly related to the true elephants, was
yet quite different from them in appearance, as will be immediately
seen on comparing 1 and 4, Fig. 113, p. 195. The most obvious external
difference was the comparative shortness of the legs in the †Mastodon,
which did not exceed and seldom attained a height of 9 ft. 6 in. at the
shoulder; the head also was lower and more flattened. The teeth were
very different from those of the elephants; the grinding teeth were much
smaller and simpler, being low-crowned and rooted and having three or
four high, transverse, enamel-covered crests, without cement. The tusks
were elephant-like except that in the male there was a single small tusk
in the lower jaw, which cannot have been visible externally; this is a
remnant of an earlier stage of development, when there were two large
tusks in the lower as well as the upper jaw. The creature was covered
with long, coarse, dun-coloured hair; such hair has been found with some
of the skeletons.
Of true elephants, the North American Pleistocene had three species.
Most interesting of these is the northern or Siberian †Mammoth (_Elephas
†primigenius_), a late immigrant from northern Asia, which came in by
way of Alaska, where Bering Land (as we may call the raised bed of Bering
Sea) connected it with Asia. The †Mammoth was abundant in Alaska, British
Columbia and all across the northern United States to the Atlantic coast.
Hardly any fossil mammal is so well known as this, for the carcasses
entombed in the frozen gravels of northern Siberia have preserved every
detail of structure. It is thus definitely known that the †Mammoth was
well adapted to a cold climate and was covered with a dense coat of wool
beneath an outer coating of long, coarse hair, while the contents of the
stomach and the partially masticated food found in the mouth show that
the animal fed upon the same vegetation as grows in northern Siberia
to-day. The grinding teeth were very high, cement-covered, and composed
of many thin plates of enamel, dentine and cement, and were closely
similar to those of the existing Indian Elephant (_E. maximus_). In size
this is the smallest of the three Pleistocene species, 9 feet at the
shoulder. The †Mammoth was not peculiar to Siberia and North America, but
extended also into Europe, where it was familiar to Palæolithic Man, as
is attested by the spirited and lifelike carvings and cave-paintings of
that date. Thus, during some part of the Pleistocene, this species ranged
around the entire northern hemisphere.
Closely related to the †Mammoth and in some cases hardly distinguishable
from it, is the †Columbian Elephant (_E. †columbi_) which, however,
attained a considerably larger size, as much as 11 feet, rivalling the
largest African elephants of the present time. The head was very high
and had a curiously peaked appearance, and the tusks in old males curved
inward, overlapping at the tips. From the likeness in teeth and skeleton
to the †Mammoth, it may be inferred, though somewhat doubtfully, that
the †Columbian Elephant was clothed with hair, but not so heavily as the
†Mammoth, which was a northern species, the Columbian form replacing
it southward, and ranging over the whole United States, including
Florida and even throughout the table-land of Mexico. The areas of the
two species overlapped along the northernmost United States, but are
elsewhere distinct.
[Illustration: FIG. 114.—Restoration of the †Columbian Elephant (_Elephas
†columbi_) from a skeleton in the American Museum of Natural History.]
A third species was the huge †Imperial Elephant (_E. †imperator_), the
largest of American forms, to which Osborn’s calculations give the almost
incredible height of 13 ft. 6 in. This great creature was characterized
not only by its enormous stature, but also by the proportionately very
large size of its grinding teeth, and was a survivor from the preceding
Pliocene epoch; it is not known to have passed beyond the middle
Pleistocene and was thus the first of the species to become extinct. In
geographical range, the †Imperial Elephant was a western form, extending
from the Pacific coast almost to the Mississippi River, east of which it
has never been found, and from Nebraska southward to the City of Mexico.
The meaning of this distribution is probably that this elephant shunned
the forests and was especially adapted to a life on the open plains. Over
most of its area the winters were severe, and this fact makes it likely
that the animal was clothed with hair, but nothing is definitely known on
this point.
[Illustration: FIG. 115.—A Horse (_Equus †scotti_) from the older
Pleistocene of Texas. Restored from a skeleton in the American Museum of
Natural History.]
Many other hoofed animals, far more than now inhabit North America, are
found in this Pleistocene fauna. The Perissodactyla were represented by
horses and tapirs, but not by rhinoceroses; it might seem superfluous
to say that there were no rhinoceroses, but, as a matter of fact, that
family had a long and varied American history and became extinct only
during or at the end of the Pliocene epoch. The horses were extremely
numerous, both individually and specifically, and ranged, apparently in
great herds, all over Mexico and the United States and even into Alaska.
All the known species (at least ten in number) belong to the genus
_Equus_, but the True Horse (_E. caballus_), to which all the domestic
breeds are referred, is not represented. The smallest known member of
the genus is the pygmy _E. †tau_ of Mexico. _E. †fraternus_, likewise a
very small species, is found especially in the southeast, but extended
as far north as Pennsylvania and west to Nebraska. On the other hand,
_E. †giganteus_ of Texas exceeded the heaviest modern draught-horses
in size and was the largest of the American species; of other Texan
forms, one (_E. †scotti_) resembled Burchell’s Zebra (_E. burchelli_)
in the proportions of head and neck, body and limbs, while another (_E.
†semiplicatus_) was more ass-like. The forest horse of the eastern states
has been named _E. †pectinatus_, an animal of moderate size. The Great
Plains must have been fairly covered with enormous herds of horses, the
countless bones and teeth of which, entombed in the Sheridan formation,
have given to it the name of “Equus beds.” The most abundant of the
plains species is _E. †complicatus_, a horse of about 14½ hands in height
(_i.e._ 4 feet 10 inches at the shoulder) which also ranged down the
Mississippi Valley nearly or quite to the Gulf of Mexico. In California
was _E. †occidentalis_, equalling _E. †complicatus_ in size, but with
much more simple teeth, and associated with it the much larger _E.
†pacificus_, which was inferior only to _E. †giganteus_ and therefore the
second largest of the American Pleistocene horses.
To one who knows nothing of the geological history of North America
it would be natural to suppose that the Pleistocene horses must
have been immigrants from the Old World, which failed to establish
themselves permanently here, since they completely disappeared before
the discovery of the continent by Europeans. This would, however, be a
mistaken inference, for North America was for long ages the chief area
of development of the equine family, which may here be traced in almost
unbroken continuity from the lower Eocene to the Pliocene. On the other
hand, it is quite possible that some of the species were immigrants.
Tapirs, which are now confined to southern Asia, Central and South
America, were not uncommon in the forested parts of eastern North America
as far north as Pennsylvania, but they have not been found west of the
Mississippi in the plains region. Two species are known, a larger and
heavier one, _Tapirus †haysii_, and a smaller one which seems to be
identical with the living _T. terrestris_ of Central and South America.
Like the horses, the tapirs had a long history of development in North
America and may well have originated here, but they withdrew from the
continent in the Pleistocene, probably yielding to the last of the
glacial advances.
There was likewise a much greater variety of Artiodactyla than North
America can boast at the present day; some were autochthonous, but, for
the most part, they were migrants from the eastern hemisphere, where the
great group of the true ruminants (Pecora) passed through the greater
part of its development and where its headquarters still are. Indigenous
were the peccaries, or American swine, which still occur from Texas south
to Brazil. In Pleistocene time they ranged over nearly all of the United
States, as far northward as Pennsylvania, and across the plains to the
Pacific coast; they were represented by two genera, now extinct, one of
which (_†Platygonus_) had crested grinding teeth and much longer legs
than the modern peccaries. Another indigenous group, strange as that may
seem, is the suborder (Tylopoda) of the camels and llamas, both of which
are represented in the North American Pleistocene, the descendants of a
very long American ancestry. Some of these tylopodans were far larger
than existing forms, and at least one species extended its range to
Alaska.
Of ultimately Old World origin, but through a considerable line of
descent in America, were the typically American deer (_Odocoileus_)
of which the Virginian and Black-tailed species are familiar modern
instances. Whether or not the Old World types, the Caribou (_Rangifer_)
and Wapiti (_Cervus canadensis_) had reached the western hemisphere,
is a matter of some doubt; if present at all, they must have been
comparatively rare. The Moose (_Alce americanus_), on the other hand,
had already appeared, but seems to have been confined to the western
half of the continent, its presence in the east being questionable. The
mistakenly named “Rocky Mountain Goat” (_Oreamnos montanus_), which is
an antelope of the chamois group, was an apparently late arrival in the
Pleistocene, while the peculiar Prong-Buck (_Antilocapra americana_),
which is very different from any of the Old World antelopes, was
present in the early part of the epoch. The descent of this remarkable
animal is still a problem, but not improbably it was derived from the
“deer-antelopes” of the Miocene and Pliocene, the last of which occurred
in the early Pleistocene. Mr. Gidley has announced the surprising
discovery in Maryland of a large antelope hardly distinguishable from the
African Eland (_Taurotragus_). Other late arrivals from the Old World
were several forms allied to the existing Musk Ox (_Ovibos_), at least
two genera of which (_†Preptoceras_ and _†Euceratherium_) have been found
in California. A surprising number of species of _Bison_ occurred in the
Pleistocene, no less than seven of which are recognized as distinct,
ranging from Florida to Alaska. It is not likely that all these species
coexisted at the same time, but we cannot yet determine their order of
succession, though the modern species, _B. bison_, was probably the
latest to arise. Most of these species were much larger than _B. bison_,
and some were gigantic, such as _B. †latifrons_, which had a spread of
horns of 6 feet and is found through the Mississippi Valley, and _B.
†crassicornis_ of Alaska.
[Illustration: FIG. 116.—Restoration of _†Preptoceras_, a musk-ox like
animal from the Californian Pleistocene. (From a skeleton in the museum
of the University of California.)]
Preying upon this great assemblage of hoofed animals was a corresponding
array of Carnivora, most of which were indigenous and derived from
American stocks, but there was a considerable migrant element also,
such as the bears and badgers. Nearly all the modern kinds of
flesh-eaters found in the North America of to-day were already here
in the Pleistocene, minks, weasels, martens, skunks, otters, badgers,
wolverenes, raccoons, foxes, wolves, coyotes, pumas, etc., etc., but
there were several others which are either now extinct or no longer
to be found in this continent. Of the extinct types much the most
striking were the several species of †sabre-tooth tigers (_†Smilodon_,
see Frontispiece) which have been found in the greater part of the
United States and no doubt ranged over the whole. These were massive,
short-tailed and rather short-legged, but very muscular and powerful,
cat-like animals, in which the upper canine teeth were converted into
great, recurved, scimitar-like tusks. These large beasts of prey, which
about equalled the Leopard in height, but were far heavier, belonged
to a group which, at one time or another, spread over nearly the whole
world and persisted much later and attained a larger size and higher
development in the western hemisphere than in the eastern. They had a
very long American ancestry, from the lower Oligocene to the end of
the Pleistocene, but the place of their origin is still unknown. In
addition to the pumas and lynxes, there were some very large true felines
(_Felis †atrox_ and _F. †imperialis_), which closely resembled the Lion
(_F. leo_) in size, appearance and structure, and have been found in
California and the Mississippi Valley; probably these great cats were
immigrants, but they may represent a native development of Miocene and
Pliocene stock; the history of the family is too imperfect for a decision
of this question.
Besides coyotes and wolves which are indistinguishable from existing
species, there were some very large wolves, now extinct, of which the
commonest and most widely distributed was _Canis †dirus_ (also called _C.
†indianensis_) so abundant in the asphalts of southern California. Bears
were not so common in the middle Pleistocene and have not been found in
the older part of that epoch, though they probably had already reached
North America from the Old World, where they originated. Their absence
from the older Pleistocene (Equus Beds) may be accounted for by the fact
that those beds contain a fauna of the open plains, while bears are
chiefly forest-living animals. An extinct type of the family is the group
of species which constitute the †short-faced bears (_†Arctotherium_),
very large and powerful creatures, with remarkably shortened jaws, which
have been found from ocean to ocean. The smaller beasts of prey, badgers,
weasels, etc., were, as intimated above, substantially the same as now.
The rodents of the Pleistocene were very nearly in their modern stage
of development, most of the genera and many of the species surviving
to present times. Just what members of the order were introduced from
the Old World, the imperfect and fragmentary history will not permit
us to say, but some interesting South American immigrants should be
noted. One of these, the Capybara or so-called Water-Hog (_Hydrochærus
capybara_), the largest of existing rodents, failed to gain a permanent
foothold, but another South American form, the Short-tailed or Canada
Porcupine (_Erethizon dorsatus_), common all over the United States in
the Pleistocene, has maintained itself to the present day. One especially
peculiar form, not derived from South America or the Old World, is the
†Giant Beaver (_†Castoroides_), one species of which, _†C. ohioensis_,
was as large as a Black Bear and occurred in the later Pleistocene, while
a smaller species (_†C. species indet._) is found in the more ancient
deposits of the epoch. In almost all respects _†Castoroides_ was simply
a gigantic beaver, but the grinding teeth were remarkably like those of
the South American Capybara (_Hydrochærus_), so much so that it has been
mistakenly referred to the same family by some authorities.
By far the strangest elements of the Pleistocene faunas were the two
suborders of gigantic edentates, the _†Gravigrada_, or †ground-sloths,
and the _†Glyptodontia_, which might well be called giant armadillos,
if that name were not already in use for a living Brazilian animal.
Both suborders are completely extinct, but they long played a very
conspicuous rôle in South America, where they originated and whence the
North American representatives migrated. The †ground-sloths were great,
unwieldy, herbivorous animals covered with long hair, and in one family
(†Mylodontidæ) there was a close-set armour of pebble-like ossicles in
the skin, not visible externally; they walked upon the outer edges of
the feet, somewhat as the Ant-Bear (_Myrmecophaga_) uses his fore paws,
and must have been very slow-moving creatures. Their enormous claws may
have served partly as weapons of defence and were doubtless used also
to drag down branches of trees and to dig roots and tubers. Apparently,
the latest of these curious animals to survive was the very large
_†Megalonyx_, which, it is interesting to note, was first discovered and
named by Thomas Jefferson. The animals of this genus were very abundant
in the forests east of the Mississippi River and on the Pacific coast,
much less common in the plains region, where they would seem to have
been confined to the wooded river valleys. The still more gigantic
_†Megatherium_, which had a body as large as that of an elephant and much
shorter, though more massive legs, was a southern animal and has not
been found above South Carolina. _†Mylodon_, smaller and lighter than
the preceding genera, would seem to have entered the continent earlier
and to have become extinct sooner; it ranged across the continent, but
was much commoner in the plains region and less so in the forested areas
than _†Megalonyx_, being no doubt better adapted to subsisting upon the
vegetation of the plains and less dependent upon trees for food.
The †Glyptodonts were undoubtedly present in the North American
Pleistocene, but the remains which have been collected so far are very
fragmentary and quite insufficient to give us a definite conception of
the number and variety of them. It will be better therefore to defer the
description of these most curious creatures until the South American
Pleistocene is dealt with, as they were incomparably more varied and
characteristic in that continent. In North America they have been found
only in Mexico and the southern United States.
The many and great climatic changes which took place in the Pleistocene
led to very extensive migrations of mammals from one part of the
continent to another, as the conditions of temperature and moisture
changed. In Interglacial stages, when the climate was much ameliorated,
southern species spread far to the north, as when the †Mastodon ranged
into Alaska, and the Manatee, or Sea-Cow, of Florida waters, came up the
coast to New Jersey, while the increasing cold of oncoming glaciation
caused a reverse movement and drove northern and even Arctic forms far
to the south. Thus, the Musk-Ox, the Caribou and the northern †Mammoth
came south beyond the Ohio and the Potomac, and the Walrus was found on
the South Atlantic coast. It is these migrations which give such a mixed
character to the Pleistocene faunas from the climatic point of view, as
it is often very difficult to correlate or synchronize the fossiliferous
deposits with the Glacial and Interglacial stages, though this has been
definitely accomplished in several very important instances.
The latest of the Pleistocene faunas is less completely known than those
of the earlier and middle portions of the epoch, for but few localities
have yet been discovered with any extensive series of fossils. As worked
out by Osborn, this fauna coincided with the last Glacial stage and was
a greatly reduced and impoverished assemblage as compared with those of
the middle and lower Pleistocene, though it is not safe to argue that all
the animals not found in this fauna were already extinct, for the known
list is still far too short to be entirely representative. The American
†Mastodon (_†Mastodon americanus_, see p. 196) was still abundant in
the forested regions and was apparently able to withstand severe winter
temperatures, as certainly was the †Mammoth (_Elephas †primigenius_, see
p. 196), which was so abundant in the coldest part of Siberia and which
extended south to the Potomac, presumably at this time. Horses were
still present in North America, though apparently in greatly diminished
numbers and variety. Tapirs have not been found, though they may have
lingered on in the southern regions. The typically North American genus
of deer (_Odocoileus_) was, of course, well represented, and Old World
types had a much more southerly distribution than at present. The Caribou
(_Rangifer caribou_) came down into Pennsylvania and Ohio, the Moose
(_Alce americanus_) into Kentucky and Kansas, and the Wapiti (_Cervus
canadensis_) is reported as far south as Florida. A very remarkable
animal is the Stag-Moose (_†Cervalces scotti_), the best preserved
skeleton of which is that in the museum of Princeton University. This was
found in a shell-marl beneath a peat-bog at Mt. Hermon, N. J., _north of
the great terminal moraine_, and therefore most probably this particular
individual dates from a time not earlier than the beginning of the final
retreat of the ice.
_†Cervalces_, as its name implies, was in some respects intermediate
between the Stag (_Cervus_) and the Moose (_Alce_); in general
proportions it most nearly resembled the latter, having a short neck,
long body and very long legs; but the skull differed in many respects
from that of the Moose, especially in parts which show that the great,
inflated snout and prehensile upper lip had no such development in the
extinct as in the living form. The antlers were unique among the known
members of the deer family, resembling those of the Moose, though much
less palmated and with the addition of great trumpet-shaped plates.
The feet were large, almost as large as in the Caribou, and the whole
structure indicates an animal well fitted to travel through deep snows
and flourish in severe winters.
[Illustration: FIG. 117.—_†Cervalces scotti_: restored from a skeleton
in the museum of Princeton University.]
Even more typically northern than the Caribou were the Musk-Oxen,
of which two genera occurred in the late Pleistocene. One of these,
_†Symbos_, is extinct and was characterized by its short horns; the
other, _Ovibos_, is the genus to which the existing species, _O.
moschatus_ and _O. wardi_, belong and is now confined to the extreme
north of the continent, the Arctic islands and Greenland. The remains of
Musk-Oxen have been found mostly along the great terminal moraine which
marks the front of the last ice-invasion, but they occurred also as
far south as Oklahoma, and in Utah they ranged far to the south of the
ice-front. Nothing could be more conclusive evidence of a climate much
colder than the modern one than the presence of Caribou and Musk-Oxen in
the United States and of the Walrus on the coast of Georgia.
The smaller animals were much as they are now, differing only in range.
The †sabre-tooth tigers, the last of a most interesting line, persisted
in the south, and an extinct genus of skunks has been discovered in
Arkansas, but otherwise the Carnivora were entirely modern in character.
Unfortunately, these smaller animals are very incompletely known, much
the richest aggregation which has yet been found being that collected by
Mr. Brown in the Conard Fissure, Arkansas. From this collection Mr. Brown
has described thirty-seven genera and fifty-one species of mammals, of
which four genera and twenty-four species are extinct. That is to say,
less than one-ninth of the genera and one-half of the species represent
extinct forms. Contrast this with the middle Pleistocene assemblage found
in the Port Kennedy cavern in eastern Pennsylvania, of sixty-four species
with at least forty extinct ones.
The foregoing sketch, brief and imperfect as it necessarily is, makes
it sufficiently plain that North America during the Pleistocene was far
richer in mammalian life than it was when the continent was first settled
by Europeans. When we make the proper allowance for the many forms which
undoubtedly remain to be discovered and for those which may have vanished
without leaving a trace behind them, the contrast becomes all the more
striking. Not only did Pleistocene North America have substantially all
the mammals that it now possesses, but it had many more. The lions and
†sabre-tooth tigers, the gigantic †short-faced bears, the tapirs and
many varieties of horses, large and small, the camels and llamas, many
species of bisons, some of enormous proportions, several forms allied to
the Musk-Ox, the elephants and †mastodons, the †giant beavers and South
American water-hogs, the huge †ground-sloths and †glyptodonts, have all
disappeared, leaving a continent, that, by contrast, is “zoölogically
impoverished.” The Pleistocene fauna was strangely mixed in character,
the free roads of migration bringing together Old World and South
American types, and mingling them with indigenous forms in a cosmopolitan
assemblage.
Turning to _South America_, we find in the pampas of Argentina a
wonderful museum of Pleistocene mammals, such as occurs nowhere else in
the known world, and this is supplemented by the very rich collections
gathered from the caverns of Brazil and from deposits of Ecuador and
Bolivia, and thus all the important regions of the continent, save the
far south, are well represented. These faunas are far stranger than the
corresponding ones of North America and differ more radically from those
of modern times, since they include a much larger proportion of extinct
types, and the extinctions have swept away not only species and genera,
but families and orders as well.
The South American Pleistocene assemblage of mammals is very clearly
divisible into two elements: (1) the immigrants from the north, which
reached the southern continent in successive waves of migration, that
have left records of themselves as early as the older Pliocene, perhaps
even the upper Miocene, and (2) the indigenous element, which had a very
long history of development in South America. To the immigrant class
belonged all of the Carnivora, which therefore resembled their North
American relatives, but were less varied in character. Of the bears,
only the huge, †short-faced kind (_†Arctotherium_, Fig. 275, p. 549)
are known, and it is not likely that true bears existed except in the
Andes, as is also the case to-day. Of the cat family, the †sabre-tooth
tigers (_†Smilodon_) were as common in South America as in North, and,
while there were no lions, there were large cats nearly allied to the
Jaguar and Puma, and smaller ones, like the Ocelot. The dogs were quite
numerously represented by species resembling closely the existing South
American fox-like wolves and the Bush-Dog (_Icticyon_) and, strange
to say, by one which seems referable to the same genus (_Cyon_) as
the Dhole of India. The weasel family (Mustelidæ) were less numerous
and varied than in the northern continent, as they still are; coatis
(_Nasua_) and raccoons (_Procyon_) were abundant and one species of the
latter was much larger than any existing one; extinct species of skunk
(_Conepatus_), tayra (_Tayra_) and otter (_Lutra_) were also present, but
the badgers, minks, martens and wolverenes were not.
[Illustration: FIG. 118.—Some of the commoner Pampean mammals, reduced
to a uniform scale, with a pointer dog (in the frame) to show the
relative sizes. 1. _†Dœdicurus clavicaudatus._ 2. _†Glyptodon clavipes_,
†glyptodonts. 3. _†Macrauchenia patachonica_, one of the †Litopterna.
4. †Pampas Horse (_†Hippidion neogæum_). 5. _†Toxodon burmeisteri_,
a †toxodont. 6. _†Megatherium americanum_. 7. _†Mylodon robustus_,
†ground-sloths.]
The hoofed animals were represented by a great variety of forms, both
immigrant and indigenous, of which the latter belonged to orders now
entirely extinct. Horses were common in all parts of the continent, where
fossils of this epoch have been obtained, and are referable to two very
distinct groups: (1) to the typical genus _Equus_, of which three species
have been described, all somewhat more primitive than the True Horse (_E.
caballus_) and, like most of the Pleistocene species of North America,
with a certain resemblance to the zebras and asses; (2) to an extinct
group of four genera, the best known of which is _†Hippidion_. The
species of this genus (which has also been reported from North America,
though upon hardly sufficient evidence) had most exceptional characters
in the skull, and the head was relatively large and clumsy, with narrow
and very high facial region. The neck was comparatively short, the limbs
heavy and the feet short. These animals can hardly have been very swift
runners. A very interesting member of this group is _†Hyperhippidium_, a
small horse found in the Andes, with remarkably short feet, well adapted
for a mountain life. The only other perissodactyls were tapirs, which
ranged down to the Argentine pampas, much farther south than now.
[Illustration: FIG. 119.—A Pampas Horse (_†Hippidion neogæum_). Restored
from a skeleton in the National Museum, Buenos Aires.]
The Artiodactyla were much more varied; there were peccaries, many
species of llamas, which then extended into Brazil, and were not
confined, as at present, to the colder portions of the continent.
There were also numerous deer, all of the South American type, and
two different antelopes have been reported, though that family has
no representatives in the southern continent now. Several species of
†mastodons have been found in Brazil, Argentina, Bolivia and elsewhere,
but none of the true elephants. Why the †mastodons were able to make
their way into South America, while the elephants were not, is one of the
puzzling questions of mammalian distribution to which no answer can be
given.
All the preceding types of hoofed animals, the horses, tapirs, peccaries,
llamas, deer, antelopes and †mastodons were migrants from the north, and
four of these, tapirs, peccaries, llamas and deer, were able to gain a
permanent footing in South America and are more or less abundant there
to-day, while the horses, antelopes and †mastodons failed to do so and
died out. In addition to these, there were the indigenous types, which
are now extinct and have never been found outside of the Neotropical
region. An extremely peculiar creature, _†Macrauchenia_, was the last
surviving member of an order, the †Litopterna, which for ages played
a very important rôle in South America. _†Macrauchenia_ was a large
animal, somewhat larger and of much heavier build than a camel, to which
it had a considerable, though entirely superficial, resemblance. The
head was relatively small and must have had quite a long proboscis; the
neck was very long, suggesting that the animal browsed upon trees, which
is also indicated by the character of the teeth; the legs were long
and stout, the feet short and each provided with three toes. Another
curious creature was _†Typotherium_, from which is named the group of the
†Typotheria, which some authorities regard as a suborder, while others
assign to it a full ordinal rank.
[Illustration: FIG. 120.—A Pampean †Litoptern (_†Macrauchenia
patachonica_). Restored from a skeleton in the Museum of La Plata.]
The †Typotheres throughout the Tertiary period were among the most
abundant and characteristic of the South American hoofed animals, and the
genus _†Typotherium_ was the last of a very long series and was an animal
of moderate size, with chisel-shaped incisor teeth so like those of the
rodents that the genus was long referred to that order. Finally, we have
_†Toxodon_, type of the order †Toxodontia, a ponderous beast, as large as
a rhinoceros, which, there is some reason to think, was largely aquatic
in its habits. The first species of this extraordinary creature was found
by Charles Darwin, who says of it: “Perhaps one of the strangest animals
ever discovered; in size it equalled an elephant or megatherium, but the
structure of its teeth, as Mr. Owen states, proves indisputably that
it was intimately related to the Gnawers [_i.e._ Rodentia] ... in many
details it is allied to the Pachydermata: judging from the position of
its eyes, ears, and nostrils, it was probably aquatic, like the Dugong
and Manatee, to which it is also allied.”[4] Modern views concerning
the relationships of _†Toxodon_ are very different from those advanced
by Darwin, but he gives a vivid picture of its diverse likenesses.
Neither _†Macrauchenia_, _†Typotherium_ nor _†Toxodon_ has been found
in the Brazilian caverns, but this is no doubt due to the accidents of
preservation, for the latter animal ranged north to Nicaragua.
[Illustration: FIG. 121.—A Pampean †Toxodont (_†Toxodon burmeisteri_).
Restored from a skeleton in the La Plata Museum.]
The rodents likewise were partially of immigrant and partially of native
stock. To the former belonged the few mice and rats and a meadow-mouse
(_Microtus_), a group not represented in present-day South America, and
a rabbit. Very much more abundant and varied were the indigenous forms,
all of which belonged to existing families and most of them to existing
genera; the tree-porcupines, cavies, agoutis, spiny-rats, vizcachas,
capybaras, coypus, etc., were abundantly represented, for the most part
by extinct species.
The monkeys were of purely Neotropical type and several modern genera,
such as _Cebus_ and _Callithrix_, and one very large extinct genus,
_†Protopithecus_, of the same family, have been found in the caverns of
Brazil, but not in the pampas of Argentina, which would seem to have been
a country of open plains.
In the South America of to-day one of the most striking and peculiar
elements of the fauna is that formed by the Edentata, the sloths,
anteaters and armadillos, and this was even more true of the same region
in Pleistocene times. Anteaters and sloths are very scantily represented,
but this is merely an accident of preservation; armadillos, on the
other hand, were very numerous both in Brazil and in Argentina, and, in
addition to many modern genera, there were several which are no longer in
existence, such as _†Chlamydotherium_, which was a huge creature almost
as large as a rhinoceros. Then there were the two extinct suborders of
the †glyptodonts (†Glyptodontia) and the †ground-sloths (†Gravigrada)
which were astonishingly abundant in Argentina and which, as was shown in
a previous page (p. 205), were also well represented in North America.
Few more fantastic-looking mammals than the †glyptodonts have ever been
found; the short, deep head, with its shield of thick, bony plates, the
huge carapace made up of innumerable plates of bone firmly united at
their edges and without the movable bands of the armadillo carapace, the
enormous tail-sheath, the short legs and massive feet with broad hoofs,
must have given these animals rather the appearance of gigantic tortoises
than of mammals. The †glyptodonts were especially numerous and varied
in the Argentine pampas, and a stately array of them is mounted in the
museums of La Plata and Buenos Aires; in length, they ranged from six to
twelve feet, including the tail. The skeleton and carapace did not differ
very greatly in appearance among the various genera, but there were great
differences in the form and size of the bony sheath enclosing the tail.
In the genus _†Glyptodon_ the sheath was composed throughout of movable
overlapping rings, with prominent spines on them; in _†Sclerocalyptus_
the hinder half of the sheath coalesced into a single piece, marked only
by the elaborate ornamentation of the horny scales, while in _†Dœdicurus_
the end had a tremendous, club-like expansion, which must have been set
with great horn-like spines. The †glyptodonts were ponderous, slow-moving
and inoffensive plant-feeders, almost invulnerable to attack, and
probably used their massive tails, which could be freely swung from side
to side, as redoubtable weapons of defence, much as the alligator uses
his tail. In comparison with the bewildering variety in South America,
the few that made their way into North America were quite insignificant.
Much the same statement applies to the †ground-sloths, and though these
ranged far more widely through the northern continent than did the
†glyptodonts, they were but few in comparison with the multitude which
inhabited alike the forests of Brazil and the plains of the south. Two
of the three genera of †ground-sloths which occur in the North American
Pleistocene, _†Megatherium_ and _†Mylodon_, are also found in South
America; and though _†Megalonyx_ has not yet been obtained there, the
family of which it is a member was represented. In size, these creatures
varied from a tapir to an elephant, though all were much shorter-legged
than any elephant; the extremely massive tail, which the larger forms
had, served to support the huge body, when erected to tear down the
branches and leaves upon which these strange creatures fed.
[Illustration: FIG. 122.—A gigantic Pampean †Ground-Sloth (_†Megatherium
americanum_). Restored from a skeleton in the Museum of La Plata.]
Opossums were extremely numerous, especially in the Brazilian caves,
where in half a cubic foot of earth 400 jaws were collected.
The Pleistocene mammalian fauna of South America was a mixture of modern
forms with ancient, vanished types similar to that which we found in
North America. The †ground-sloths and †glyptodonts, the †litopterns,
†toxodonts and †typotheres, the antelopes, horses and †mastodons have all
disappeared from the continent, or vanished altogether from the face of
the earth.
II. TERTIARY FAUNAS
1. _Pliocene_
_North America._—No part of the Cenozoic history of North America is
so imperfectly recorded and so unsatisfactorily known as that of the
Pliocene, and the later portion of that epoch is especially obscure. If
the Peace Creek formation of Florida is properly referred to the upper
Pliocene, it would show that the mammals of that time were substantially
the same as those of the early Pleistocene.
The only fauna, as yet discovered, which can be referred to the middle
Pliocene, is that of the Blanco beds of northwestern Texas, which have
yielded but a scanty list of mostly ill-preserved fossils. Obviously,
these give us a very incomplete picture of the life of that time. The
great †ground-sloths had already reached North America, and the genus
_†Megalonyx_, so common in the forested areas of Pleistocene North
America, was perhaps already in existence. The †glyptodonts were likewise
represented by one genus (_†Glyptotherium_) which was distinguished
by the simple rings of the tail-sheath. No rodents have yet been found
and only a few of the Carnivora, though a large cat, a musteline and a
large “†bear-dog” are known. There were no true elephants, but several
species of †mastodons, all of which were different from those of the
Pleistocene; and in some, grinding teeth, though still low-crowned, had
become much larger and more complex, marking a stage of advance toward
the elephantine dentition. Horses of primitive type, the feet having
three functional toes instead of one, were relatively abundant. Very
large llama-like animals were present, but nothing has been ascertained
with regard to the deer and antelopes of the time, and the only
other representative of the Artiodactyla yet recovered is a peccary,
interesting as being a species of the genus (_†Platygonus_) which became
so abundant and widespread in the Pleistocene. Scanty and incomplete as
this fauna is, it suffices to show that the middle Pliocene mammals were
much more primitive than those of the Pleistocene.
[Illustration: FIG. 123.—†Horned Gopher (_†Epigaulus hatcheri_), lower
Pliocene, Nebraska. Restored from a skeleton in the U.S. National Museum.]
The fauna of the Snake Creek formation in western Nebraska and that
of the presumably somewhat later beds of northwestern Nevada, which
are referable to the lower Pliocene, may be considered together. The
rodents, which are not very fully represented, were quite modern in
character and belonged mostly to extinct species of modern genera, such
as hares, pocket-gophers, beavers, forerunners of the †Giant Beaver,
marmots, sewellels, etc. A remnant of a more ancient world, especially
characteristic of the Miocene, is found in the remarkable burrowers,
the horned †mylagaulids which have been extinct since the lower
Pliocene. Carnivora were abundant, and members of all the families which
inhabit North America to-day have been obtained; wolves, “†bear-dogs,”
“†hyena-dogs” and forms like the Dhole of India were common. The terms
“†bear-dogs” and “†hyena-dogs” are not to be understood as implying any
relationships of these animals to bears or hyenas, but merely a certain
superficial resemblance; these were very large members of the dog family
(Canidæ), now extinct. Mustelines, large and small, are found, and
possibly some bears had already made their way from the Old World, but
this is still uncertain. †Sabre-tooth tigers and true cats, some as large
as lions and one species fairly gigantic, were likewise characteristic
of the time. There was a great wealth of horses, though the modern genus
_Equus_ was not among them; all the genera are now extinct and all were
three-toed. Several distinct phyla were represented, some progressive and
advancing toward the modern forms, others conservative and stationary.
Browsing horses with low-crowned teeth, grazing horses with prismatic,
cement-covered teeth, heavier and lighter, larger and smaller, must have
covered the plains and thronged the woods. Ancestral tapirs were present,
though far less common. A family which seems to be utterly exotic to
North America, that of the rhinoceroses, was present, and of these there
were three or four series, mostly without horns, or with a very small
horn on the tip of the snout. The extremely aberrant perissodactyls
(†Ancylopoda), in which the hoofs were converted into great claws,
perhaps persisted, but the evidence is not conclusive.
The Artiodactyla were, for the most part, totally different from those
of modern times, though several forms were ancestral to some now
living. Peccaries more primitive than the living genus were the only
representatives of the swine-like suborder; ancestral camels and llamas
were among the commonest of the hoofed animals and an extinct phylum,
that of the “†giraffe-camels” (_†Alticamelus_) continued over from the
Miocene. The giraffe-camels are so called, not because of any actual
relationships with the giraffes, but on account of certain likenesses
in the proportions of the animals compared. _†Alticamelus_ was a very
large, camel-like creature, with remarkably elongate neck and limbs
and comparatively small head, which no doubt resembled the giraffes in
browsing upon trees which were above the reach of the ordinary camels and
llamas of the time. It was the terminal member of a series, or phylum,
which branched off from the main stock in Oligocene times and pursued a
course of development which was independent of the principal series, but
curiously parallel with it.
The deer of the lower Pliocene were little, graceful creatures
(_†Blastomeryx_) which had no antlers, but the males were armed with
sabre-like upper canine tusks, so that they must have resembled the
Musk-Deer of Tibet, but were smaller and more slender. The remarkable
group of “†deer-antelopes,” now extinct, was represented by _†Merycodus_,
a dainty little creature, less than two feet high at the shoulder,
which had the antlers and general appearance of a small deer, but the
high-crowned grinding teeth which most antelopes have. True antelopes
of two different lines were also present, though they are as yet known
from little more than the bony horn-cores; of these, one is the
flat-horned and the other the twisted-horned or strepsicerine type, such
as is illustrated by the Eland and Kudu of modern Africa. The latter
may, however, be related to the peculiarly North American Prong-Buck
(_Antilocapra_) and not to the strepsicerine antelopes of the Old
World. The last survivors of an exclusively North American family, the
†oreodonts, which were wonderfully numerous and varied from the upper
Eocene onward, are found here.
The †mastodons (_†Gomphotherium_) of this formation had well-developed
tusks in the lower as well as in the upper jaw, and in one species the
chin-region or symphysis of the lower jaw was greatly prolonged, an
ancient feature.
That the South American edentates had already reached the northern
continent is sufficiently proved by remains of †ground-sloths, which
are, however, too incomplete to permit identification of the genus.
†Glyptodonts have not yet been found, but this fact does not demonstrate
that they had not accompanied the †ground-sloths in their migration,
for at no time did they range so far north as Nebraska or northwestern
Nevada, and the only mammal-bearing formation of lower Pliocene date
known in the south, the Alachua Clay of Florida, has yielded too scanty a
list of fossils to make its negative evidence at all conclusive on this
point.
The mammals of the middle and especially of the lower Pliocene were much
stranger and more primitive than might be inferred from the foregoing
brief account. Except several of the Rodentia and perhaps one or two
of the Carnivora, the _genera_ are all extinct and such familiar terms
as horses, rhinoceroses, camels, etc., can be employed only in a very
comprehensive sense, as equivalent to families.
The Pliocene of _South America_ is involved in some obscurity; not
that there is any question as to the formations, or their order of
succession, but there is much doubt as to the limits of the epoch both
above and below. The latest Pliocene fauna, that of the Tarija Valley
in Bolivia, was essentially the same as that of the Pleistocene and
contained a similarly large proportion of migrant elements from the
north, but it was evidently older and many of the species were different.
The two divisions of the Araucanian fauna, contained in the beds of
Catamarca and Monte Hermoso respectively, are very much alike and need
not be given separate consideration. In one respect these presumably
upper Pliocene faunas formed a very strong contrast to the mammalian
assemblage of the Pleistocene, and that is in the quite insignificant
part taken by the migrants from North America. Of the Carnivora there
were but two representatives, one referable to the raccoon family and
one to the dogs, while a hare and a small member of the Artiodactyla, of
indeterminate family, complete the list of northern forms, though this
list will doubtless be extended by future discovery. The peccaries, deer,
antelopes, tapirs, horses, †mastodons, cats, weasels, otters, squirrels,
mice, etc. had not reached the southern continent, or were still so
rare that remains of them have not been found. This rarity and relative
insignificance of the northern forms gave a very different aspect to the
fauna.
On the other hand, the indigenous South American groups were very fully
represented. Many kinds of opossums and a few large carnivorous types,
much like the so-called Tasmanian Wolf (_Thylacynus_), were the remnants
of a much larger assemblage of marsupials which inhabited South America
in the Miocene. Of the Edentata, there were great abundance and variety,
many large †glyptodonts and some gigantic armadillos, as well as numerous
examples of normal size; the †ground-sloths, though somewhat smaller
than those of the Pleistocene, were mostly of gigantic size, and true
or arboreal sloths (Tardigrada) have been reported. The very numerous
rodents, with the exception of the intrusive hare, all belonged to
typically South American families. Some of the rodents were gigantic
and one (_†Megamys_), a member of the Chinchilla family, was equal to a
rhinoceros in size and the largest known representative of the order.
Especially characteristic was the abundance of the cavy family (Caviidæ).
The hoofed animals, with the single known exception of the immigrant
artiodactyl, all belonged to the autochthonous orders, all of which
are extinct at the present time. Forerunners of the extraordinary
genus _†Macrauchenia_, which was one of the most conspicuous elements
of Pleistocene life, were quite common in the Pliocene and differed
from the Pampean genus chiefly in their smaller size and less advanced
specialization. We find here also the last survivors of another family of
the †Litopterna, the †proterotheres (†Proterotheriidæ), which imitated
the horses in such a surprising manner that some authorities believe
them to have been actually related to those perissodactyls. The Monte
Hermoso genus (_†Epitherium_) had feet which were wonderfully, though
but superficially, like those of the three-toed horses. The †Toxodonta
were numerous and most of them were large, ponderous animals; one genus
(_†Trigodon_) had the interesting peculiarity of a single median horn
on the forehead, much like that of a rhinoceros. Horned species were
always rare among the indigenous groups of South American ungulates, and
all that have been discovered so far belonged to the †toxodonts. The
remaining group, that of the †Typotheria, was also well represented,
both by larger and by very small forms, some no larger than a rabbit
(_†Pachyrukhos_).
The presumably lower Pliocene (perhaps upper Miocene) fauna of the
Paraná formation is as yet known only from very fragmentary material.
Representatives of the dogs, raccoons and bears have been reported, but
the identifications are doubtful; at all events, these would seem to
have been the most ancient of the northern immigrants. A considerable
number of marsupials, both opossums and large predaceous types, have been
found. The rodents were very numerous, all belonging to South American
families and some of them very large. The edentates were gigantic
†ground-sloths and †glyptodonts, with numerous armadillos of ordinary
size. The hoofed animals all belonged to the indigenous South American
orders, the predominant place being taken by the †toxodonts, some of
which were large. There were many †typotheres, both of the larger and
smaller kinds. The †Litopterna were represented both by the horse-like
†proterotheres and the long-necked †macrauchenids, the latter smaller and
less specialized than those of the Pampean.
[Illustration: FIG. 124.—Head of Horned †Toxodont (_†Trigodon gaudryi_).
Pliocene of Monte Hermoso. Restored from a skull in the Ameghino
collection.]
2. _Miocene_
_North America._—Upper Miocene beds cover extensive areas of the Great
Plains region and are scattered from Montana far into Mexico. The rich
fauna is an outgrowth and development of that of the middle Miocene, with
but few immigrant additions and, on the other hand, passes so gradually
into that of the lower Pliocene, that any line of separation between
them is very difficult to draw. The rodents, numerous as they are among
the fossils, are almost certainly very incompletely represented in the
collections; the families are almost all still in existence, but nearly
every genus is extinct, and thus the vernacular names used to designate
them must be understood in a broad sense. Hares, mice, pocket-gophers,
squirrels, marmots, beavers and the extraordinary †mylagaulids were all
abundant.
In even more strongly marked sense must the broad meaning for the
vernacular names of the other mammals be emphasized, for we have to deal
almost exclusively with extinct genera, which differed much from their
modern descendants. Many of the Carnivora have been obtained; there were
numerous dogs, some rivalling the largest of existing bears in size, true
felines and †sabre-tooth tigers, which were smaller and lighter animals
than the great beasts of the Pleistocene; weasels, martens, otters and
raccoons, but no bears. The bears, a family of Old World origin, are
not certainly known in America before the Pleistocene, but had probably
reached this continent in the Pliocene.
As is so very generally true, the commonest and best-preserved of the
fossils are those of the hoofed animals. The †mastodons were of the
four-tusked kind (_†Gomphotherium_ or _†Trilophodon_), the skull and
teeth of which differed so markedly from those of the true elephants.
The relatively small, low-crowned and simple grinding teeth were common
to all the †mastodons, but the tusks were different from those of the
larger members of the group. The upper tusks were comparatively short
and nearly straight and retained a band of enamel, while the lower tusks
were still shorter, chisel-shaped and so worn as to prove that they were
regularly used, no doubt in cropping leaves; the shortness of these lower
tusks was compensated for by the great elongation of the lower jaw. The
head was proportionately broad and low and, for Proboscidea, these were
small animals, not more than five or six feet high at the shoulder.
The body, limbs and feet had already attained substantially their
modern grade of structure, advance among the Proboscidea being chiefly
restricted to the teeth and skull.
[Illustration: FIG. 125.—_†Teleoceras fossiger_, a short-legged
rhinoceros, with small nasal horn; lower Pliocene and upper Miocene of
Nebraska. Restored from a skeleton in the American Museum of Natural
History.]
Four families of Perissodactyla were represented in the upper Miocene.
The rhinoceroses, which were very abundant, were present in considerable
variety; some were hornless, others had a single small horn on the end
of the nose. Among these rhinoceroses there was much difference in
bodily proportions, some being extremely heavy, with very short legs
and feet, and these were the commonest, while others had longer legs and
less massive bodies. Tapirs, on the contrary, would seem to have been
scantily represented; at least, they are rare among the fossils. The
extraordinary, aberrant †chalicotheres, perissodactyls with claws instead
of hoofs, still persisted, but are far better known from the lower
Miocene, in connection with which they will be described. The dominant
perissodactyl family was that of the horses, of which no less than five
genera are already known. There were some with very low-crowned teeth,
which must have fed principally by browsing upon leaves and such soft
diet; but the grazing kinds, which had high-crowned, cement-covered and
very complex grinding teeth, had come to the fore. Still retaining three
toes in each foot, with the middle toe so enlarged as to bear nearly the
entire weight, save in snow or soft ground, these eminently cursorial
animals, which had the slender limbs of a deer, must have roamed the
plains in great herds.
Still commoner were the Artiodactyla. Many species of grazing camels,
which were the predominant artiodactyl family in North America during
upper Miocene times, were the ancestors both of the true camels of
the Old World and the South American llamas. †Giraffe-camels have not
yet been found and no doubt they were much less abundant than in the
middle Miocene, but that they had not completely disappeared is shown
by their recurrence in the Pliocene. As compared with earlier ages, the
†oreodonts had begun a rapid decline and had lost notably both in numbers
and variety, but one most curious beast (_†Pronomotherium_, Fig. 197,
p. 375) marked the final step in the development of the short-faced,
proboscis-bearing series, which may be traced back to its beginnings in
the Oligocene. In this wonderful creature the skull was so short and
deep as to suggest that of a gorilla or some other great ape. No other
artiodactyls even approximate these later proboscis-bearing †oreodonts
in the altogether exceptional form of the skull. Grazing †oreodonts
(_†Merychyus_), of moderate and small size with high-crowned teeth,
were evidently quite common on the upper Miocene plains. The †hornless
deer and “†deer-antelopes” differed but little from those of the lower
Pliocene. Peccaries were fairly abundant.
[Illustration: FIG. 126.—_†Procamelus elrodi_, a large camel from the
upper Miocene. Restored from specimens in the Carnegie Museum.]
The upper Miocene fauna was especially characterized by the large number
of mammals, belonging to several different orders, which had acquired the
high-crowned, persistently growing pattern of grinding teeth. Many of the
horses, camels, ruminants and rodents displayed this structure, and, as
was first pointed out by Kowalevsky, the explanation is probably to be
found in the spread of grassy plains at the expense of the forests. On
account of the silica which they contain, the grasses are very abrasive
and rapidly wear the teeth down. In adaptation to this new source of
abundant and nutritious food, many kinds of mammals developed a form
of tooth which was fitted to compensate by growth for the loss through
abrasion.
The middle Miocene, small areas of which occur in Montana, eastern Oregon
and northeastern Colorado, has received various local names, the typical
one being the Deep River of Montana. Very probably, these scattered
areas are not strictly contemporaneous, but form a closely connected
series. That a land-connection with the eastern hemisphere existed, is
made clear by the appearance of several unmistakably Old World types of
animals and the beginnings of migration from South America are perhaps
also to be noted, though this cannot be positively stated. The evidence
for the South American connection is the finding in the middle Miocene of
Oregon of what are believed to be the earliest remains of †ground-sloths
yet discovered in North America, but the material is too scanty for
altogether certain determination.
The smaller animals are not very well represented in the middle
Miocene faunas, as conditions appear to have been unfavourable to
their preservation; something is known of them, nevertheless. The
very curious extinct family of rodents known as the †Mylagaulidæ, the
presence of which was noted in the upper Miocene and lower Pliocene,
first appeared here. These †mylagaulids, which were distantly related
to the modern Sewellel (_Aplodontia rufa_), were characterized by the
great enlargement and complication of one of the grinding teeth in
each jaw and the consequent reduction of the others. One genus of this
family, as in the Pliocene, had the peculiarity, unique among rodents, of
developing a large horn upon the nose, like a miniature rhinoceros. Among
the Carnivora, we find a great variety of dogs, large and small, all
belonging to extinct genera, as indeed is true of the other carnivores
also. True felines have been found, but as yet, none of the †sabre-tooth
series; the abundance of the latter, however, in both preceding and
succeeding formations, is sufficient proof that the discovery of them in
the middle Miocene is merely a question of time. Mustelines were present,
and especially noteworthy is the appearance of the first American otters,
immigrants from the Old World.
Of the hoofed animals, the most interesting are the Proboscidea, the
most ancient of which that are definitely determinable in America occur
in this horizon. The place of origin and ancestry of these animals were
long exasperating puzzles. Appearing suddenly in the Miocene of Europe
and North America, in which regions nothing was known that could, with
any plausibility, be regarded as ancestral to them, they might as well
have dropped from the moon, for all that could be told concerning their
history. The exploration of the Eocene and Oligocene beds of Egypt has
dispelled the mystery and shown that Africa was the original home of the
group, whence they gradually spread to every continent except Australia.
Little is known of these earliest American proboscideans, but they were
doubtless small †mastodons of the four-tusked type.
Among the Perissodactyla, the rhinoceroses were perhaps the most
conspicuous; the native American stocks of this family appear to have
mostly died out and to have been replaced by two or more phyla of
immigrants from the Old World, some of which were hornless, others had
a small horn on the tip of the nose and others again had a second and
smaller horn on the forehead. Tapirs, though unquestionably present, are
rare as fossils and not well known. Several distinct phyla of horses
may be distinguished, which were like small ponies in size, but of more
slender form; they were all three-toed, but there were marked differences
among them with regard to the degree to which the middle toe (the
third of the original five) had been enlarged to carry the whole weight
and the lateral toes (second and fourth) reduced to mere “dew-claws.”
While browsing horses, with low-crowned teeth, still persisted in large
numbers, we find also the extremely interesting beginnings of the highly
complex, cement-covered and high-crowned teeth of the grazing kinds. The
clawed †chalicotheres were present, though very little is known about
them because of the fragmentary character of the remains.
The Artiodactyla were much more varied and abundant, though they did
not rival the great assemblage of these animals found in the European
Miocene. Of the peccaries little more can be said than that they were
present in these faunas. The †oreodonts were very numerous, both
individually and generically; two stages of the proboscis-bearing kind
are found here together, the older, long-faced genus (_†Promerycochœrus_)
surviving from the Oligocene, while the newer Miocene type was
short-faced and had a moderate proboscis (see Fig. 196, p. 373). Others
had more the proportions of peccaries and still others were very small
and presumably aquatic in habits. Camels abounded, both the grazing kinds
which were ancestral to the modern forms of South America and Asia, and
the great, browsing †giraffe-camels. The †hornless deer and the antlered
†deer-antelopes were much like those of the Upper Miocene, slender and
graceful little creatures, and there were also considerably larger
ruminants (_†Dromomeryx_) with straight, simple and non-deciduous horns,
which may be called antelopes.
[Illustration: FIG. 127.—Gigantic †giraffe-camel (_†Alticamelus altus_)
from the middle Miocene of Colorado. Restored from specimens in the
American Museum of Natural History.]
The line of division between the lower Miocene and the uppermost
Oligocene is a very obscure and difficult one to draw. Personally, I
prefer to begin the Miocene with the widespread formation of the Great
Plains, which has been variously named Arikaree, Harrison, Rosebud,
etc., but this is a moot point. Concerning the lower part of these beds
Osborn says: “They may be either: (1) Upper Oligocene or (2) transitional
from Oligocene to Miocene, or (3) of pure Lower Miocene age.” The upper
division is referred to the Miocene without question by any one, but
for the purposes of this rapid sketch it will be best to treat the two
faunas together. This many-named formation, for which the term _Arikaree_
is here employed, as having priority, is found over extensive areas of
South Dakota, northern Nebraska and central Wyoming. The fauna was almost
entirely a development from that of the North American Oligocene, with
very little admixture of foreign elements, so that the land communication
with the eastern hemisphere must have been difficult. In this, as in
most of the Miocene formations, the smaller mammals are not fairly
represented, and it is evident that much remains to be learned with
regard to them; this is especially true of the upper division of this
stage.
[Illustration: FIG. 128.—Most ancient American Antelope (_†Dromomeryx
antilopina_), middle Miocene. Restored from specimens in the Carnegie
Museum and Princeton University.]
The rodents, which were fairly numerous, were directly continuous with
those of the upper Oligocene and included forms which were more or
less distantly connected with the modern hares, squirrels, beavers,
sewellels, pocket-gophers and kangaroo-rats. A few Insectivora of
doubtful reference have been found. Among the Carnivora there was also
considerable variety: dogs, large and small, were abundant, but all of
them were decidedly primitive from the modern standpoint; the cats were
represented both by the true felines, which were probably immigrants,
and by the †sabre-tooth series. There were several large and powerful
mustelines, or members of the weasel family, which were likewise
immigrants, one of which resembles in many ways the modern Wolverene
(_Gulo_). Very interesting is the beginning of the raccoon family
(Procyonidæ) or, at least, what is believed to be such, which arose from
a branch of the dogs; this most ancient of the raccoons was _†Phlaocyon_,
a small and slender animal.
The earliest traces of the Proboscidea in America have been reported
from this formation, but the fragmentary specimens are inconclusive.
The Perissodactyla are among the commonest fossils. The rhinoceroses
belonged to native stocks, including both the horned and hornless forms.
The horned genus (_†Diceratherium_) differed from all other rhinoceroses
in having a transverse _pair_ of horns on the nose, and the species
of the lower Miocene were quite small and light; the hornless genus
(_†Cænopus_) was a larger and heavier animal. Tapirs are rare as fossils
and consequently not well known. While there were several kinds of
horses, they all agreed in having short-crowned and relatively simple
grinding teeth and three-toed feet; they were smaller and of lighter,
more slender build than those of the middle Miocene. The wonderful
aberrant perissodactyls with clawed feet, the †chalicotheres (suborder
†Ancylopoda), appear to have been more abundant in the Arikaree than at
any other time in North America, though their history in this continent
extends from the middle Eocene to the lower Pliocene. _†Moropus_, the
lower Miocene genus, was as grotesque a creature as could well be
imagined and, in advance of experience, no one ever did imagine such a
beast. With rather small and somewhat horse-like head, long neck, long
fore limbs and shorter hind limbs, these extraordinary animals united
short, three-toed feet, which were armed with enormous claws. The long
persistence (to the Pleistocene of Asia) and wide geographical range
of the †chalicotheres are sufficient evidence that their very unusual
structure must have been advantageous to them, but the problem of their
habits and mode of life is still unsolved. From the character of the
teeth, the long neck and fore limbs, it may, however, be inferred that
they fed chiefly upon the leaves of trees.
[Illustration: FIG. 129.—The small, †paired-horned rhinoceros
(_†Diceratherium cooki_) of the lower Miocene. Restored from a skeleton
in the Carnegie Museum, Pittsburgh.]
[Illustration: FIG. 130.—A †chalicothere (_†Moropus elatus_) of the lower
Miocene. Restored from a skeleton in the Carnegie Museum, Pittsburgh.]
Even more numerous and varied were the Artiodactyla. Peccaries of a
primitive sort were common, and we find the last of the series of Ҡgiant
pigs,” which had been a very conspicuous group throughout the Oligocene.
The lower Miocene genus, _†Dinohyus_, was a monstrous beast, six feet
or more in height, with formidable canine tusks and a very long head
made grotesque by bony excrescences upon the skull and jaws. For a pig,
the legs were very long and the feet slender, having but two toes. The
†oreodonts were present in great numbers, both small and large forms;
except for bodily stature and modifications of the head, they all
looked very much alike; _†Merycochœrus_, with its incipient proboscis,
here made its first appearance. The last representatives of a family
(†Hypertragulidæ) of small and graceful artiodactyls are found in this
formation. One of these (_†Syndyoceras_, _see_ Fig. 215, p. 403), an
animal considerably larger than the existing Musk-Deer, was in its way
even more bizarre-looking than the †chalicotheres; with an antelope-like
head, it had four horns, one pair over the eyes, curving inward, and
a shorter pair, with outward curvature, on the muzzle. Another genus
(_†Hypertragulus_) was very much smaller and very slender.