THE TERTIARY
THE CENOZOIC ERA. The last stages of the Cretaceous are marked by
a decadence of the reptiles. By the end of that period the
reptilian forms characteristic of the time had become extinct one
after another, leaving to represent the class only the types of
reptiles which continue to modern times. The day of the ammonite
and the belemnite also now drew to a close, and only a few of
these cephalopods were left to survive the period. It is therefore
at the close of the Cretaceous that the line is drawn which marks
the end of the Middle Age of geology and the beginning of the
Cenozoic era, the era of modern life,--the Age of Mammals.
In place of the giant reptiles, mammals now become masters of the
land, appearing first in generalized types which, during the long
ages of the era, gradually evolve to higher forms, more
specialized and ever more closely resembling the mammals of the
present. In the atmosphere the flying dragons of the Mesozoic give
place to birds and bats. In the sea, whales, sharks, and teleost
fishes of modern types rule in the stead of huge swimming
reptiles. The lower vertebrates, the invertebrates of land and
sea, and the plants of field and forest take on a modern aspect,
and differ little more from those of to-day than the plants and
animals of different countries now differ from one another. From
the beginning of the Cenozoic era until now there is a steadily
increasing number of species of animals and plants which have
continued to exist to the present time.
The Cenozoic era comprises two divisions,--the TERTIARY period and
the QUATERNARY period.
In the early days of geology the formations of the entire
geological record, so far as it was then known, were divided into
three groups,--the PRIMARY, the SECONDARY (now known as the
Mesozoic), and the TERTIARY, When the third group was subdivided
into two systems, the term Tertiary was retained for the first
system of the two, while the term QUATERNARY was used to designate
the second.
DIVISIONS OF THE TERTIARY. The formations of the Tertiary are
grouped in three divisions,--the PLIOCENE (more recent), the
MIOCENE (less recent), and the EOCENE (the dawn of the recent).
Each of these epochs is long and complex. Their various sub-
divisions are distinguished each by its own peculiar organisms,
and the changes of physical geography recorded in their strata. In
the rapid view which we are compelled to take we can note only a
few of the most conspicuous events of the period.
PHYSICAL GEOGRAPHY OF THE TERTIARY IN EASTERN NORTH AMERICA. The
Tertiary rocks of eastern North America are marine deposits and
occupy the coastal lowlands of the Atlantic and Gulf states (Fig.
260). In New England, Tertiary beds occur on the island of
Martha's Vineyard, but not on the mainland; hence the shore line
here stood somewhat farther out than now. From New Jersey
southward the earliest Tertiary sands and clays, still
unconsolidated, leave only a narrow strip of the edge of the
Cretaceous between them and the Triassic and crystalline rocks of
the Piedmont oldland; hence the Atlantic shore here stood farther
in than now, and at the beginning of the period the present
coastal plain was continental delta. A broad belt of Tertiary sea-
laid limestones, sandstones, and shales surrounds the Gulf of
Mexico and extends northward up the Mississippi embayment to the
mouth of the Ohio River; hence the Gulf was then larger than at
present, and its waters reached in a broad bay far up the
Mississippi valley.
Along the Atlantic coast the Mesozoic peneplain may be traced
shoreward to where it disappears from view beneath an
unconformable cover of early Tertiary marine strata. The beginning
of the Tertiary was therefore marked by a subsidence. The wide
erosion surface which at the close of the Mesozoic lay near sea
level where the Appalachian Mountains and their neighboring
plateaus and uplands now stand was lowered gently along its
seaward edge beneath the Tertiary Atlantic to receive a cover of
its sediments.
As the period progressed slight oscillations occurred from time to
time. Strips of coastal plain were added to the land, and as early
as the close of the Miocene the shore lines of the Atlantic and
Gulf states had reached well-nigh their present place. Louisiana
and Florida were the last areas to emerge wholly from the sea,--
Florida being formed by a broad transverse upwarp of the
continental delta at the opening of the Miocene, forming first an
island, which afterwards was joined to the mainland.
THE PACIFIC COAST. Tertiary deposits with marine fossils occur
along the western foothills of the Sierra Nevadas, and are
crumpled among the mountain masses of the Coast Ranges; it is
hence inferred that the Great Valley of California was then a
border sea, separated from the ocean by a chain of mountainous
islands which were upridged into the Coast Ranges at a still later
time. Tertiary marine strata are spread over the lower Columbia
valley and that of Puget Sound, showing that the Pacific came in
broadly there.
THE INTERIOR OF THE WESTERN UNITED STATES. The closing stages of
the Mesozoic were marked, as we have seen, by the upheaval of the
Rocky Mountains and other western ranges. The bases of the
mountains are now skirted by widespread Tertiary deposits, which
form the highest strata of the lofty plateaus from the level of
whose summits the mountains rise. Like the recent alluvium of the
Great Valley of California, these deposits imply low-lying lands
when they were laid, and therefore at that time the mountains rose
from near sea level. But the height at which the Tertiary strata
now stand--five thousand feet above the sea at Denver, and twice
that height in the plateaus of southern Utah--proves that the
plateaus of which the Tertiary strata form a part have been
uplifted during the Cenozoic. During their uplift, warping formed
extensive basins both east and west of the Rockies, and in these
basins stream-swept and lake-laid waste gathered to depths of
hundreds and thousands of feet, as it is accumulating at present
in the Great Valley of California and on the river plains of
Turkestan. The Tertiary river deposits of the High Plains have
already been described. How widespread are these ancient river
plains and beds of fresh-water lakes may be seen in the map of
Figure 260.
THE BAD LANDS. In several of the western states large areas of
Tertiary fresh-water deposits have been dissected to a maze of
hills whose steep sides are cut with innumerable ravines. The
deposits of these ancient river plains and lake beds are little
cemented and because of the dryness of the climate are unprotected
by vegetation; hence they are easily carved by the wet-weather
rills of scanty and infrequent rains. These waterless, rugged
surfaces were named by the early French explorers the BAD LANDS
because they were found so difficult to traverse. The strata of
the Bad Lands contain vast numbers of the remains of the animals
of Tertiary times, and the large amount of barren surface exposed
to view makes search for fossils easy and fruitful. These desolate
tracts are therefore frequently visited by scientific collecting
expeditions.
MOUNTAIN MAKING IN THE TERTIARY. The Tertiary period included
epochs when the earth's crust was singularly unquiet. From time to
time on all the continents subterranean forces gathered head, and
the crust was bent and broken and upridged in lofty mountains.
The Sierra Nevada range was formed, as we have seen, by strata
crumpling at the end of the Jurassic. But since that remote time
the upfolded mountains had been worn to plains and hilly uplands,
the remnants of whose uplifted erosion surfaces may now be traced
along the western mountain slopes. Beginning late in the Tertiary,
the region was again affected by mountain-making movements. A
series of displacements along a profound fault on the eastern side
tilted the enormous earth block of the Sierras to the west,
lifting its eastern edge to form the lofty crest and giving to the
range a steep eastern front and a gentle descent toward the
Pacific.
The Coast Ranges also have had a complex history with many
vicissitudes. The earliest foldings of their strata belong to the
close of the Jurassic, but it was not until the end of the Miocene
that the line of mountainous islands and the heavy sediments which
had been deposited on their submerged flanks were crushed into a
continuous mountain chain. Thick Pliocene beds upon their sides
prove that they were depressed to near sea level during the later
Tertiary. At the close of the Pliocene the Coast Ranges rose along
with the upheaval of the Sierra, and their gradual uplift has
continued to the present time.
The numerous north-south ranges of the Great Basin and the Mount
Saint Elias range of Alaska were also uptilted during the
Tertiary.
During the Tertiary period many of the loftiest mountains of the
earth--the Alps, the Apennines, the Pyrenees, the Atlas, the
Caucasus, and the Himalayas--received the uplift to which they owe
most of their colossal bulk and height, as portions of the
Tertiary sea beds now found high upon their flanks attest. In the
Himalayas, Tertiary marine limestones occur sixteen thousand five
hundred feet above sea level.
VOLCANIC ACTIVITY IN THE TERTIARY. The vast deformations of the
Tertiary were accompanied on a corresponding scale by outpourings
of lava, the outburst of volcanoes, and the intrusion of molten
masses within the crust. In the Sierra Nevadas the Miocene river
gravels of the valleys of the western slope, with their placer
deposits of gold, were buried beneath streams of lava and beds of
tuff. Volcanoes broke forth along the Rocky Mountains and on the
plateaus of Utah, New Mexico, and Arizona.
Mount Shasta and the immense volcanic piles of the Cascades date
from this period. The mountain basin of the Yellowstone Park was
filled to a depth of several thousand feet with tuffs and lavas,
the oldest dating as far back as the beginning of the Tertiary.
Crandall volcano was reared in the Miocene and the latest
eruptions of the Park are far more recent.
THE COLUMBIA AND SNAKE RIVER LAVAS. Still more important is the
plateau of lava, more than two hundred thousand square miles in
area, extending from the Yellowstone Park to the Cascade
Mountains, which has been built from Miocene times to the present.
Over this plateau, which occupies large portions of Idaho,
Washington, and Oregon, and extends into northern California and
Nevada, the country rock is basaltic lava. For thousands of square
miles the surface is a lava plain which meets the boundary
mountains as a lake or sea meets a rugged and deeply indented
coast. The floods of molten rock spread up the mountain valleys
for a score of miles and more, the intervening spurs rising above
the lava like long peninsulas, while here and there an isolated
peak was left to tower above the inundation like an island off a
submerged shore.
The rivers which drain the plateau--the Snake, the Columbia, and
their tributaries--have deeply trenched it, yet their canyons,
which reach the depth of several thousand feet, have not been worn
to the base of the lava except near the margin and where they cut
the summits of mountains drowned beneath the flood. Here and there
the plateau has been deformed. It has been upbent into great
folds, and broken into immense blocks of bedded lava, forming
mountain ranges, which run parallel with the Pacific coast line.
On the edges of these tilted blocks the thickness of the lava is
seen to be fully five thousand feet. The plateau has been built,
like that of Iceland, of innumerable overlapping sheets of lava.
On the canyon walls they weather back in horizontal terraces and
long talus slopes. One may distinguish each successive flow by its
dense central portion, often jointed with large vertical columns,
and the upper portion with its mass of confused irregular columns
and scoriaceous surface. The average thickness of the flows seems
to be about seventy-five feet.
The plateau was long in building. Between the layers are found in
places old soil beds and forest grounds and the sediments of
lakes. Hence the interval between the flows in any locality was
sometimes long enough for clays to gather in the lakes which
filled depressions in the surface. Again and again the surface of
the black basalt was reddened by oxidation and decayed to soil,
and forests had time to grow upon it before the succeeding
inundation sealed the sediments and soils away beneath a sheet of
stone. Near the edges of the lava plain, rivers from the
surrounding mountains spread sheets of sand and gravel on the
surface of one flow after another. These pervious sands,
interbedded with the lava, become the aquifers of artesian wells.
In places the lavas rest on extensive lake deposits, one thousand
feet deep, and Miocene in age as their fossils prove. It is to the
middle Tertiary, then, that the earliest flows and the largest
bulk of the great inundation belong. So ancient are the latest
floods in the Columbia basin that they have weathered to a
residual yellow clay from thirty to sixty feet in depth and
marvelously rich in the mineral substances on which plants feed.
In the Snake River valley the latest lavas are much younger. Their
surfaces are so fresh and undecayed that here the effusive
eruptions may well have continued to within the period of human
history. Low lava domes like those of Iceland mark where last the
basalt outwelled and spread far and wide before it chilled (Fig.
341). In places small mounds of scoria show that the eruptions
were accompanied to a slight degree by explosions of steam. So
fluid was this superheated lava that recent flows have been traced
for more than fifty miles.
The rocks underlying the Columbia lavas, where exposed to view,
are seen to be cut by numerous great dikes of dense basalt, which
mark the fissures through which the molten rock rose to the
surface.
The Tertiary included times of widespread and intense volcanic
action in other continents as well as in North America. In Europe,
Vesuvius and Etna began their career as submarine volcanoes in
connection with earth movements which finally lifted Pliocene
deposits in Sicily to their present height,--four thousand feet
above the sea. Volcanoes broke forth in central France and
southern Germany, in Hungary and the Carpathians. Innumerable
fissures opened in the crust from the north of Ireland and the
western islands of Scotland to the Faroes, Iceland, and even to
arctic Greenland; and here great plateaus were built of flows of
basalt similar to that of the Columbia River. In India, at the
opening of the Tertiary, there had been an outwelling of basalt,
flooding to a depth of thousands of feet two hundred thousand
square miles of the northwestern part of the peninsula, and
similar inundations of lava occurred where are now the table-lands
of Abyssinia. From the middle Tertiary on, Asia Minor, Arabia, and
Persia were the scenes of volcanic action. In Palestine the rise
of the uplands of Judea at the close of the Eocene, and the
downfaulting of the Jordan valley were followed by volcanic
outbursts. In comparison with the middle Tertiary, the present is
a time of volcanic inactivity and repose.
EROSION OF TERTIARY MOUNTAINS AND PLATEAUS. The mountains and
plateaus built at various times during the Tertiary and at its
commencement have been profoundly carved by erosive agents. The
Sierra Nevada Mountains have been dissected on the western slope
by such canyons as those of King's River and the Yosemite. Six
miles of strata have been denuded from parts of the Wasatch
Mountains since their rise at the beginning of the era. From the
Colorado plateaus, whose uplift dates from the same time, there
have been stripped off ten thousand feet of strata over thousands
of square miles, and the colossal canyon of the Colorado has been
cut after this great denudation had been mostly accomplished.
On the eastern side of the continent, as we have seen, a broad
peneplain had been developed by the close of the Cretaceous. The
remnants of this old erosion surface are now found upwarped to
various heights in different portions of its area. In southern New
England it now stands fifteen hundred feet above the sea in
western Massachusetts, declining thence southward and eastward to
sea level at the coast. In southwestern Virginia it has been
lifted to four thousand feet above the sea. Manifestly this upwarp
occurred since the peneplain was formed; it is later than the
Mesozoic, and the vast dissection which the peneplain has suffered
since its uplift must belong to the successive cycles of Cenozoic
time.
Revived by the uplift, the streams of the area trenched it as
deeply as its elevation permitted, and reaching grade, opened up
wide valleys and new peneplains in the softer rocks. The
Connecticut valley is Tertiary in age, and in the weak Triassic
sandstones has been widened in places to fifteen miles. Dating
from the same time are the valleys of the Hudson, the Susquehanna,
the Delaware, the Potomac, and the Shenandoah.
In Pennsylvania and the states lying to the south the Mesozoic
peneplain lies along the summits of the mountain ridges. On the
surface of this ancient plain, Tertiary erosion etched out the
beautifully regular pattern of the Allegheny mountain ridges and
their intervening valleys. The weaker strata of the long, regular
folds were eroded into longitudinal valleys, while the hard
Paleozoic sandstones, such as the Medina and the Pocono, were left
in relief as bold mountain walls whose even crests rise to the
common level of the ancient plain. From Virginia far into Alabama
the great Appalachian valley was opened to a width in places of
fifty miles and more, along a belt of intensely folded and faulted
strata where once was the heart of the Appalachian Mountains. In
Figure 70 the summit of the Cumberland plateau (ab) marks the
level of the Mesozoic peneplain, while the lower erosion levels
are Tertiary and Quaternary in age.
LIFE OF THE TERTIARY PERIOD
VEGETATION AND CLIMATE. The highest plants in structure, the
DICOTYLS (such as our deciduous forest trees) and the MONOCOTYLS
(represented by the palms), were introduced during the Cretaceous.
The vegetable kingdom reached its culmination before the animal
kingdom, and if the dividing line between the Mesozoic and the
Cenozoic were drawn according to the progress of plant life, the
Cretaceous instead of the Tertiary would be made the opening
period of the modern era.
The plants of the Tertiary belonged, for the most part, to genera
now living; but their distribution was very different from that of
the flora of to-day. In the earlier Tertiary, palms flourished
over northern Europe, and in the northwestern United States grew
the magnolia and laurel, along with the walnut, oak, and elm. Even
in northern Greenland and in Spitzbergen there were lakes covered
with water lilies and surrounded by forests of maples, poplars,
limes, the cypress of our southern states, and noble sequoias
similar to the "big trees" and redwoods of California. A warm
climate like that of the Mesozoic, therefore, prevailed over North
America and Europe, extending far toward the pole. In the later
Tertiary the climate gradually became cooler. Palms disappeared
from Europe, and everywhere the aspect of forests and open lands
became more like that of to-day. Grasses became abundant,
furnishing a new food for herbivorous animals.
ANIMAL LIFE OF THE TERTIARY. Little needs to be said of the
Tertiary invertebrates, so nearly were they like the invertebrates
of the present. Even in the Eocene, about five per cent of marine
shells were of species still living, and in the Pliocene the
proportion had risen to more than one half.
Fishes were of modern types. Teleosts were now abundant. The ocean
teemed with sharks, some of them being voracious monsters seventy-
five feet and even more in length, with a gape of jaw of six feet,
as estimated by the size of their enormous sharp-edged teeth.
Snakes are found for the first time in the early Tertiary. These
limbless reptiles, evolved by degeneration from lizardlike
ancestors, appeared in nonpoisonous types scarcely to be
distinguished from those of the present day.
MAMMALS OF THE EARLY TERTIARY. The fossils of continental deposits
of the earliest Eocene show that a marked advance had now been
made in the evolution of the Mammalia. The higher mammals had
appeared, and henceforth the lower mammals--the monotremes and
the marsupials--are reduced to a subordinate place.
These first true mammals were archaic and generalized in
structure. Their feet were of the primitive type, with five toes
of about equal length. They were also PLANTIGRADES,--that is, they
touched the ground with the sole of the entire foot from toe to
heel. No foot had yet become adapted to swift running by a
decrease in the number of digits and by lifting the heel and sole
so that only the toes touch the ground,--a tread called
DIGITIGRADE. Nor was there yet any foot like that of the cats,
with sharp retractile claws adapted to seizing and tearing the
prey. The forearm and the lower leg each had still two separate
bones (ulna and radius, fibula and tibia), neither pair having
been replaced with a single strong bone, as in the leg of the
horse. The teeth also were primitive in type and of full number.
The complex heavy grinders of the horse and elephant, the sharp
cutting teeth of the carnivores, and the cropping teeth of the
grass eaters were all still to come.
Phenacodus is a characteristic genus of the early Eocene, whose
species varied in size from that of a bulldog to that of an animal
a little larger than a sheep. Its feet were primitive, and their
five toes bore nails intermediate in form between a claw and a
hoof. The archaic type of teeth indicates that the animal was
omnivorous in diet. A cast of the brain cavity shows that, like
its associates of the time, its brain was extremely small and
nearly smooth, having little more than traces of convolutions.
The long ages of the Eocene and the following epochs of the
Tertiary were times of comparatively rapid evolution among the
Mammalia. The earliest forms evolved along diverging lines toward
the various specialized types of hoofed mammals, rodents,
carnivores, proboscidians, the primates, and the other mammalian
orders as we know them now. We must describe the Tertiary mammals
very briefly, tracing the lines of descent of only a few of the
more familiar mammals of the present.
THE HORSE. The pedigree of the horse runs back into the early
Eocene through many genera and species to a five-toed, [Footnote:
Or, more accurately, with four perfect toes and a rudimentary
fifth corresponding to the thumb.] short-legged ancestor little
bigger than a cat. Its descendants gradually increased in stature
and became better and better adapted to swift running to escape
their foes. The leg became longer, and only the tip of the toes
struck the ground. The middle toe (digit number three), originally
the longest of the five, steadily enlarged, while the remaining
digits dwindled and disappeared. The inner digit, corresponding to
the great toe and thumb, was the first to go. Next number five,
the little finger, was also dropped. By the end of the Eocene a
three-toed genus of the horse family had appeared, as large as a
sheep. The hoof of digit number three now supported most of the
weight, but the slender hoofs of digits two and four were still
serviceable. In the Miocene the stature of the ancestors of the
horse increased to that of a pony. The feet were still three-toed,
but the side hoofs were now mere dewclaws and scarcely touched the
ground. The evolution of the family was completed in the Pliocene.
The middle toe was enlarged still more, the side toes were
dropped, and the palm and foot bones which supported them were
reduced to splints.
While these changes were in progress the radius and ulna of the
fore limb became consolidated to a single bone; and in the hind
limb the fibula dwindled to a splint, while the tibia was
correspondingly enlarged. The molars, also gradually lengthened,
and became more and more complex on their grinding surface; the
neck became longer; the brain steadily increased in size and its
convolutions became more abundant. The evolution of the horse has
made for greater fleetness and intelligence.
THE RHINOCEROS AND TAPIR. These animals, which are grouped with
the horse among the ODD-TOED (perissodactyl) mammals, are now
verging toward extinction. In the rhinoceros, evolution seems to
have taken the opposite course from that of the horse. As the
animal increased in size it became more clumsy, its limbs became
shorter and more massive, and, perhaps because of its great
weight, the number of digits were not reduced below the number
three. Like other large herbivores, the rhinoceros, too slow to
escape its enemies by flight, learned to withstand them. It
developed as its means of defense a nasal horn.
Peculiar offshoots of the line appeared at various times in the
Tertiary. A rhinoceros, semiaquatic in habits, with curved tusks,
resembling in aspect the hippopotamus, lived along the water
courses of the plains east of the Rockies, and its bones are now
found by the thousands in the Miocene of Kansas. Another developed
along a line parallel to that of the horse, and herds of these
light-limbed and swift-footed running rhinoceroses ranged the
Great Plains from the Dakotas southward.
The tapirs are an ancient family which has changed but little
since it separated from the other perissodactyl stocks in the
early Tertiary. At present, tapirs are found only in South America
and southern Asia,--a remarkable distribution which we could not
explain were it not that the geological record shows that during
Tertiary times tapirs ranged throughout the northern hemisphere,
making their way to South America late in that period. During the
Pleistocene they became extinct over all the intervening lands
between the widely separated regions where now they live. The
geographic distribution of animals, as well as their relationships
and origins, can be understood only through a study of their
geological history.
THE PROBOSCIDIANS. This unique order of hoofed mammals, of which
the elephant is the sole survivor, began, so far as known, in the
Eocene, in Egypt, with a piglike ancestor the size of a small
horse, with cheek teeth like the Mastodon's, but wanting both
trunk and tusks. A proboscidian came next with four short tusks,
and in the Miocene there followed a Mastodon (Fig. 346) armed with
two pairs of long, straight tusks on which rested a flexible
proboscis.
The DINOTHERE was a curious offshoot of the line, which developed
in the Miocene in Europe. In this immense proboscidian, whose
skull was three feet long, the upper pair of tusks had
disappeared, and those of the lower jaw were bent down with a
backward curve in walrus fashion.
In the true ELEPHANTS, which do not appear until near the close of
the Tertiary, the lower jaw loses its tusks and the grinding teeth
become exceedingly complex in structure. The grinding teeth of the
mastodon had long roots and low crowns crossed by four or five
peaked enameled ridges. In the teeth of the true elephants the
crown has become deep, and the ridges of enamel have changed to
numerous upright, platelike folds, their interspaces filled with
cement. The two genera--Mastodon and Elephant--are connected by
species whose teeth are intermediate in pattern. The proboscidians
culminated in the Pliocene, when some of the giant elephants
reached a height of fourteen feet.
THE ARTIODACTYLS comprise the hoofed Mammalia which have an even
number of toes, such as cattle, sheep, and swine. Like the
perissodactyls, they are descended from the primitive five-toed
plantigrade mammals of the lowest Eocene. In their evolution,
digit number one was first dropped, and the middle pair became
larger and more massive, while the side digits, numbers two and
five, became shorter, weaker, and less serviceable. The FOUR-TOED
ARTIODACTYLS culminated in the Tertiary; at present they are
represented only by the hippopotamus and the hog. Along the main
line of the evolution of the artiodactyls the side toes, digits
two and five, disappeared, leaving as proof that they once existed
the corresponding bones of palm and sole as splints. The TWO-TOED
ARTIODACTYLS, such as the camels, deer, cattle, and sheep, are now
the leading types of the herbivores.
SWINE AND PECCARIES are two branches of a common stock, the first
developing in the Old World and the second in the New. In the
Miocene a noticeable offshoot of the line was a gigantic piglike
brute, a root eater, with a skull a yard in length, whose remains
are now found in Colorado and South Dakota.
CAMELS AND LLAMAS. The line of camels and llamas developed in
North America, where the successive changes from an early Eocene
ancestor, no larger than a rabbit, are traced step by step to the
present forms, as clearly as is the evolution of the horse. In the
late Miocene some of the ancestral forms migrated to the Old World
by way of a land connection where Bering Strait now is, and there
gave rise to the camels and dromedaries. Others migrated into
South America, which had now been connected with our own
continent, and these developed into the llamas and guanacos, while
those of the race which remained in North America became extinct
during the Pleistocene.
Some peculiar branches of the camel stem appeared in North
America. In the Pliocene arose a llama with the long neck and
limbs of a giraffe, whose food was cropped from the leaves and
branches of trees. Far more generalized in structure was the
Oreodon, an animal related to the camels, but with distinct
affinities also with other lines, such as those of the hog and
deer. These curious creatures were much like the peccary in
appearance, except for their long tails. In the middle Eocene they
roamed in vast herds from Oregon to Kansas and Nebraska.
THE RUMINANTS. This division of the artiodactyls includes
antelopes, deer, oxen, bison, sheep, and goats,--all of which
belong to a common stock which took its rise in Europe in the
upper Eocene from ancestral forms akin to those of the camels. In
the Miocene the evolution of the two-toed artiodactyl foot was
well-nigh completed. Bonelike growths appeared on the head, and
the two groups of the ruminants became specialized,--the deer with
bony antlers, shed and renewed each year, and the ruminants with
hollow horns, whose two bony knobs upon the skull are covered with
permanent, pointed, horny sheaths.
The ruminants evolved in the Old World, and it was not until the
later Miocene that the ancestors of the antelope and of some deer
found their way to North America. Mountain sheep and goats, the
bison and most of the deer, did not arrive until after the close
of the Tertiary, and sheep and oxen were introduced by man.
The hoofed mammals of the Tertiary included many offshoots from
the main lines which we have traced. Among them were a number of
genera of clumsy, ponderous brutes, some almost elephantine in
their bulk.
THE CARNIVORES. The ancestral lines of the families of the flesh
eaters--such as the cats (lions, tigers, etc.), the bears, the
hyenas, and the dogs (including wolves and foxes)--converge in the
creodonts of the early Eocene,--an order so generalized that it
had affinities not only with the carnivores but also with the
insect eaters, the marsupials, and the hoofed mammals as well.
From these primitive flesh eaters, with small and simple brains,
numerous small teeth, and plantigrade tread, the different
families of the carnivores of the present have slowly evolved.
DOGS AND BEARS. The dog family diverged from the creodonts late in
the Eocene, and divided into two branches, one of which evolved
the wolves and the other the foxes. An offshoot gave rise to the
family of the bears, and so closely do these two families, now
wide apart, approach as we trace them back in Tertiary times that
the Amphicyon, a genus doglike in its teeth and bearlike in other
structures, is referred by some to the dog and by others to the
bear family. The well-known plantigrade tread of bears is a
primitive characteristic which has survived from their creodont
ancestry.
CATS. The family of the cats, the most highly specialized of all
the carnivores, divided in the Tertiary into two main branches.
One, the saber-tooth tigers (Fig. 351), which takes its name from
their long, saberlike, sharp-edged upper canine teeth, evolved a
succession of genera and species, among them some of the most
destructive beasts of prey which ever scourged the earth. They
were masters of the entire northern hemisphere during the middle
Tertiary, but in Europe during the Pliocene they declined, from
unknown causes, and gave place to the other branch of cats,--which
includes the lions, tigers, and leopards. In the Americas the
saber-tooth tigers long survived the epoch.
MARINE MAMMALS. The carnivorous mammals of the sea--whales,
seals, walruses, etc.--seem to have been derived from some of the
creodonts of the early Tertiary by adaptation to aquatic life.
Whales evolved from some land ancestry at a very early date in the
Tertiary; in the marine deposits of the Eocene are found the bones
of the Zeuglodon, a whalelike creature seventy feet in length.
PRIMATES. This order, which includes lemurs, monkeys, apes, and
man, seems to have sprung from a creodont or insectivorous
ancestry in the lower Eocene. Lemur-like types, with small, smooth
brains, were abundant in the United States in the early Tertiary,
but no primates have been found here in the middle Tertiary and
later strata. In Europe true monkeys were introduced in the
Miocene, and were abundant until the close of the Tertiary, when
they were driven from the continent by the increasing cold.
ADVANCE OF THE MAMMALIA DURING THE TERTIARY. During the several
millions of years comprised in Tertiary time the mammals evolved
from the lowly, simple types which tenanted the earth at the
beginning of the period, into the many kinds of highly specialized
mammals of the Pleistocene and the present, each with the various
structures of the body adapted to its own peculiar mode of life.
The swift feet of the horse, the horns of cattle and the antlers
of the deer, the lion's claws and teeth, the long incisors of the
beaver, the proboscis of the elephant, were all developed in
Tertiary times. In especial the brain of the Tertiary mammals
constantly grew larger relatively to the size of body, and the
higher portion of the brain--the cerebral lobes--increased in size
in comparison with the cerebellum. Some of the hoofed mammals now
have a brain eight or ten times the size of that of their early
Tertiary predecessors of equal bulk. Nor can we doubt that along
with the increasing size of brain went a corresponding increase in
the keenness of the senses, in activity and vigor, and in
intelligence.