THE MESOZOIC
With the close of the Permian the world of animal and vegetable
life had so changed that the line is drawn here which marks the
end of the old order and the beginning of the new and separates
the Paleozoic from the succeeding era,--the Mesozoic, the Middle
Age of geological history. Although the Mesozoic era is shorter
than the Paleozoic, as measured by the thickness of their strata,
yet its duration must be reckoned in millions of years. Its
predominant life features are the culmination and the beginning of
the decline of reptiles, amphibians, cephalopod mollusks, and
cycads, and the advent of marsupial mammals, birds, teleost
fishes, and angiospermous plants. The leading events of the long
ages of the era we can sketch only in the most summary way.
The Mesozoic comprises three systems,--the _Triassic_, named from
its threefold division in Germany; the _Jurassic_, which is well
displayed in the Jura Mountains; and the _Cretaceous_, which
contains the extensive chalk (Latin, _creta_) deposits of Europe.
In eastern North America the Mesozoic rocks are much less
important than the Paleozoic, for much of this portion of the
continent was land during the Mesozoic era, and the area of the
Mesozoic rocks is small. In western North America, on the other
hand, the strata of the Mesozoic--and of the Cenozoic also--are
widely spread. The Paleozoic rocks are buried quite generally from
view except where the mountain makings and continental uplifts of
the Mesozoic and Cenozoic have allowed profound erosion to bring
them to light, as in deep canyons and about mountain axes. The
record of many of the most important events in the development of
the continent during the Mesozoic and Cenozoic eras is found in
the rocks of our western states.
The Triassic and Jurassic
=Eastern North America.= The sedimentary record interrupted by the
Appalachian deformation was not renewed in eastern North America
until late in the Triassic. Hence during this long interval the
land stood high, the coast was farther out than now, and over our
Atlantic states geological time was recorded chiefly in erosion
forms of hill and plain which have long since vanished. The area
of the later Triassic rocks of this region, which take up again
the geological record, is seen in the map of Figure 260. They lie
on the upturned and eroded edges of the older rocks and occupy
long troughs running for the most part parallel to the Atlantic
coast. Evidently subsidence was in progress where these rocks were
deposited. The eastern border of Appalachia was now depressed. The
oldland was warping, and long belts of country lying parallel to
the shore subsided, forming troughs in which thousands of feet of
sediment now gathered.
These Triassic rocks, which are chiefly sandstones, hold no marine
fossils, and hence were not laid in open arms of the sea. But
their layers are often ripple-marked, and contain many tracks of
reptiles, imprints of raindrops, and some fossil wood, while an
occasional bed of shale is filled with the remains of fishes. We
may conceive, then, of the Connecticut valley and the larger
trough to the southwest as basins gradually sinking at a rate
perhaps no faster than that of the New Jersey coast to-day, and as
gradually aggraded by streams from the neighboring uplands. Their
broad, sandy flats were overflowed by wandering streams, and when
subsidence gained on deposition shallow lakes overspread the
alluvial plains. Perhaps now and then the basins became long,
brackish estuaries, whose low shores were swept by the incoming
tide and were in turn left bare at its retreat to receive the rain
prints of passing showers and the tracks of the troops of reptiles
which inhabited these valleys.
The Triassic rocks are mainly red sandstones,--often feldspathic,
or arkose, with some conglomerates and shales. Considering the
large amount of feldspathic material in these rocks, do you infer
that they were derived from the adjacent crystalline and
metamorphic rocks of the oldland of Appalachia, or from the
sedimentary Paleozoic rocks which had been folded into mountains
during the Appalachian deformation? If from the former, was the
drainage of the northern Appalachian mountain region then, as now,
eastward and southeastward toward the Atlantic? The Triassic
sandstones are voluminous, measuring at least a mile in thickness,
and are largely of coarse waste. What do you infer as to the
height of the lands from which the waste was shed, or the
direction of the oscillation which they were then undergoing? In
the southern basins, as about Richmond, Virginia, are valuable
beds of coal; what was the physical geography of these areas when
the coal was being formed?
[Illustration: Fig. 315. Section of Triassic Sandstones of the
Connecticut Valley
_ss_, sandstones; _ll_, lava sheets; _cc_, crystalline igneous
and metamorphic rocks]
Interbedded with the Triassic sandstones are contemporaneous lava
beds which were fed from dikes. Volcanic action, which had been
remarkably absent in eastern North America during Paleozoic times,
was well-marked in connection with the warping now in progress.
Thick intrusive sheets have also been driven in among the strata,
as, for example, the sheet of the Palisades of the Hudson,
described on page 269.
The present condition of the Triassic sandstones of the
Connecticut valley is seen in Figure 315. Were the beds laid in
their present attitude? What was the nature of the deformation
which they have suffered? When did the intrusion of lava sheets
take place relative to the deformation? What effect have these
sheets on the present topography, and why? Assuming that the
Triassic deformation went on more rapidly than denudation, what
was its effect on the topography of the time? Are there any of its
results remaining in the topography of to-day? Do the Triassic
areas now stand higher or lower than the surrounding country, and
why? How do the Triassic sandstones and shales compare in hardness
with the igneous and metamorphic rocks about them? The Jurassic
strata are wanting over the Triassic areas and over all of eastern
North America. Was this region land or sea, an area of erosion or
sedimentation, during the Jurassic period? In New Jersey,
Pennsylvania, and farther southwest the lowest strata of the next
period, the Cretaceous, rest on the eroded edges of the earlier
rocks. The surface on which they lie is worn so even that we must
believe that at the opening of the Cretaceous the oldland of
Appalachia, including the Triassic areas, had been baseleveled at
least near the coast. When, therefore, did the deformation of the
Triassic rocks occur?
=Western North America.= Triassic strata infolded in the Sierra
Nevada Mountains carry marine fossils and reach a thickness of
nearly five thousand feet. California was then under water, and
the site of the Sierra was a subsiding trough slowly filling with
waste from the Great Basin land to the east.
Over a long belt which reaches from Wyoming across Colorado into
New Mexico no Triassic sediments are found, nor is there any
evidence that they were ever present; hence this area was high
land suffering erosion during the Triassic. On each side of it, in
eastern Colorado and about the Black Hills, in western Texas, in
Utah, over the site of the Wasatch Mountains, and southward into
Arizona over the plateaus trenched by the Colorado River, are
large areas of Triassic rocks, sandstones chiefly, with some rock
salt and gypsum. Fossils are very rare and none of them marine.
Here, then, lay broad shallow lakes often salt, and warped basins,
in which the waste of the adjacent uplands gathered. To this
system belong the sandstones of the Garden of the Gods in
Colorado, which later earth movements have upturned with the
uplifted mountain flanks.
The Jurassic was marked with varied oscillations and wide changes
in the outline of sea and land.
Jurassic shales of immense thickness--now metamorphosed into
slates--are found infolded into the Sierra Nevada Mountains. Hence
during Jurassic times the Sierra trough continued to subside, and
enormous deposits of mud were washed into it from the land lying
to the east. Contemporaneous lava flows interbedded with the
strata show that volcanic action accompanied the downwarp, and
that molten rock was driven upward through fissures in the crust
and outspread over the sea floor in sheets of lava.
=The Sierra deformation.= Ever since the middle of the Silurian, the
Sierra trough had been sinking, though no doubt with halts and
interruptions, until it contained nearly twenty-five thousand feet
of sediment. At the close of the Jurassic it yielded to lateral
pressure and the vast pile of strata was crumpled and upheaved
into towering mountains. The Mesozoic muds were hardened and
squeezed into slates. The rocks were wrenched and broken, and
underground waters began the work of filling their fissures with
gold-bearing quartz, which was yet to wait millions of years
before the arrival of man to mine it. Immense bodies of molten
rock were intruded into the crust as it suffered deformation, and
these appear in the large areas of granite which the later
denudation of the range has brought to light.
The same movements probably uplifted the rocks of the Coast Range
in a chain of islands. The whole western part of the continent was
raised and its seas and lakes were for the most part drained away.
=The British Isles.= The Triassic strata of the British Isles are
continental, and include breccia beds of cemented talus, deposits
of salt and gypsum, and sandstones whose rounded and polished
grains are those of the wind-blown sands of deserts. In Triassic
times the British Isles were part of a desert extending over much
of northwestern Europe.
The Cretaceous
The third great system of the Mesozoic includes many formations,
marine and continental, which record a long and complicated
history marked by great oscillations of the crust and wide changes
in the outlines of sea and land.
=Early Cretaceous.= In eastern North America the lowest Cretaceous
series comprises fresh-water formations which are traced from
Nantucket across Martha's Vineyard and Long Island, and through
New Jersey southward into Georgia. They rest unconformably on the
Triassic sandstones and the older rocks of the region. The
Atlantic shore line was still farther out than now in the northern
states. Again, as during the Triassic, a warping of the crust
formed a long trough parallel to the coast and to the Appalachian
ridges, but cut off from the sea; and here the continental
deposits of the early Cretaceous were laid.
Along the Gulf of Mexico the same series was deposited under like
conditions over the area known as the Mississippi embayment,
reaching from Georgia northwestward into Tennessee and thence
across into Arkansas and southward into Texas.
In the Southwest the subsidence continued until the transgressing
sea covered most of Mexico and Texas and extended a gulf northward
into Kansas. In its warm and quiet waters limestones accumulated
to a depth of from one thousand to five thousand feet in Texas,
and of more than ten thousand feet in Mexico. Meanwhile the
lowlands, where the Great Plains are now, received continental
deposits; coal swamps stretched from western Montana into British
Columbia.
=The Middle Cretaceous.= This was a land epoch. The early Cretaceous
sea retired from Texas and Mexico, for its sediments are overlain
unconformably by formations of the Upper Cretaceous. So long was the
time gap between the two series that no species found in the one
occurs in the other.
=The Upper Cretaceous.= There now began one of the most remarkable
events in all geological history,--the great Cretaceous subsidence.
Its earlier warpings were recorded in continental deposits,--wide
sheets of sandstone, shale, and some coal,--which were spread from
Texas to British Columbia. These continental deposits are overlain by
a succession of marine formations whose vast area is shown on the map,
Figure 260. We may infer that as the depression of the continent
continued the sea came in far and wide over the coast lands and the
plains worn low during the previous epochs. Upper Cretaceous
formations show that south of New England the waters of the Atlantic
somewhat overlapped the crystalline rocks of the Piedmont Belt and
spread their waste over the submerged coastal plain. The Gulf of
Mexico again covered the Mississippi embayment, reaching as far north
as southern Illinois, and extended over Texas. A mediterranean sea now
stretched from the Gulf to the arctic regions and from central Iowa to
the eastern shore of the Great Basin land at about the longitude of
Salt Lake City, the Colorado Mountains rising from it in a chain of
islands. Along with minor oscillations there were laid in the interior
sea various formations of sandstones, shales, and limestones, and from
Kansas to South Dakota beds of white chalk show that the clear, warm
waters swarmed at times with foraminiferal life whose disintegrating
microscopic shells accumulated in this rare deposit.
[Illustration: Fig. 316. Hypothetical Map of Upper Cretaceous
Epicontinental Seas
Shaded areas, probable seas; broken lines, approximate shore
lines]
[Illustration: Fig. 317. Foraminifera from Cretaceous Chalk,
Iowa]
At this epoch a wide sea, interrupted by various islands, stretched
across Eurasia from Wales and western Spain to China, and spread
southward over much of the Sahara. To the west its waters were clear
and on its floor the crumbled remains of foraminifers gathered in
heavy accumulations of calcareous ooze,--the white chalk of France and
England. Sea urchins were also abundant, and sponges contributed their
spicules to form nodules of flint.
=The Laramie.= The closing stage of the Cretaceous was marked in
North America by a slow uplift of the land. As the interior sea
gradually withdrew, the warping basins of its floor were filled with
waste from the rising lands about them, and over this wide area
there were spread continental deposits in fresh-water lakes like the
Great Lakes of the present, in brackish estuaries, and in river
plains, while occasional oscillations now and again let in the sea.
There were vast marshes in which there accumulated the larger part
of the valuable coal seams of the West. The Laramie is the
coal-bearing series of the West, as the Pennsylvanian is of the
eastern part of our country.
=The Rocky Mountain deformation.= At the close of the Cretaceous we
enter upon an epoch of mountain-making far more extensive than any which
the continent had witnessed. The long belt lying west of the ancient
axes of the Colorado Islands and east of the Great Basin land had been
an area of deposition for many ages, and in its subsiding troughs
Paleozoic and Mesozoic sediments had gathered to the depth of many
thousand feet. And now from Mexico well-nigh to the Arctic Ocean this
belt yielded to lateral pressure. The Cretaceous limestones of Mexico
were folded into lofty mountains. A massive range was upfolded where the
Wasatch Mountains now are, and various ranges of the Rockies in Colorado
and other states were upridged. However slowly these deformations were
effected they were no doubt accompanied by world-shaking earthquakes,
and it is known that volcanic eruptions took place on a magnificent
scale. Outflows of lava occurred along the Wasatch, the laccoliths of
the Henry Mountains (p. 271) were formed, while the great masses of
igneous rock which constitute the cores of the Spanish Peaks (p. 271)
and other western mountains were thrust up amid the strata. The high
plateaus from which many of these ranges rise had not yet been uplifted,
and the bases of the mountains probably stood near the level of the sea.
North America was now well-nigh completed. The mediterranean seas
which so often had occupied the heart of the land were done away
with, and the continent stretched unbroken from the foot of the
Sierras on the west to the Fall Line of the Atlantic coastal plain
on the east.
=The Mesozoic peneplain.= The immense thickness of the Mesozoic
formations conveys to our minds some idea of the vast length of time
involved in the slow progress of its successive ages. The same
lesson is taught as plainly by the amount of denudation which the
lands suffered during the era.
The beginning of the Mesozoic saw a system of lofty mountain ranges
stretching from New York into central Alabama. The end of this long
era found here a wide peneplain crossed by sluggish wandering rivers
and overlooked by detached hills as yet unreduced to the general
level. The Mesozoic era was long enough for the Appalachian
Mountains, upridged at its beginning, to have been weathered and
worn away and carried grain by grain to the sea. The same plain
extended over southern New England. The Taconic range, uplifted
partially at least at the close of the Ordovician, and the block
mountains of the Triassic, together with the pre-Cambrian mountains
of ancient Appalachia, had now all been worn to a common level with
the Allegheny ranges. The Mesozoic peneplain has been upwarped by
later crustal movements and has suffered profound erosion, but the
remnants of it which remain on the upland of southern New England
and the even summits of the Allegheny ridges suffice to prove that
it once existed. The age of the Mesozoic peneplain is determined
from the fact that the lower Tertiary sediments were deposited on
its even surface when at the close of the era the peneplain was
depressed along its edges beneath the sea.
Life of the Mesozoic
=Plant life of the Triassic and Jurassic.= The Carboniferous forests
of lepidodendrons and sigillarids had now vanished from the earth.
The uplands were clothed with conifers, like the Araucarian pines
of South America and Australia. Dense forests of tree ferns throve
in moist regions, and canebrakes of horsetails of modern type, but
with stems reaching four inches in thickness, bordered the lagoons
and marshes. Cycads were exceedingly abundant. These gymnosperms,
related to the pines and spruces in structure and fruiting, but
palmlike in their foliage, and uncoiling their long leaves after
the manner of ferns, culminated in the Jurassic. From the view
point of the botanist the Mesozoic is the Age of Cycads, and after
this era they gradually decline to the small number of species now
existing in tropical latitudes.
[Illustration: Fig. 318. A Living Cycad of Australia]
[Illustration: Fig. 319. Stem of a Mesozoic Cycad]
=Plant life of the Cretaceous.= In the Lower Cretaceous the woodlands
continued of much the same type as during the Jurassic. The
forerunners now appeared of the modern dicotyls (plants with two seed
leaves), and in the Middle Cretaceous the monocotyledonous group of
palms came in. Palms are so like cycads that we may regard them as the
descendants of some cycad type.
In the _Upper Cretaceous_, cycads become rare. The highest types of
flowering plants gain a complete ascendency, and forests of modern
aspect cover the continent from the Gulf of Mexico to the Arctic
Ocean. Among the kinds of forest trees whose remains are found in
the continental deposits of the Cretaceous are the magnolia, the
myrtle, the laurel, the fig, the tulip tree, the chestnut, the
oak, beech, elm, poplar, willow, birch, and maple. Forests of
Eucalyptus grew along the coast of New England, and palms on the
Pacific shores of British Columbia. Sequoias of many varieties
ranged far into northern Canada. In northern Greenland there were
luxuriant forests of magnolias, figs, and cycads; and a similar
flora has been disinterred from the Cretaceous rocks of Alaska and
Spitzbergen. Evidently the lands within the Arctic Circle enjoyed
a warm and genial climate, as they had done during the Paleozoic.
Greenland had the temperature of Cuba and southern Florida, and
the time was yet far distant when it was to be wrapped in glacier
ice.
[Illustration: Fig. 320. A Jurassic Long-Tailed Crustacean]
=Invertebrates.= During the long succession of the ages of the
Mesozoic, with their vast geographical changes, there were many
and great changes in organisms. Species were replaced again and
again by others better fitted to the changing environment. During
the Lower Cretaceous alone there were no less than six successive
changes in the faunas which inhabited the limestone-making sea
which then covered Texas. We shall disregard these changes for the
most part in describing the life of the era, and shall confine our
view to some of the most important advances made in the leading
types.
Stromatopora have disappeared. Protozoans and sponges are
exceedingly abundant, and all contribute to the making of Mesozoic
strata. Corals have assumed a more modern type. Sea urchins have
become plentiful; crinoids abound until the Cretaceous, where they
begin their decline to their present humble station.
[Illustration: Fig. 321. A Fossil Crab]
[Illustration: Fig. 322. Cretaceous Mollusks
_A_, Ostrea (oyster); _B_, Exogyra; _C_. Gryphæa]
Trilobites and eurypterids are gone. Ten-footed crustaceans abound of
the primitive long-tailed type (represented by the lobster and the
crayfish), and in the Jurassic there appears the modern short-tailed
type represented by the crabs. The latter type is higher in
organization and now far more common. In its embryological development
it passes through the long-tailed stage; connecting links in the
Mesozoic also indicate that the younger type is the offshoot of the
older.
Insects evolve along diverse lines, giving rise to beetles, ants,
bees, and flies.
Brachiopods have dwindled greatly in the number of their species,
while mollusks have correspondingly increased. The great oyster family
dates from here.
Cephalopods are now to have their day. The archaic Orthoceras lingers
on into the Triassic and becomes extinct, but a remarkable development
is now at hand for the more highly organized descendants of this
ancient line. We have noticed that in the Devonian the sutures of some
of the chambered shells become angled, evolving the Goniatite type.
The sutures now become lobed and _corrugated_ in _Ceratites_. The process
was carried still farther, and the sutures were elaborately frilled in
the great order of the Ammonites. It was in the Jurassic that the
Ammonites reached their height. No fossils are more abundant or
characteristic of their age. Great banks of their shells formed beds
of limestone in warm seas the world over.
[Illustration: Fig. 323. Ceratites]
[Illustration: Fig. 324. An Ammonite
A portion of the shell is removed to show frilling of suture]
[Illustration: Fig. 325. Slab of Rock covered with
Ammonites,--a Bit of a Mesozoic Sea Bottom]
[Illustration: Fig. 326. Representative Species of Different
Families of Ammonites]
The ammonite stem branched into a most luxuriant variety of forms.
The typical form was closely coiled like a nautilus. In others the
coil was more or less open, or even erected into a spiral. Some
were hook-shaped, and there were members of the order in which the
shell was straight, and yet retained all the internal structures
of its kind. At the end of the Mesozoic the entire tribe of
ammonites became extinct.
The Belemnite (Greek, _belemnon_, a dart) is a distinctly higher
type of cephalopod which appeared in the Triassic, became numerous
and varied in the Jurassic and Cretaceous, and died out early in
the Tertiary. Like the squids and cuttlefish, of which it was the
prototype, it had an internal calcareous shell. This consisted of
a chambered and siphuncled cone, whose point was sheathed in a
long solid guard somewhat like a dart. The animal carried an ink
sac, and no doubt used it as that of the modern cuttlefish is
used,--to darken the water and make easy an escape from foes.
Belemnites have sometimes been sketched with fossil sepia, or
india ink, from their own ink sacs. In the belemnites and their
descendants, the squids and cuttlefish, the cephalopods made the
radical change from external to the internal shell. They abandoned
the defensive system of warfare and boldly took up the offensive.
No doubt, like their descendants, the belemnites were exceedingly
active and voracious creatures.
[Illustration: Fig. 327. Internal Shell of Belemnite]
=Fishes and amphibians.= In the Triassic and Jurassic, little
progress was made among the fishes, and the ganoid was still the
leading type. In the Cretaceous the teleosts, or bony fishes, made
their appearance, while ganoids declined toward their present
subordinate place.
The amphibians culminated in the Triassic, some being formidable
creatures as large as alligators. They were still of the primitive
Paleozoic types. Their pygmy descendants of more modern types are
not found until later, salamanders appearing first in the
Cretaceous, and frogs at the beginning of the Cenozoic.
No remains of amphibians have been discovered in the Jurassic. Do
you infer from this that there were none in existence at that
time?
Reptiles of the Mesozoic
The great order of Reptiles made its advent in the Permian, culminated
in the Triassic and Jurassic, and began to decline in the Cretaceous.
The advance from the amphibian to the reptile was a long forward step
in the evolution of the vertebrates. In the reptile the vertebrate
skeleton now became completely ossified. Gills were abandoned and
breathing was by lungs alone. The development of the individual from
the egg to maturity was uninterrupted by any metamorphosis, such as
that of the frog when it passes from the tadpole stage. Yet in
advancing from the amphibian to the reptile the evolution of the
vertebrate was far from finished. The cold-blooded, clumsy and
sluggish, small-brained and unintelligent reptile is as far inferior
to the higher mammals, whose day was still to come, as it is superior
to the amphibian and the fish.
The reptiles of the Permian, the earliest known, were much like
lizards in form of body. Constituting a transition type between the
amphibians on the one hand, and both the higher reptiles and the
mammals on the other, they retained the archaic biconcave vertebra of
the fish and in some cases the persistent notochord, while some of
them, the theromorphs, possessed characters allying them with mammals.
In these the skull was remarkably similar to that of the carnivores,
or flesh-eating mammals, and the teeth, unlike the teeth of any later
reptiles, were divisible into incisors, canines, and molars, as are
the teeth of mammals (Fig. 328).
[Illustration: Fig. 328. Skull of a Permian Theromorph]
At the opening of the Mesozoic era reptiles were the most highly
organized and powerful of any animals on the earth. New ranges of
continental extent were opened to them, food was abundant, the climate
was congenial, and they now branched into very many diverse types
which occupied and ruled all fields,--the land, the air, and the sea.
The Mesozoic was the Age of Reptiles.
=The ancestry of surviving reptilian types.= We will consider first
the evolution of the few reptilian types which have survived to the
present.
Crocodiles, the highest of existing reptiles, are a very ancient
order, dating back to the lower Jurassic, and traceable to earlier
ancestral, generalized forms, from which sprang several other orders
also.
Turtles and tortoises are not found until the early Jurassic, when
they already possessed the peculiar characteristics which set them off
so sharply from other reptiles. They seem to have lived at first in
shallow water and in swamps, and it is not until after the end of the
Mesozoic that some of the order became adapted to life on the land.
The largest of all known turtles, _Archelon_, whose home was the great
interior Cretaceous sea, was fully a dozen feet in length and must
have weighed at least two tons. The skull alone is a yard long.
Lizards and snakes do not appear until after the close of the
Mesozoic, although their ancestral lines may be followed back into the
Cretaceous.
We will now describe some of the highly specialized orders peculiar to
the Mesozoic.
=Land reptiles.= The _dinosaurs_ (terrible reptiles) are an extremely
varied order which were masters of the land from the late Trias until
the close of the Mesozoic era. Some were far larger than elephants,
some were as small as cats; some walked on all fours, some were
bipedal; some fed on the luxuriant tropical foliage, and others on the
flesh of weaker reptiles. They may be classed in three divisions,--the
_flesh-eating dinosaurs_, the _reptile-footed dinosaurs_, and the
_beaked dinosaurs_,--the latter two divisions being herbivorous.
The _flesh-eating dinosaurs_ are the oldest known division of the
order, and their characteristics are shown in Figure 329. As a class,
reptiles are egg layers (_oviparous_); but some of the flesh-eating
dinosaurs are known to have been _viviparous_, i.e. to have brought
forth their young alive. This group was the longest-lived of any of
the three, beginning in the Trias and continuing to the close of the
Mesozoic era.
[Illustration: Fig. 329. Ceratosaurus]
Contrast the small fore limbs, used only for grasping, with the
powerful hind limbs on which the animal stalked about. Some of the
species of this group seem to have been able to progress by
leaping in kangaroo fashion. Notice the sharp claws, the ponderous
tail, and the skull set at right angles with the spinal column.
The limb bones are hollow. The ceratosaurs reached a length of
some fifteen feet, and were not uncommon in Colorado and the
western lands in Jurassic times.
[Illustration: Fig. 330. Diplodocus]
The _reptile-footed dinosaurs_ (Sauropoda) include some of the
biggest brutes which ever trod the ground. One of the largest,
whose remains are found entombed in the Jurassic rocks of Wyoming
and Colorado, is shown in Figure 330.
Note the five digits on the hind feet, the quadrupedal gait, the
enormous stretch of neck and tail, the small head aligned with the
vertebral column. Diplodocus was fully sixty-five feet long and
must have weighed about twenty tons. The thigh bones of the
Sauropoda are the largest bones which ever grew. That of a genus
allied to the Diplodocus measures six feet and eight inches, and
the total length of the animal must have been not far from eighty
feet, the largest land animal known.
The Sauropoda became extinct when their haunts along the rivers
and lakes of the western plains of Jurassic times were invaded by
the Cretaceous interior sea.
The _beaked dinosaurs_ (Predentata) were distinguished by a beak
sheathed with horn carried in front of the tooth-set jaw, and
used, we may imagine, in stripping the leaves and twigs of trees
and shrubs. We may notice only two of the most interesting types.
[Illustration: Fig. 331. Stegosaurus]
_Stegosaurus_ (plated reptile) takes its name from the double row of
bony plates arranged along its back. The powerful tail was armed
with long spines, and the thick skin was defended with irregular
bits of bone implanted in it. The brain of the stegosaur was
smaller than that of any land vertebrate, while in the sacrum the
nerve canal was enlarged to ten times the capacity of the brain
cavity of the skull. Despite their feeble wits, this well-armored
family lived on through millions of years which intervened between
their appearance, at the opening of the Jurassic, and the close of
the Cretaceous, when they became extinct.
A less stupid brute than the stegosaur was _Triceratops_, the
dinosaur of the three horns,--one horn carried on the nose, and a
massive pair set over the eyes (Fig. 332). Note the enormous wedge-shaped
skull, with its sharp beak, and the hood behind resembling a
fireman's helmet. Triceratops was fully twenty-five feet long, and
of twice the bulk of an elephant. The family appeared in the Upper
Cretaceous and became extinct at its close. Their bones are found
buried in the fresh-water deposits of the time from Colorado to
Montana and eastward to the Dakotas.
[Illustration: Fig. 332. Restoration of Triceratops
By courtesy of the American Museum of Natural History]
=Marine reptiles.= In the ocean, reptiles occupied the place now
held by the aquatic mammals, such as whales and dolphins, and
their form and structure were similarly modified to suit their
environment. In the Ichthyosaurus (fish reptile), for example, the
body was fishlike in form, with short neck and large, pointed head
(Fig. 333).
[Illustration: Fig. 333. Ichthyosaurus]
A powerful tail, whose flukes were set vertical, and the lower one
of which was vertebrated, served as propeller, while a large
dorsal fin was developed as a cutwater. The primitive biconcave
vertebræ of the fish and of the early land vertebrates were
retained, and the limbs degenerated into short paddles. The skin
of the ichthyosaur was smooth like that of a whale, and its food
was largely fish and cephalopods, as the fossil contents of its
stomach prove.
These sea monsters disported along the Pacific shore over northern
California in Triassic times, and the bones of immense members of
the family occur in the Jurassic strata of Wyoming. Like whales
and seals, the ichthyosaurs were descended from land vertebrates
which had become adapted to a marine habitat.
[Illustration: Fig. 334. Plesiosaurus]
_Plesiosaurs_ were another order which ranged throughout the
Mesozoic. Descended from small amphibious animals, they later
included great marine reptiles, characterized in the typical genus
by long neck, snakelike head, and immense paddles. They swam in
the Cretaceous interior sea of western North America.
[Illustration: Fig. 335. Restoration of a Mosasaur]
_Mosasaurs_ belong to the same order as do snakes and lizards, and
are an offshoot of the same ancestral line of land reptiles. These
snakelike creatures--which measured as much as forty-five feet in
length--abounded in the Cretaceous seas. They had large conical
teeth, and their limbs had become stout paddles.
The lower jaw of the mosasaur was jointed; the quadrate bone,
which in all reptiles connects the bone of the lower jaw with the
skull, was movable, and as in snakes the lower jaw could be used
in thrusting prey down the throat. The family became extinct at
the end of the Mesozoic, and left no descendants. One may imitate
the movement of the lower jaw of the mosasaur by extending the
arms, clasping the hands, and bending the elbows.
=Flying reptiles.= The atmosphere, which had hitherto been tenanted
only by insects, was first conquered by the vertebrates in the
Mesozoic. _Pterosaurs_, winged reptiles, whose whole organism was
adapted for flight through the air, appeared in the Jurassic and
passed off the stage of existence before the end of the
Cretaceous. The bones were hollow, as are those of birds. The
sternum, or breastbone, was given a keel for the attachment of the
wing muscles. The fifth finger, prodigiously lengthened, was
turned backward to support a membrane which was attached to the
body and extended to the base of the tail. The other fingers were
free, and armed with sharp and delicate claws, as shown in Figures
336 and 337.
[Illustration: Fig. 336. Restoration of a Pterosaur]
[Illustration: Fig. 337. Skeletons of Pterosaur Ornithostoma,
_A_, and of the Condor, _B_
After Lucas]
These "dragons of the air" varied greatly in size; some were as
small as sparrows, while others surpassed in stretch of wing the
largest birds of the present day. They may be divided into two
groups. The earliest group comprises genera with jaws set with
teeth, and with long tails sometimes provided with a rudderlike
expansion at the end. In their successors of the later group the
tail had become short, and in some of the genera the teeth had
disappeared. Among the latest of the flying reptiles was
_Ornithostoma_ (bird beak), the largest creature which ever flew,
and whose remains are imbedded in the offshore deposits of the
Cretaceous sea which held sway over our western plains.
Ornithostoma's spread of wings was twenty feet. Its bones were a
marvel of lightness, the entire skeleton, even in its petrified
condition, not weighing more than five or six pounds. The sharp
beak, a yard long, was toothless and bird-like, as its name
suggests.
[Illustration: Fig. 338. Archæopteryx]
=Birds.= The earliest known birds are found in the Jurassic, and
during the remainder of the Mesozoic they contended with the
flying reptiles for the empire of the air. The first feathered
creatures were very different from the birds of to-day. Their
characteristics prove them an offshoot of the dinosaur line of
reptiles. _Archæopteryx_ (_ancient bird_) (Fig. 338) exhibits a
strange mingling of bird and reptile. Like birds, it was fledged
with perfect feathers, at least on wings and tail, but it retained
the teeth of the reptile, and its long tail was vertebrated,
a pair of feathers springing from each joint. Throughout the
Jurassic and Cretaceous the remains of birds are far less common
than those of flying reptiles, and strata representing hundreds of
thousands of years intervene between Archæopteryx and the next
birds of which we know, whose skeletons occur in the Cretaceous
beds of western Kansas.
=Mammals.= So far as the entries upon the geological record show,
mammals made their advent in a very humble way during the Trias.
These earliest of vertebrates which suckle their young were no
bigger than young kittens, and their strong affinities with the
theromorphs suggest that their ancestors are to be found among
some generalized types of that order of reptiles.
[Illustration: Fig. 339. Jawbone of a Jurassic Mammal]
During the long ages of the Mesozoic, mammals continued small and
few, and were completely dominated by the reptiles. Their remains
are exceedingly rare, and consist of minute scattered teeth,--with
an occasional detached jaw,--which prove them to have been flesh
or insect eaters. In the same way their affinities are seen to be
with the lowest of mammals,--the _monotremes_ and _marsupials_.
The monotremes,--such as the duckbill mole and the spiny ant-eater
of Australia, reproduce by means of eggs resembling those of
reptiles; the marsupials, such as the opossum and the kangaroo,
bring forth their young alive, but in a very immature condition,
and carry them for some time after birth in the marsupium, a pouch
on the ventral side of the body.