Life of the Past
The Mesozoic (Middle Life) Era
covers a period of about 165 million years, during
which reptiles so overshadowed all other animals
that it is often called the “Age of Reptiles.” Great
changes took place in some invertebrates too. New
forms replaced those which had become extinct at the
end of the Paleozoic. Ammonites developed rapidly
until countless numbers lived in the seas. Birds,
mammals, flowering plants and many modern insects
appeared for the first time. Elm, oak, maple and
other modern broad- leaved trees became common. The
development and spread of some flowering plants
depended on the parallel development of insects
which pollinated the flowers. Other important
geographic changes were also taking place. New
patterns of lands and seas formed. New mountain
ranges slowly emerged. As the result of several
related geologic processes, great mineral deposits
were formed. Sixty million years later, we still
depend on many of these deposits for our metals and
our fuels. THE TRIASSIC PERIOD
The Triassic period (230 to 180 million years
ago) was named from a threefold division of its
rocks. In many places Triassic rocks resemble
those of the Permian Period—thick sequences of
red shales and sandstones, deposited in
temporary lakes, deserts and basins. Volcanic
activity was considerable, as in eastern North
America from Virginia to Connecticut. Against
this background the reptiles developed and
established their mastery. Their advanced body
structure and shell-protected eggs enabled them to
survive changing and often adverse climates, and to
colonize new areas which were forbidden to the
water-tied amphibia. The first dinosaurs appeared;
their footprints are abundant in some rocks, as in
the Connecticut Valley. Nor was the dominance of
reptiles confined to land, for in open oceans
dolphin-like ichthyosaurs swept through the water.
Later 15- to 20-foot long plesiosaurs paddled their
way through Triassic seas. New types of sponges
and protozoans developed. The modern hexacorals
appeared, and new groups of brachiopods replaced
their Permian forebears. Gastropods and pelecypods
increased in numbers. Ammonites flourished and
underwent considerable change. Lobster-like
arthropods and modern echinoids and crinoids first
appeared in the Triassic. Cycads and primitive
conifers flourished on upland areas. The Petrified
Forest of Arizona contains fossils of these trees.
Ferns and scouring rushes prospered in
lower, moist areas. THE JURASSIC PERIOD
Named
for the Jura Mountains, this period began about 180
million years ago and lasted about 45 million years.
Of all its abundant and exotic life, none was more
typical than the dinosaurs of which there were three
main Jurassic groups: first, the sauropods,
long-necked, long-tailed, four-legged monsters,
which included the largest land animal (Diplodocus,
87 ft. long); second, stegosaurs, armored reptiles
that weighed up to 10 tons (with only a 3-oz.
brain); third, the carnivorous theropods which
walked on their hind legs, including Allosaurus, a
savage, 35-ft. creature. Others were more slender,
and some were only 3 feet long. A few early,
duck-billed herbivorous dinosaurs lumbered across
the swampy lowlands. Flying reptiles gliding
through the air included sparrow- sized species and
others up to 4 feet long with slender club-like
tails. Ichthyosaurs and plesiosaurs were the
carnivorous masters of the oceans. Hosts of
ammonites (some up to 6 feet in diameter) thronged
the shallow seas, together with gastropods,
pelecypods, squids (belemnites), echinoids, crinoids
and foraminifera. The Jurassic also saw the
development of two groups that were later to
establish their dominance. The oldest mammals are
known from fossil fragments of rat-sized jaws and
teeth from western United States and Europe. The
Solenhofen Jurassic limestone of Bavaria contains
remains of Archaeopteryx, the oldest known bird.
Jurassic plants included the now extinct cycadeoids
with short, thick trunks. These were crowned with
frond- like leaves and ornate reproductive
structures which closely resembled modern flowers.
Cycads, conifers, ferns and ginkgos were common.
Ginkgos were widespread all through the Mesozoic but
later became almost extinct. Today only a single
species survives but this is widely planted. Over a
thousand species of insects are known, including
many modern forms. CRETACEOUS PERIOD
The Cretaceous Period (named from chalk, its
most characteristic deposit) began about 135
million years ago and lasted some 70 million
years. It was one of the most important of all
geologic periods, marked by a major advance of
the sea in many parts of the world, and by the
great thickness of both marine and continental
sediments.
A great depression connected the Arctic Ocean
with the Gulf of Mexico. Middle Europe was also
submerged except for a central land mass.
Towards the close of this period earth movements
produced mountain ranges which are now the Andes
and the Rockies, as well as mountains in
Antarctica and northeastern Asia.
The Cretaceous Period marked both the
culmination of Mesozoic life and the
foreshadowing of animals and plants that were
later to displace it. The most important new
arrivals were the flowering plants
(angiosperms). They first appeared in the Lower
Cretaceous, but eventually they became the
dominant plants on every continent.
Many familiar living trees and shrubs,
including the poplar, magnolia, oak, maple;
beech, holly, ivy and laurel appeared during the
Cretaceous. The spread of the flowering plants
also had important effects on animal life, for
they provided new sources of food for mammals,
birds. reptiles and insects. The subsequent
expansion of mammals and birds depended very
largely upon these new food supplies.
Dinosaurs extended their dominance across Cretaceous
lands during this time. They are known from every
continent, and included many unusual types. Horned
dinosaurs (ceratopsians) were common, as were the
armored ankylosaurs, and the bizarre duck-bills with
their striking amphibious adaptations. The great
quadruped dinosaurs declined in the Cretaceous but
savage carnivores were common. Tyrannosaurus stood
20 feet high and had a skull over 3 feet long. Other
carnivores were much smaller. Flying reptiles were
represented by Pleranodon, a toothless, hammer-
headed creature with a wingspan of 25 feet, the
largest animal ever to fly. In the seas, giant
turtles (Archelon) reached a length of 12 feet, and
some plesiosaurs grew over 40 feet long.
Ichthyosaurs declined. Savage, serpent-like
mosasaurs, some 35 feet long, were sea-going
lizards. Two well-known types of fossil birds
occur in Cretaceous rocks. Ichthyornis, a slender,
tern-like bird about 8 inches high, was a strong
flier. Hesperornis, in complete contrast, was about
4% feet high, a diving bird, with powerful swimming
legs but only vestiges of wings. It also had long,
toothed jaws. Mammals were small and relatively
insignificant. Their remains are rare, represented
by small primitive forms that survived from the
Jurassic and also by two new groups, the
opossum-like pouched marsupials and the
insectivores, forerunners of the shrews. The fossils
are mostly teeth and parts of lower jaws, which,
because of their unique structures, are sufficient
to distinguish these true mammals from mammal-like
reptiles. In the shallow seas invertebrates lived
in great diversity. The dominant group was the
ammonites, which showed many unusual forms.
Belemnites, pelecypods and gastropods of rather
modern appearance, corals, sea urchins and
foraminifera also flourished. Modern bony fish (teleosts)
were common. The corals, abundant locally in
Cretaceous beds, show a basic sixfold symmetry.
Crinoids also developed new forms, including a
tree-swimming, stemless crinoid with slender arms up
to 4 ft. long. Inoceramus mollusks, sometimes 3to 5
ft. across, were widely distributed. The close of
the Cretaceous saw the widespread extinction of many
of the dominant animals of the Mesozoic Era.
Dinosaurs, pterosaurs, ichthyosaurs, plesiosaurs,
mosasaurs, ammonites, true belemnites, many
pelecypods and corals all became extinct. It was as
truly the end of an era in the long history of life
as was the Permian, and the causes for this
widespread decline are no less difficult to
identify. It is probable, however, that the great
geological changes, and the changes in plants
exercised a profound effect on many groups of
animals. |
