Life of the Past
The Cenozoic Era
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The Cenozoic Era includes the last 70 million
years of earth history. It is far more familiar than
previous eras, for although some animals and plants
died out other kinds have survived without drastic
change and are alive today. Slow but striking
changes in climate took place during this time.
Polar regions cooled and the general warm temperate
climate gave way to a wider climatic range.
The
continents were similar to those of today, although
there was mountain building, continental warping and
volcanic activity. Cenozoic strata in the Gulf
Coast, California, the Middle East and the East
Indies are now important petroleum producers.
Cenozoic life (the Age of Mammals) is dominated by
the mammals and flowering plants. Mammals replaced
the ruling reptiles of the Mesozoic in every
environment. The flowering plants became broadly
similar to living forms. Amphibia and reptiles
became relatively inconspicuous. Birds continued to
expand in numbers and variety. The bony fish (teleosts)
outnumbered all other fish by twenty to one.
Marine invertebrates took on a modern look.
Gastropods and pelecypods became the most abundant,
and cephalopods and brachiopods were greatly
reduced. This then is the last great era in the long
history of life—the prelude to the present.
THE LOWER TERTIARY PERIOD
The Lower Tertiary period (Paleocene, Eocene, and
Oligocene) in North America is represented by great
continental, badland deposits which contain many
mammalian fossils. Thick marine deposits formed in
south-eastern U.S. and along the Pacific Coast where
volcanoes also erupted.
Lower Tertiary fossils are strikingly different from
those of the Cretaceous. Mammals increased
explosively, spreading into all environments and
becoming adapted to many ways of life on land, water
and in the air. Mammalian history varied. In
isolated Tertiary South America a great diversity of
marsupial mammals developed. Other marsupials still
survive in the isolation of Australia.
Early Tertiary mammals included primitive
rodent-sized forms, insectivores and marsupials,
that had persisted from the Cretaceous. New species
included two main groups, hoofed mammals and
carnivores.
Creodonts, forerunners of the carnivores, and
condylarths, forerunners of hoofed mammals, were
somewhat similar, differing mainly in details of
teeth and feet. Both were squat, heavy-limbed, about
the size of a collie, with dog-like heads and five
bluntly clawed toes on each limb. Their brains were
small and primitive. Other new arrivals were
ancestral rodents and primates, represented by
lemur-like animals.
Amblypods were heavy, clumsy, hoofed mammals, with
broad feet. The early ones were sheep-sized, but the
later kinds (uintatheres) were seven feet high, as
big as a large rhinoceros, with three pairs of blunt
horns. The males had large down-curved tusks.
During Eocene times more advanced mammals displaced
many older forms in North America and Europe. These
new arrivals included true rodents, mainly squirrel-
like species, and a variety of rhinoceroses (one
Oligocene giant, Baluchitherium, measured 18 feet
high at the shoulders). Ancestral tapirs,
titanotheres and the first even- toed hoofed mammals
also appeared. The early titanotheres were small,
hornless browsers with clumsy bodies and tiny
brains. Condylarths, creodonts and uintatheres
persisted for a time, but lost out to later arrivals
such as early horses (the three-toed, collie-sized
Mesohippus), giant pigs, ancestral camels, oreodonts,
mastodons and large saber-toothed cats.
Oreodonts were sheep-sized, long-tailed herbivores
that survived until the Pliocene. Titanotheres were
huge, grotesque horned beasts. The cat family first
appeared in the form of the earliest saber-tooth,
Hoplophoneus, about the size of a mountain lion.
Lower Tertiary birds were modern in appparance, but
included numbers of large, ground-living genera, one
of which, Dial ryma, was 7 feet high. Fish, too,
were of modern aspect. The fine fresh-water
sediments of the Green River beds of Wyoming have
yielded thousands of beautiful, well-preserved fish
fossils.
Marine invertebrates were very much like modern
forms. Large foraminifera (“Nummuliles”) abounded in
the shallow seas of the Mediterranean and Caribbean.
Plants resembled living forms, but palms grew in
Canada, and temperate oak and walnut forests in
Alaska. There was mountain building and crustal
disturbance in the Alps, Carpathians, Pyrenees,
Apennines, and Himalayas. The Coast Range of western
North America was the scene of mountain building
too, with volcanic activity.
THE UPPER TERTIARY PERIOD
The Upper Tertiary Period (Miocene and Pliocene)
lasted about 25 million years, ending about a
million years ago. It is marked by the continued
rise of modern mammals.
Changes involving brain, limbs and teeth and the
accompanying expansion of mammals as a group were
closely related to climatic changes. Over wide areas
of North America continental uplift produced drier
climates, and converted lush lowland forests into
grassy prairies. The oldest common grasses come from
the Miocene. Many mammalian changes were associated
with the change from browsing to grazing habits.
Changes of this kind are particularly well
illustrated in the horse family. Some of these
changes were correlated with increase in overall
size, but others were not. Thus horse teeth became
larger and deeper, but they also became
high-crowned, with a square, infolded, chewing
surface. The limbs became longer and changed in
relative proportions, but the number of
ground-touching toes was reduced. This reflects a
radical change from a flat-footed to a tip-toe,
spring-hoofed posture. Feeding on tough prairie
grasses demanded tougher teeth, and the advantage of
speed on the hard, open prairies favored the new
foot structure.
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Evolution of the Horse |
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Widespread changes also took place in other groups
of mammals. Ancestral elephants, camels,
rhinoceroses, dogs and smaller carnivores abounded.
Other forms have no living descendants. Moropv’s
resembled a clumsily constructed horse with claws.
One giant pig had a skull four feet long. There were
giraffes, camels, and the pathetic antelope,
Syndyoceras, with the strangest horns of any of its
tribe. Saber-toothed cats continued, and apelike
creatures (Dryopllhecus) spread across Europe and
Africa. Many of the older type mammals became
extinct towards the close of Pliocene times.
Most Upper Tertiary marine invertebrates and plants
are barely distinguishable from modern species.
Renewed earth movements in the Alps, Himalayas and
along the Pacific Coast of North America complicated
and extended existing mountain ranges.
Our hurried excursion through geologic time has
taken us, with giant steps, over a period of more
than half a billion years from the late Pre-Cambrian
to the end of the Tertiary. The late Tertiary world
is modern except for minor species development
including the explosive dominance of man.
Though fossil clues are meager, the panorama of life
is understandable because of the length of geologic
time. The slow organic changes which led to
extinction or survival are the building blocks of
evolution and, for their operation, time is needed.
Man’s development is a matter of only a few million
years, but a billion years of preparation lie behind
it.
THE QUATERNARY PERIOD
The Quaternary period includes the moment in which
we now live, together with the Pleistocene Epoch,
which began about one million years ago. Continental
ice sheets, up to ten thousand feet thick, spread
over much of the Northern Hemisphere in at least
four glacial advances, the last of which retreated
about 11,000 years ago in North America. Antarctica
and the mountains of the Southern Hemisphere were
also glaciated.
There is evidence of repeated floral and faunal
migrations in response to climatic changes. During
the colder episodes vast herds of wild pigs, camels,
bison and elephants ranged across North America,
Europe and Asia. There were four American species of
elephants, including the Imperial Mammoth, 14 feet
high at the shoulder, with curved tusks 13 feet
long.
Majestic woolly mammoths, which roamed across the
tundra of Europe, Asia and North America, are
pictured in early cave paintings. Most of these
large mammals became extinct near the end of the
Pleistocene. Carnivores, including wolves, foxes,
badgers, and the terrible saber-tooth Smilodon, are
well known from the Pleistocene tar pools of
California. Huge armadillo-like glyptodonts and
giant (20 ft. high) ground sloths had evolved in
South America and spread into North America when the
land bridge between the two continents was
re-established in late Pliocene times. In this arid
and partly frozen world, man emerged.
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THE EMERGENCE OF
HUMANS |
 CRO-MAGNON
Cro-Magnon man was finely built, tall,
muscular, with modern brain and facial
features, replaced Neanderthal. He
manufactured finely worked tools from stone,
ivory and bone, practiced ceremonial burial
and was probably advanced socially as well.
Cro-Magnon cave paintings, drawings and
sculpture are beautifully executed. |
 NEANDERTHAL
Neanderthal man lived over a wide area
of Europe and North Africa during the last
glacial advance. He was short, stocky,
stooped, with heavy brows, retreating
forehead and a prominent but chinless jaw. A
cave dweller and skilled hunter, he
practiced ceremonial burial. He became
extinct after about 100,000 years. Not
ancestral to modern man, but probably from
same stock. |
 PITHECANTHROPUS
Pithecanthropus (sometimes referred to as the
"ape man") is known
from specimens about 500,000 years old from
Java and china.
Typical height was about 5 ft.,
semi-erect posture, heavy brows, powerful
jaws with manlike teeth and a brain capacity
between that of the large apes and modern
man, charred bones found with simple stone
tools suggest the possibility for cannibalism. |
 AUSTRALOPITHECUS
Australopithecus of South Africa and his
relative, Zinjanthropus, of Olduvai Gorge, E.
Africa, are more than 1 million yrs. old.
A
recent Olduvai discovery is an advanced
fossil man, Homo habilis, a hunter who
probably built simple shelters and made most
of the tools found with the split bones of
game he ate.
Other discoveries have pushed
the age of man’s direct ancestors back to
about 20 million years. |
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