Cenozoic
Era (65 million years ago to present)
 The
KT Event set the stage for the Cenozoic Era Cenozoic Era that
began 65 million years ago. As
the dinosaurs perished at the end of the Cretaceous, the mammals
took center stage. Even as mammals increased in numbers and diversity,
so too did the birds, reptiles, fish, insects, trees, grasses,
and other forms of life. Species changed as the epochs of the
Cenozoic Era rolled by, with the mammals eventually becoming
the
largest land animals of the Era, as the dinosaurs had been during
the Mesozoic. Flowering plants strongly influenced the evolution
of both birds and herbivors throughout the Cenozoic era by providing
a rich abundance of food. Those that could adapt to the changes
in the environment survived; those that could not were doomed
to extinction. A rich fossil record in
rocks relatively undisturbed by geological forces reveals the
history of both. The Cenozoic is when the continents moved to
their current positions. Australia-New Guinea that split
from Gondwana during the early Cretaceous
drifted north eventually collidedine Asia.
Antarctica moved into its current position over the South Pole.
The Atlantic Ocean widened and South America became attached
to
North America.
Paleogene
Period (65
to 23 million years ago);
Also see: Tertiary Fossils
The Paleogene
Period contains the Paleocene, Eocene, and Oligocene epochs.
Invertebrates, fish and reptiles were similar to those of modern
types, but
mammals,
birds, protozoa and flowering plants would undergo considerable
evolutionary change.
Paleocene
The Paleocene Period
began after the extinction of the dinosaurs. Mainly nocturnal
mammals that had cowered in the shadows of dinosaurs for millions
of years eventually evolved into a vast number of different
forms to fill the newly vacant environmental niches. At the
beginning of the Paleocene, most mammals were tiny and rodent-like.
With time, mammals grew in size, number, and diversity. Many
early mammal designs of this time would soon become extinct,
but others would survive and then evolve into other forms.
The diversity of birds, other animals, and plants increased,
and species became more specialized. Although dinosaurs were
gone, their reptile cousins lived on in the form of turtles,
crocodiles, lizards, and snakes.
Eocene
The
first grasses appeared in the Eocene Epoch (from about 54
to 37 million years ago) with
growth near the root  as
opposed to the tip,
providing a vastly expanded and renewable food resource
for the herbovores; this allowed adaptation to
life on the savanna and prairie and the
evolution of running animals such as the Equiidae (the horse family).
The grazing mammals evolved the teeth enabling a diet of harsh
grass. The
Eocene Epoch was a period when flowering plants continued a massive
radiation that began in the Paleocene Epoch. Plants thrived, and
with that many animals as new environmental niches were filled.
The first grasses also provided a refuge for many animals. Small
mammals radiate. Many new species of shrubs, trees and small plants
appeared. A variety of trees thrived in a warm Eocene climate,
including beech, elm, chestnut, magnolia, redwood, birch, and
cedar, and more. The evolution of plants was providing a powerful
selective pressure across the entire animal Kingdom, and many
new symbiotic systems appeared.
Oligocene
The Oligocene Epoch extends from about 34 million to 23 million
years years ago. The name Oligocene comes from the Greek
oligos
(meaning few) and ceno (meaning new) and is in reference to
the paucity of new mammalian animals after their radiation
during
the preceding Eocene Epic. The Oligocene is often considered
as an important window of environmental transition from
the tropical
Eocene and the cooler Miocene. The start of the Oligocene is
marked by a major extinction event that might have been
caused by a meteor
impact in Siberia or near the Chesapeake Bay. Angiosperms continued
their expansion throughout the world, as did grasses. Temperate
deciduous woodlands mostly replaced tropical and sub-tropical
forests, while plains and deserts became more commonplace.
Among
the animals, mammals diversified markedly, and marine fauna
evolved to forms closely resembling those extant today.
Ancestors of modern
elephants and rhinoceros grew to large size in Africa, where
the first apes primate belonging to suborder Anthropoidea
that includes
monkeys,
apes, and humans, also appeared.
The
Neogene Period contains the Miocene and Pliocene epochs. During
this long period, while some mammal groups evolved markedly,
others changed little. Importantly, the earliest hominids arose
on the continent of Africa.
Miocene
The
Miocene Epoch extends from about 23 to 5 million years ago.
The name comes from the Greek words meion (less) and
ceno (new)
because of the smaller proportion of modern sea invertebrates
than the subsequent Pliocene Epoch. The Miocene is thus
a very
long 18 million years, and generally marks the transition from
the far prehistoric world to a pseudo-modern world. A major
expansion
of grasslands occurred as forests declined in the cooler and
dryer climate, driving selection and radiation of large
herbivores, including the ruminants which are ancestors of
modern cattle
and deer. Mammals such as wolves, horses and deer as well as birds
also generally evolved to closely resemble forms extant today.
Pliocene
The
Pliocene Epoch extends from 5.3 million to 1.8 million years
before present. The name comes from the Greek words pleion
(more)
and ceno (new) and roughly means the continuation of the recent
in reference to the fact that mammals were essentially modern
in form. The Pliocene climate was also relative cool and dry
as
in modern  times. These modern climates reduced tropical vegetation
and shrank tropical forest to a band near the equator. Concurrently,
deciduous and coniferous forests, tundra, grasslands, dry savannahs
and deserts filled the space.
Continental
drift would play a major role is how animals, and particularly
terrestrial mammals were to distribute. South America linked to
North America through the Isthmus of Panama, to the detriment
of South American marsupials, and precipitating a drop in Atlantic
Ocean temperatures. The collision of Africa and Europe formed
the Mediterranean Sea, and disconnected part of the former Tethys
Ocean. Receding sea levels formed a land bridge between Alaska
and Asia.
Both
marine and terrestrial life was for the most part modern, though
discernibly more primitive. Herbivores grew in size, as did their
predators. The first recognizable human ancestors, the australopithecines,
appeared in the Pliocene. Mammalian life evolved in continent-dependent
ways, and some migration occurred between continents. In North
America, rodents, mastodonts, elephant-like gomphotheres, and
opossums were notably prolific, while hoofed animals generally
declined. Africa’s hoofed animals and primates were notably
successful, and the australopithecines (some of the first hominids)
appeared late in the Pliocene The Pliocene seas were thrived with
mammals such as seals and sea lions.
Quaternary
Period
(2.6
million years ago to present)
The
Quaternary Period that began some 2.6 million years ago marked
the origin of the close human ancestors as well as the modern
forms of the animals we see today. The period includes two
geologic epochs: the Pleistocene and Holocene, and contained
a series of glaciations. Huge glaciers advanced and
retreated
over
much of North America and Europe, parts of South America and
Asia, and over all of Antarctica. The Great Lakes formed and
giant mammals thrived in parts of North America and Eurasia
not covered
in ice. These mammals became extinct when the last Ice Age
ended some 11,700 years ago. Modern humans evolved about 190,000
years ago. During the Quaternary period, mammals,
flowering plants, and insects dominated the land.
Pleistocene
The
Pleistocene is the geological epoch which lasted from about
2.6 million years ago to about 11,700 years
ago, comprising the most period of
repeated glaciations. Both marine and land faunas were
essentially like today, albeit mammals were
generally larger than their modern descendants. The dramatic
climatic changes during the ice age impacted
fauna and
flora in majoe ways. When glaciers advanced south, so did plants
and animals reduced living space and
food availability. A major extinction event of large mammals
(megafauna) that included mammoths, mastodons, saber-toothed
cats, glyptodons,
ground sloths, Irish elk, and cave bears
began late in the Pleistocene and continued into the Holocene.
Neanderthals also became extinct during this period. By the
end of the last ice age, smaller and swifter mammals and migratory
birds, had displaced the megafauna
and then returned northward.
Holocene
Animal
and plant life have not evolved much during the relatively
short Holocene, and are essentially as they are today. A
number of large animals including mammoths and mastodons, saber-toothed
cats and giant sloths disappeared in the late Pleistocene
and
early Holocene, especially in North America, where animals
that survived elsewhere (including horses and camels) became
extinct. |
