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The Mesozoic Era
The “Age of Dinosaurs” is the Mesozoic Era, which is divided into three
periods: the Triassic (245-208 million years ago), Jurassic (208-145 million
years ago), and Cretaceous (145-66 million years ago). The earliest dinosaur
fossils, recently discovered in Argentina's Ischigualasto basin, have been
radiometrically-dated as 228 million years old. (This discovery has led
some scientists to characterize the southern region of South America as
“The Cradle of Dinosauria.”) After existing for 163 million years, the
dinosaurs disappeared at the end of the Cretaceous Period, 66 million years
ago.
The Earth's appearance during the Mesozoic Era was quite different from
its appearance today. The reason for the difference is that although the
surface of our Earth seems rock solid, it is not. The surface of the Earth
is changing constantly, although most of the change is extremely slow.
For example, the east coast of the United States is moving away from Europe
and Africa at a rate of one to three inches per year. This movement is
known as “continental drift.”
The rate of continental drift, one to three inches per year, is insignificant
to humans, who might see their continent move six feet (1.82 m) or so during
their 70- or 80-year lifetimes. But when we speak of the Earth's past,
we are talking about hundreds of millions of years. A lot can happen in
a million years.
When the Triassic Period began, 245 million years ago, there was only one
continent on Earth, a super-continent called Pangea. Pangea separated
into two lesser continents, Laurasia and
Gondwanaland, during the Jurassic
Period; and during the Cretaceous Period, the modern continents began to
appear (although they were not in their present-day locations).
You could think of today's world maps as “snap-shots” of the way the continents
look today. In the future, they will look quite different.
Environment
In general, the climate of the Mesozoic
Era was warmer than that of
today, and there appears to have been little difference between summer
and winter. It is incomplete, however, to picture dinosaurs as moving through
either a tropical or desert landscape. These settings did exist, but there
were forests, as well, and a range of mild climates. In whatever mental
picture of dinosaurs and their environment that one develops, there should
be one very common plant missing -- throughout the Mesozoic Era, there
were no grasses! Small ferns provided the only ground cover there was.
Evidence suggests that at the beginning of the Triassic Period (245-208
million years ago), before the appearance of dinosaurs, the global temperature
averaged around 50º to 60º F (10º to 15º C). Toward
the end of period, however, the global climate began to become drier and
hotter; deserts began to appear on much of Pangaea's surface area. In the
northern hemisphere, gingko and tree fern forests flourished, while near
the equator were forests of conifers and cycads. Horsetails grew near bodies
of water.
Rainfall increased and the oceans rose during the Jurassic Period (208-145
million years ago). Lush vegetation, resembling the profuse growth in today's
rain forests, covered much of Earth's surface, displacing former desert
areas. Evidence of the abundant plant life can be found in the extensive
coal fields that date from this period. Benettitalean cycads, ferns, and
tree ferns (leaves perched high atop thin trunks), conifers, and giant
club moss dominated the forests and marshlands.
As the Cretaceous Period (145-66 million years ago) progressed, the giant
forests of the Jurassic began to disappear. Toward the middle of the period,
a gradual global cooling began, and different vegetation began to develop.
Flowering plants (angiosperms) appeared, including beech, fig, and magnolia,
and seed ferns became extinct. In the Late Cretaceous, dinosaurs inhabited
forests that would have closely resembled those of today.
Footnote: In December, 1994, scientists at the Royal Botanic Gardens
in Sydney Australia, announced the discovery of 39 prehistoric pine trees,
thought to have been extinct since the Jurassic Period. One of the most
significant botanical finds of the century, the trees have been peacefully
thriving in a secluded rain forest for 150 million years -- a real-life
Jurassic Park.
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