Geoscience Reference
In-Depth Information
The Chinle Group
B ACKGROUND : P ANGEA AND THE
END -P ERMIAN EXTINCTION
In the last two chapters, we saw how
the terrestrial environment became green
with plant life, closely followed by animals
rustling through the undergrowth; by the
Pennsylvanian Period lush tropical forests
were widespread. These forests were
dominated by lycopod trees and
pteridosperm (seed-ferns) understorey,
and animal life included amphibians,
insects, and arachnids. The Paleozoic seas
had witnessed the rise of arthropods
(e.g. trilobites, eurypterids), planktonic
graptolites, and brachiopods, and coral
reefs were widespread in tropical regions.
However, the end of both the Permian
Period and the Paleozoic Era was defined
by the sudden change in fauna and
flora which resulted from a major mass
extinction event - the greatest the Earth
has ever witnessed, wiping out 95% of all
life. As we now enter the Triassic Period,
and the Mesozoic Era, life on Earth is
quite different from what it was before.
Trilobites, eurypterids, and graptolites
are extinct; bivalved mollusks, not
brachiopods, are now the dominant
shelled animals on the sea floor; there are
new types of corals forming reefs; reptiles
have diversified greatly and increased in
size, and gymnospermous trees, especially
conifers, now dominate the flora on land.
The Earth at this time was a very
different place. The vast Panthalassa
Ocean covered two-thirds of its surface
and a supercontinent called Pangea,
occupied the rest of the globe stretching
from pole to pole. There were, however,
no ice caps - instead the polar regions
were monsoonal, while the rest of the
Earth was hot and dry. As life on this
hostile land began to recover following
the extinction, the surviving reptiles
diversified again. The synapsid or
'mammal-like' reptiles, so successful
during the Permian, flourished again
briefly and eventually gave rise in the
late Triassic to the first small mammals;
these would have to wait, however,
until after the end of the Mesozoic before
they dominated the Earth. It was the
diapsid archosaurs that were to dominate
the Mesozoic; initially the semiaquatic
crocodile-like reptiles were top predators,
but soon these were overshadowed by
another group that appeared in the late
Triassic wilderness - the dinosaurs.
The Permian and Triassic periods are
both relatively barren of Fossil-
Lagerstätten. Nevada has a locality (Buck
Mountain), which preserves soft-part
morphology of cephalopods, but for
terrestrial fossils the late Triassic Chinle
Group of Arizona and New Mexico is
justly famous for its preservation of huge
permineralized trees in the Petrified
Forest National Park. It is perhaps more
important paleontologically, however, for
its fauna of Triassic reptiles, typically
 
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