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In the early Eocene Epoch over
50 million years ago, drainage
from the newly uplifted Rocky
Mountains filled an intermountain
basin to form what geologists
call Fossil Lake.
The climate of Fossil Lake
was subtropical, similar to
the climate of Florida today.
The lake's paleoenvironment
persisted for about 2 million
years, and was home to palm
trees, crocodiles, turtles,
and an abundance of fish.
On numerous occasions, unique
conditions came together to
result in some of the best
preserved fossils ever discovered.
The sediments of fossil lake
were first discovered in the
1850's near the town of Green
River Wyoming, thus the name
"Green River Formation"
which has become famous. Unlike
other fossil formations around
the world where fragments
of specimens are prevalent,
Green River fish are most
often perfectly preserved
in entirety.
Paleontologists and sedimentologists
theorize that
the lake was deep enough to
be anoxic (devoid of oxygen)
at the bottom.
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This prevented scavengers from
disturbing the fish, and inhibited
decomposition. Several of
the laminated sedimentary
layers contain so many fish
fossils that scientists believe
there were relatively frequent
events such as algal blooms
which resulted in large scale
fish kills. Algae and other
plant and animal life would
die and fall to the bottom
as in lakes and ponds today.
Storms brought runoff from
the mountains, covering the
fish with mineral rich material
that would ensure their preservation.
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