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exceptional preservation, " said Arvid Aase, park
manager and museum curator at Fossil Butte.
The lake was primarily freshwater, but a layer of
low-oxygen saltwater was located at the bottom.
This prevented freshwater scavengers living in the
lake from entering the saltwater layer, meaning they
didn't feast on or spread the remains of anything
that sank in the lake. Not only was scavenging
prevented, but a microbial mat comprised of photosynthesizing
cyanobacteria grew at the bottom.
The mat grew quickly and pinned dead organisms to
the lake bottom, preventing them from floating or
breaking apart as they decayed. Periodically storms
stirred the water and released gases that allowed
calcite to form, which settled to the bottom and
later became limestone. After sediment settled, the
microbial mat sent tendrils through the thin layer
seeking sunlight, then recolonized the lake bottom
within days. These paper-thin, alternating layers
preserved the organisms.
" We estimate at least 4 million fossils have been
excavated from Fossil Lake in quarries on private
and state-owned land and distributed around the
world by commercial fossil businesses, " Aase said.
" We still have trillions of fossils in the ground. "
FISH OF FOSSIL LAKE
Fish are the most common fossils found at Fossil
Lake with 27 different species identified from the
formation. Some of the fish have living relatives
today, while others are unique. The majority of
the fossilized fish at this location do not have close
relatives in the Cowboy State.
" The kinds of fish we see in Wyoming today
have changed because it was warmer and wetter
here, much like the Gulf Coast states, " Aase said.
" Our latitude and atmospheric CO2 are lower and
the altitude higher than 52 million ago, creating a
much cooler and drier climate. "
The most common fish found are two Knightia
species: Knightia eocaena and Knightia alta. These
extinct freshwater fish belong to the same family as
modern marine herring. They were small, with most
fossils around 3-4 inches long - although some
reached 10 inches. Fossil evidence shows Knightia
were schooling fish that were subject to mass dieoff
events suggesting that they were sensitive to
environmental changes, not unlike their modern
relatives. These mass die-offs sometimes resulted
in large numbers of fish fossilized near each other
with as many as 100 fish per square meter in some
areas of Fossil Lake. Knightia earned the title of
Wyoming's State Fossil in 1987.
One fossil with living relatives in Wyoming today
is a pike, Esox kronneri. This fish is in the same
genus as northern pike still swimming in Wyoming's
Knightia eocaena was a schooling freshwater herring. The fish in this fossil, which is in a private collection, are
approximately 10 centimeters long.
Knightia alta are the less common of the two Knightia species found at Fossil Lake. This fossil is about 8
centimeters long.
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January 2025

Table of Contents for the Digital Edition of January 2025