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( 189 ), thus extending the formation back
into late Paleocene time. Not surprisingly
the deposits of this lake are also the
thickest, reaching more than 2,100 m
(7,000 ft) in places. These include vast
quantities of economically viable oil-shale,
many deltaic horizons, and rich insect-
and plant-beds of middle Eocene age, but
the fish fauna is less well known than that
of the two Wyoming Green River lakes.
All three lakes show evidence of mass
mortalities of fish populations, but Grande
(1984) concluded that the cause may be
different in each lake. In the deeper
waters of Fossil Lake mixing of stratified
water is the most likely cause. During the
cold season the water temperature of a
lake is homogenous as surface winds set
up circulation currents throughout the
water body. In the hot season, however,
the surface water heats up and becomes
less dense, forming a distinct layer of
warm, light water, the epilimnion,
resting on a deeper, colder, denser body of
water known as the hypolimnion. The
thermocline separating these layers acts as
a seal between the oxygenated surface
waters and the hypolimnion, such that the
bottom waters become deoxygenated,
stagnant, and rich in lethal H 2 S. The
seasonal mixing of such stratified water,
bringing toxic waters to the surface, would
cause mass mortalities, while during
the stratified season the anoxic bottom
waters would protect carcasses from
decomposition by scavengers and bacteria,
explaining their fine preservation. The
dark layers in the varves of the 18-inch
layer represent the organic-rich layers
deposited during stagnation, while the
lighter layers are calcium carbonate which
precipitated out on mixing when the pH
of the acidic bottom waters was suddenly
raised. Grande (1984) concluded that the
undisturbed preservation of these fine
varves, together with the scarcity of any
benthic fauna (catfish, suckers, rays,
crustaceans) in the 18-inch layer suggests
that Fossil Lake was thermally and/or
chemically stratified in the deepest water
at its centre. On the other hand, the
poorly laminated split-fish layer, with
common bottom-dwelling stingrays and
crayfish, indicates better-oxygenated
bottom conditions in the shallow waters at
the lake edge (Grande and Buchheim,
1994).
Mixing of stratified water is less likely
to have been the cause of mortality in the
shallower lakes of Gosiute or Uinta. Lake
Gosiute is known to have been saline
at times (see section on Paleoecology)
and Grande (1984) favored periods of
excessive salinity as a likely cause of mass
mortalities in this lake. It is also likely
that this shallower lake was more
susceptible to temperature changes
which may have caused mass mortalities
of small fish such as the herring
Gosiutichthys (see below).
In Lake Uinta, there are no mass
mortalities, but there are limited
mortalities affecting a single group of fish,
the gars (see below). Grande (1984)
suggested that Lake Uinta's lagoonal
nature (see section on Paleoecology) and
fluctuating water level led to ponding of
small bodies of water which gradually
evaporated away, leaving only the most
hardy fish, such as the gar, remaining. Gar
can actually breathe air and can survive in
stagnant water for long periods. Grande
(1984) suggested that they would have
scavenged other fish already killed by the
stagnation until they eventually succumbed
themselves.
D ESCRIPTIONS OF THE G REEN
R IVER F ORMATION BIOTA
Fish
The Green River Formation is best known
for its exquisitely preserved fish, many
thousands of which are quarried every year
by commercial dealers. Despite their
abundance only 21 genera (belonging to
16 families) are recognized (Grande,
2001) and all but two belong to the teleost
group of bony fish which are essentially
modern in nature. Up to 60% of all
specimens collected are freshwater
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