The Green River Formation - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 191

mammal tracks including those of the
three-toed horse. Coprolites of fish are also
ubiquitous and are found in various
shapes. The common rope-shaped masses
were probably produced by herring, while
the larger, drop-shaped masses containing
fish bones are clearly of carnivorous origin.

using long gill rakers on the inner sides
of the gills, while the sandfish, with its
ventrally-placed mouth and toothless
jaws, was a suction feeding herbivore.
The mooneye, Eohiodon , like modern
counterparts, probably fed mainly on
insects and insect larvae which at times
were prolific in the lakes.
Apart from those areas of the lakes with
anoxic bottom waters (see section on
Taphonomy) the lake beds were home
either to rays or to catfish and suckers.
Catfish, like their modern counterparts,
would have eaten smaller fish, crayfish,
mollusks, and plant material, while the
suckers, feeding on plant and decaying
material on the lakebed, also crushed
invertebrates using the teeth on its gill
arches. The stingrays, with flat crushing
teeth, also fed on crustaceans, snails, and
small fish, and it is notable that the split-
fish horizon with its rich fauna of rays is
also the only horizon yielding crayfish and
shrimps.
The three distinct lakes which together
make up the Green River Formation
(see section on Stratigraphic setting
and taphonomy, p. 188) are, however, all
ecologically different and should not be
considered as a single community. Lake
Uinta, which persisted for the longest time,
was of considerable geographical extent,
but was consistently shallow and was
typically lagoonal (with common garfish) to
shallow lacustrine. Baer (1969) described
common deltaic horizons of mudstone,
sandstone, and siltstone, and interpreted
the interbedded limestones and mudcracks
as indicative of a fluctuating shoreline.
Lake Gosiute was also broad and
shallow and was periodically saline
(Surdam and Wolfbauer, 1975). Moreover,
it supported thick algal mats and
abundant plant growth, which would have
depleted oxygen levels and led to
eutrophic conditions. Modern eutrophic
lakes tend to support low species diversity
and this is certainly true for Lake Gosiute.
Grande (1984) showed that the bottom-
dwelling suckers and catfish were
plentiful, but that fish confined to the

P ALEOECOLOGY OF THE G REEN
R IVER F ORMATION
The Green River Formation represents a
freshwater lake system (with brief periods
of higher salinity; Grande, 1994), situated
in the subtropical to tropical zone
somewhat south of its present latitude,
with an annual rainfall of 75-100 cm
(30-40 in) and an overall average annual
temperature of 15-21°C (60-70°F;
Grande, 1984). Subtropical/tropical
conditions are clearly indicated by the
fauna of crocodiles, alligators, a
neotropical wood snake, and fish with
tropical extant members (such as
Phareodus ), and also the flora of palm
trees and balloon vines (Grande, 1994).
The lakes supported a rich and diverse
fish fauna, dominated in their upper
waters by schools of small herring which
fed mainly on algae and zooplankton.
Their abundance indicates a low position
on the food chain and they were prey to
larger fish, such as the surface-feeding,
herring-like Diplomystus . These were in
turn preyed on by the voracious gars and
bowfins which were near the top of the
food chain. Both these fish belong to the
more primitive holostean group of bony
fishes and were protected from predation
by their armour-like ganoid scales; their
only enemies were the crocodiles and
alligators. Other large predators were the
sharp-toothed Phareodus and Mioplosus
(often found with herring preserved in
their throats and stomachs) and, of
course, the voracious and cannibalistic
dogfish, although some Green River
species possessed crushing teeth
suggesting they fed on mollusks and
crustaceans, as did Priscacara . An unusual
feeding strategy was that of the
paddlefish which was a plankton strainer,