Molluscs, ostracodes, and insects comprise the invertebrate fauna of
Fossil Basin.
A large number of marine forms are found in the earlier marine deposits,
while fresh-water and terrestrial forms are common in the Tertiary
units. It is these later forms which are considered here.
The fresh-water and terrestrial molluscs from the Wasatch and Green
River formations include bivalves (clams) and gastropods (snails).
Aquatic clams from the Green River Formation include _Unio_ and
_Plesiellipta_. _Pisidium_ and _Sphaerium_ are common in the Wasatch
Formation.
Land snails from the Wasatch include such genera as _Grangerella_,
_Discus_, _Oreoconus_, and _Glypterpres_. _Oreoconus_ is also known from
the Green River Formation.
Fresh-water snails are very abundant from both the Wasatch and Green
River formations. _Physa_, _Planorbis_, _Elimia_, _Bioniphalaria_,
_Virpains_, and _Omalodiscus_ are from the Wasatch. _Physa_, _Elimia_,
_Diomphalaria_, and _Goniobasis_ are recorded from the lacustrine Green
River sediments. In some areas of the Green River Formation, these snail
fossils are so abundant that they form a major constituent of the
limestones. The presence of land snails in the Green River and
fresh-water molluscs in the Wasatch is evidence that the two formations
are not totally representative of a single environment, but represent
several related ones.
Most of these fresh-water snails were herbivorous and frequented the
shallow, well-lighted areas of Fossil Lake which had a good growth of
plants and algae. This naturally occurred close to shore, hence the
gastropodal limestones reported by Bradley (1926) and Oriel and Tracey
(1970) are interpreted as being near-shore facies.
Ostracodes are extremely small arthropods that produce a two-piece,
hinged shell within which they live. _Pseudocypus_ is the most common
form. Like the snails, the ostracodes were so abundant in areas that
they, too, formed a large part of some Green River Formation limestones.
And like the snails they lived in a near-shore environment.
Within the laminated shales of the Green River Formation is preserved an
abundant and diverse assemblage of fossil insects (Fig. 13).
These insects are important because they demonstrate that many modern
families and even genera were in existence during the Eocene. In
contrast to this, knowledge of insect evolution prior to the Eocene is
rather poorly known. Scudder (1890) and Cockerell (1920) have described
most of the Green River insects.
Beetles are the most common forms, followed by dragonflies. Maggots and
larvae of flies are commonly preserved.
Except for these insects preserved as whole "mummies" in the oil shales,
the majority of the insect fossils are preserved as distilled outlines.
This distillation process resulted from the weight and heat of overlying
sediments of driving off the volatile substances from the buried
insects, leaving a hydrocarbon outline. The process is so precise that
the fine hairs and wing veins and even body-color markings are
preserved.
[Illustration: Fig. 13. An impression of a horse fly from the Fossil
Lake deposits. Length of original, 12 mm. _Collection of University
of Wyoming._]