FOSSIL BUTTE The Geologic History of Fossil Butte National Monument and Fossil Basin NPS Occasional Paper No. 3

Invertebrates

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).

Fig. 13. An impression of a horse fly from the Fossil Lake deposits. Length of original, 12 mm. Collection of University of Wyoming.

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.