PALEONTOLOGY OF “TURRITELLA AGATE”, A PLEUROCERID GASTROPOD-DOMINATED ASSEMBLAGE FROM THE EOCENE GREEN RIVER FORMATION OF WYOMING

As is well known to professionals and most serious amateurs, “turritella agate” is not composed of turritellid gastropods (a marine group), but of turritelliform freshwater pleurocerid gastropods most commonly assigned to Elimia (=Goniobasis) tenera. The stone is a slightly to completely chalcedonized coquinite that occurs in several horizons in the Lower and Middle Eocene Green River Formation of southwestern Wyoming, northwestern Colorado, and northeastern Utah. Despite its popularity as a gemstone, the paleobiology and paleoecology of “Elimia agate” remain poorly known.

Most of the commercially available material comes from south of Wamsutter, Sweetwater County, Wyoming, but many other localities are known. The Wamsutter material comes from a 1-2 ft thinck layer and has a characteristic black to dark brown color and dense silicification; material from other sites is frequently less silicified and in thinner layers.

Preliminary work on the Green River Elimia concentrations from several Wyoming localities suggests that the concentrations of gastropods forming these layers are mostly autochthonous, and similar in abundance to those observed in association with very high levels of primary productivity in modern lakes. Elimia today is a herbivorous grazer in shallow lakes and streams throughout North America, occasionally reaching very high abundances. It is not clear, however, that it reaches abundances similar to those seen in the Green River Formation. The stratigraphic and geographic patterns of occurrence of Elimia concentrations in the Green River are still incompletely documented. It appears, however, that the conditions conducive to extremely high abundance occurred several times during the history of ancient Lakes Gosiute and Uinta. Further work is needed to refine this important aspect of the paleoenvironmental history of these lakes, and to understand more completely the mechanisms by which these shell beds formed.