ORDOVICIAN MONOTYPIC FOSSIL CONCENTRATIONS: AN EXAMPLE OF AN OPPORTUNISTIC RESPONSE TO STRESSED ENVIRONMENTS

The Middle Ordovician of the Great Basin, records not only a dramatic change in the taxonomic composition of shell beds, but also an increase in the abundance and thickness of shell beds. Two units, however, the Kanosh Formation and overlying Lehman Formation, from the classic Ibex area of western Utah have an unusually high abundance of fossil concentrations, in particular, monotypic and nearly monotypic fossil concentrations.

The succession has been interpreted as representing deposition in a restricted basin with fluctuating oxygen conditions shallowing up to a very shallow restricted lagoon (McDowell, 1987). Shell beds are polytaxic to virtually monospecific in composition and range from pavements to tens of centimeters in thickness. The composition of fossil concentrations includes echinoderms, ostracods, gastropods, bivalves, trilobites, bryozoans, and cephalopods, with orthid brachiopods dominating most beds. Several species of orthids as well as some gastropods, bivalves and ostracods form monotypic beds. The orthid beds are found throughout the succession except in the very shallow lagoon facies where bivalves and ostracods are common. One orthid brachiopod, Shoshonorthis michaelis is found exclusively in nearly monospecific beds up to tens of centimeters in thickness and traceable at the outcrop scale. These beds have not been sorted or created by other mechanical processes as their size distribution, lack of abrasion, and the presence of articulated specimens supports. Monotypic bivalve beds, up to 10 centimeters in thickness are present in the shallowest facies.

These fossil concentrations are representative of local population blooms of opportunist species in a stressed environment. While situations such as these are common in Modern settings, they are not recognized from the fossil record as often as they likely occur.