Actualistic Tests of Paleoecological Hypotheses: Reconstructing An Existing Lacustrine Environment on the Utah-Idaho Border

The distributions and abundances of fossil organisms are often used in paleoecology to reconstruct aspects of those organisms' physical environment. The molluscan shell beds surrounding Bear Lake on the Utah-Idaho border provide an opportunity to assess how accurately these distributions truly reflect the physical environment.

The shores of Bear Lake are littered with a diverse assemblage of molluscan shells. The lake itself, however, has been devoid of mollusks for millennia. Consequently, these shell beds represent a rare opportunity. They have been transported, winnowed, and time-averaged in a manner consistent with a typical fossil horizon; however, the physical environment that created these deposits is still present and subject to direct observation. These shell beds therefore give us the opportunity to use purely paleontological data and methods to attempt to reconstruct a known physical environment.

We collected bulk samples of the Bear Lake shell beds at one-mile intervals around the entire forty-mile perimeter of the lake. These samples included over 100,000 individuals from over a dozen different species of bivalve and gastropod. We then used classic paleoecological techniques to see how accurately we could reconstruct the environment from which these specimens were collected.

We examined the relative abundance of ecologically distinct species, rates of bivalve disarticulation and differences in the ratios of left to right valves from different sites around the lake perimeter. We also compared the results of tumbling experiments in the laboratory with the actual pattern of shell breakage found at these different locations. Using these data we were able to predict shoreline gradients, current patterns, and the location of streams feeding into the lake, and then verify each of these predictions.