SUBFOSSIL MOLLUSKS OF THE BEAR LAKE BASIN, UTAH-IDAHO BORDER

Bear Lake is a fault-bounded lake spanning the Utah-Idaho border. Presently, the invertebrate population of the lake is limited to insect nymphs. In the recent past, however, Bear Lake was home to a diverse assemblage of gastropod and pelecypod mollusks that now exist as subfossils in the sediment surrounding the lake. All of these species are now locally, and in some cases regionally, extinct. While the most common molluscan species is by far the planorbid snail Helisoma newberryi, this assemblage contains fossils of at least a dozen different species of gastropod representing several different taxonomic families. These shellbeds also include specimens of several different species of sphaeriid bivalve, mostly from the genus Pisidium. In addition to the diverse collection of mollusks found as body fossils in these shellbeds, there is also strong ichnological evidence for the presence of some aperture-entry durophagous predator co-occurring in the lake. These predation scars were most likely caused by crayfish, although they are also currently extinct in this lake.