| OCR Text |
Show Robert Hershler, Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, D. C. 20560. BIOGEOGRAPHY OF GREAT BASIN SPRINGSNAILS ( PYRGULOPSIS) Snails of the genus Pyrgulopsis ( Caenogastropoda: Hydrobiidae) are among the richest elements of aquatic biodiversity in the Great Basin, totalling 78 Recent species. Most of this diversity has only been documented during the past decade as a result of field surveys throughout the region. Pyrgulopsis long has been present in the Great Basin region, with a fossil record extending to the Miocene ( Truckee Formation). These small, gill- breathing gastropods ( commonly known as springsnails) are obligately aquatic and highly sensitive to terrestrial barriers, as evidenced by their speciose nature and extensive local endemism. There is little evidence of human- mediated transfer of Pyrgulopsis within the Great Basin although considerable extinction attributable to man has occurred. Although this is a highly complex group for which monophyly and phylogenetic structure have not been well established, distributions of species and putative species groups nevertheless are informative with respect to hydrographic history. The following areas of endemism within the Great Basin are defined by distributions of Pyrgulopsis: Death Valley system ( including Owens Valley, Ash Meadows sub- areas), Lahontan System ( including Soldier Meadows, Antelope Valley [ Lander County], Carico Lake basin sub- areas), Railroad Valley, Steptoe Valley, Dixie Valley, Bonneville Basin ( including Thousand Spring, Snake Valley, Sevier sub- areas), and disrupted drainage of the Colorado River in southern Nevada. Historical relationships among areas ( drainages) implied by distributions of these species and species groups are complex. In some cases biogeographic patterns closely conform to a " classical model" in which dispersal of ancestral biota within a pluvial drainage is followed by local differentiation as drainage fragments in response to onset of aridity. In other situations species distributions and inferred phylogenetic relationships bear little or no relation to configuration of pluvial drainage and suggest operation of other, older pathways of dispersal and vicariant events. |
