Late Ordovician through Middle Silurian gastropoda of the eastern Great Basin.

摘要

The Ordovician and Silurian gastropod fauna preserved within the Great Basin strata has not been as well studied as the complex geologic history of the Great Basin. This study deals with gastropods recovered by Dr. Peter Sheehan during his work on Great Basin brachiopods and stratigraphy in 1980 and 1982. Collections made by Dr. Sheehan are from the Late Ordovician, Ely Springs, and Fish Haven Dolomites, as well as the Middle Silurian, Laketown Dolomite in Utah and eastern Nevada. In addition, the Hanson Creek Dolomite of the Late Ordovician and Early Silurian age was sampled southwest of Eureka, Nevada. The gastropods are silicified and were etched from dolomitized rock. The collection shows various degrees of preservation as well as a diversity of genera.;Late Ordovican gastropods include Phragmolites cellulosus (Ulrich and Scofield, 1897), murchisonids, Gyronema Salter, 1859, raphistomatids, and Cyclonema Hall, 1852. Silurian taxa include- Euomphalopterus alatus (Wahlenberg, 1821), Trochonema Salter, 1859, Pachystrophia Perner, 1903, Fusispira Hall, 1872, Oriostoma Munier-Chalmas, 1876, Stylonema Perner, 1907, and Platyostoma Conrad, 1842. These are the first reported occurrences of Phragmolites, Oriostoma, Pachystrophia, Euomphalopterus, Onychochilus, and Platyostoma from cratonic western North America. The gastropod faunas have North American (Laurentian) biogeographic affinities and lack any of the characteristic gastropods found in the terranes of northern California or Alaska.;Most gastropod specimens are found in conjunction with brachiopods or dasycladacean algea within the stratigraphic section. These associations between fauna help researchers derive an understanding of water depth or paleobathymetry. This understanding is represented in Benthic Assemblages numbers. Benthic Assemblages are designated 1 to 5 with 1 being intertidal and 5 being at or below storm-wave base. This constitutes water depths of 0 to 90m as limited in range, due to the dasycladacean algae.