MIDDLE ORDOVICIAN BIVALVES OF THE BASIN AND RANGE

Some of the oldest representatives of Laurentian bivalves occur in Whiterockian (Middle Ordovician) strata of the Basin and Range of Nevada and Utah. Bivalve-bearing strata from six localities within this region provide insight into the paleoecology and paleoenvironments of Middle Ordovician bivalves. The bivalves occur in great abundance within particular beds within shallow marine argillaceous carbonate settings. Modiomorphid bivalves (including Modiolopsis) are present in the Lehman Formation of Utah and central Nevada, dominating an assemablage that includes rare gastropods, lingulids, ostracodes and trilobites. Ctenodontid bivalves (including Deceptrix) comprise the nearly monotaxic bivalve beds of the Pogonip Group (member f) in the Arrow Canyon Range, southern Nevada. Bivalves are not incorporated into typical brachiopod-dominated Middle Ordovician shallow marine communities. Bivalves in both the Lehman and the Pogonip Group appear as loosely to densely packed accumulations, as well as dispersed shells throughout some of the beds. Some of the bivalve-bearing accumulations reflect minor exposure to taphonomic processes, such as mud-winnowing, while others appear to represent in situ assemblages. There is a variety of preservational styles of the Great Basin bivalves, including molds and calcified and silicified material suggesting that the rareness of Middle Ordovician bivalves on Laurentia is not solely the result of preservational barriers in the nearshore depositional environments.