GASTROPOD-DOMINATED SHELLBEDS FROM THE MIDDLE ORDOVICIAN ANTELOPE VALLEY LIMESTONE (IKES CANYON, TOQUIMA RANGE, NV)

Gastropods are both diverse and abundant in Ordovician strata across the Great Basin, yet the clade is often overlooked in paleontological studies because of typically minor ecological importance, lack of biostratigraphic application and poor preservation. Early studies of the Middle Ordovician Antelope Valley Limestone at Ikes Canyon (Toquima Range, NV) reported the presence of gastropods and noted the oncolitic “Maclurites-Girvanella” zone found within the Antelope Valley Limestone across the Great Basin, but offer no ecological interpretation (Brassler, 1941; Kay, 1962; Yochelson, 1986; Rohr, 1994).

Abundant packstones, grainstones and shell pavements dominated by gastropods occur within a 19 m section of the lower portion of the Antelope Valley Limestone, just below the oncolotic “Maclurites-Girvanella” zone at Ikes Canyon. This interval is characterized by thin to medium beds of carbonate with clay partings. These gastropod shell beds are diverse and were deposited in mid to outer shelf conditions below fair-weather wave base. Taxa identified include Barnesella measuresae, Liospira americana, Malayaspira hintzeii, Monitorella auricula, Murchisonia sp., Raphistomina lapicida, while others remain to be identified. Other fauna preserved within this interval include trilobites, brachiopods, sponges, echinoderms and cephalopods, but gastropods are dominant in most shell beds.

Gastropod-dominated shell beds are rare in the Ordovician of the Great Basin and are most often found in very shallow extreme environments like the hypersaline lagoon represented by the Middle Ordovician Lehman Formation to the east in Ibex, UT. Furthermore, Ordovician gastropod shell beds are typically monospecific. The occurrence of multiple gastropod taxa forming shell beds in deeper water, open ocean conditions is atypical and contradicts the commonly-held theory that Ordovician gastropods were ecologically successful only in shallow water environments.