PALEOECOLOGY OF THE BIOTIC RECOVERY FROM THE END-TRIASSIC MASS EXTINCTION, LOWER JURASSIC SUNRISE FORMATION, NEW YORK CANYON, WEST-CENTRAL NEVADA

Despite its place among the five largest extinction events of the Phanerozoic, relatively little is known about the marine mass extinction at the end of the Triassic or the biotic recovery interval that followed. While taxonomic loss resulting from this event has been estimated (e.g., Sepkoski, 1986; Raup and Sepkoski, 1982), its impact on ecological structure is still unclear. This study focuses on the paleoecology of the biotic recovery from the end-Triassic mass extinction in the New York Canyon area of west-central Nevada in order to assess the severity of this event among marine ecosystems.

Taylor et al. (2000) discuss high rates of turnover among ammonites, nautilids, and bivalves near the top of the Upper Triassic Gabbs Formation at New York Canyon, but the taxonomic severity of the end-Triassic event does not appear to produce large-scale benthic ecological changes. Of the bivalve genera described from the Gabbs Formation whose temporal ranges are known (Laws, 1982), 46% do not appear within the Lower Jurassic Sunrise Formation. However, epifaunal to shallow infaunal forms are dominant within the Gabbs Formation as well as within limestone facies of the Sunrise Formation. Both the Gabbs and Sunrise Formations are characterized for the most part by a low diversity ichnofauna and shallow tiering. It seems, therefore, that the taxonomic losses recorded from this section do not bring about benthic ecological changes higher than the third or fourth level, according to the Droser et al. (1997) scheme of paleoecological levels.

In contrast to other major extinction events of the Phanerozoic, very little geochemical data exists for the end-Triassic mass extinction. Recently, several groups of workers have successfully retrieved carbon isotopic records from sections in British Columbia (Ward et al., 2001) and Hungary (Pálfy et al., 2001) that exhibit negative excursions at the boundary. Carbon isotope analysis of the boundary section at New York Canyon is currently under way and may provide information regarding the timing of, and the environmental conditions associated with, the end-Triassic extinction.