COMMUNITY PALEOECOLOGY OF THE JURASSIC (BAJOCIAN-OXFORDIAN) SUNDANCE SEAWAY IN THE BIGHORN BASIN OF WYOMING AND MONTANA, UNITED STATES

The biotic record of the Sundance Seaway, a Jurassic epicontinental seaway that covered portions of the US Western Interior, is preserved in the Bighorn Basin of northern Wyoming and southern Montana in the Gypsum Spring Formation (Bajocian) and Sundance Formation (Bathonian-Oxfordian). Marine invertebrate communities were investigated to determine the factors controlling community composition and variation throughout the 15 million year marine history of the seaway. Field observation and quantitative analysis of 51 taxa from 82 samples, identified to the genus level, found that communities are typically dominated by a single, highly abundant taxon that changes over time. Both Detrended Correspondence Analysis (DCA) and Nonmetric Multidimensional Scaling (NMDS) identified the primary factor driving the distribution of fauna as a complex gradient reflecting a carbonate-siliciclastic transition, salinity, water depth, and possibly temperature. NMDS identifies a secondary substrate-related driving factor, with a separation of infaunal, mobile taxa from epifaunal, stationary taxa. Both DCA and NMDS show that older units, the Gypsum Spring Formation and Canyon Springs Member, display a wider range of community compositions than younger units, which show much less variation and are heavily dominated by a few taxa. This is taken to the extreme in the Stockade Beaver Member, which overlies the Canyon Springs and is dominated by the oyster Gryphaea (up to 98.9% of individuals). High dominance by a few taxa continues through the overlying Hulett (Gryphaea), Redwater (belemnite Pachyteuthis, oyster Liostrea, or scallop Camptonectes), and Windy Hill (Liostrea and regionally the brachiopod Kallirhynchia) Members. The primary faunal gradient is likely caused by terrestrial input from the south that eventually filled the basin, as well as the continued northward shift of North America during this time. The high dominance and low diversity of these communities are atypical of many Jurassic faunas globally, though the reasons for this are still under investigation.