Integrated sequence stratigraphy and paleobiology of the Ellis Group of Montana: implications for the history and communities of the Sundance Seaway

Few studies have quantitatively demonstrated the effects of large-scale environmental gradients on the fauna of ancient epicontinental seas. Here, quantitative abundance estimates integrated with a sequence-stratigraphic framework for the Jurassic Sundance Seaway are used to compare communities in the southern part of the seaway to those in the north. In central Montana, the history of the Sundance Seaway is preserved by the Piper, Rierdon and Swift formations of the Ellis Group. The Ellis Group contains five depositional sequences with four facies associations. The J-1 sequence preserves desert depositional environments, the J-1a, J-2, and J-3 sequences preserve carbonate depositional environments, and the J-4 sequence preserves siliciclastic, wave-dominated shelf and tide-dominated coastal depositional environments. The marine communities in central Montana are distinct from those in the same depositional environment in Wyoming and ordination of fossil occurrence data suggests these differences reflect latitudinal gradients such as temperature and salinity in the seaway.