PALEOBIOLOGY OF THE ELLIS GROUP OF MONTANA: IMPLICATIONS FOR THE HISTORY AND COMMUNITIES OF THE SUNDANCE SEAWAY

Ancient epicontinental seas are a widely studied geologic environment, but few studies have quantitatively demonstrated the effects of large-scale environmental gradients on the fauna of these seas beyond gradients related to water depth. In order to isolate these gradients, I used quantitative abundance estimates integrated with a sequence-stratigraphic framework for the Jurassic Sundance Seaway to compare benthic communities in the southern Sundance Seaway to those in the northern Sundance Seaway. In central Montana, the history of the Sundance Seaway is preserved by the Piper, Rierdon and Swift formations of the Ellis Group, which contain abundant and diverse invertebrate fossil assemblages. Multivariate analysis shows that the invertebrate communities in the Ellis Group differ from those in Wyoming. Assemblages from Wyoming and from Montana show strong ties to water depth and age, but the Montana samples separate from the Wyoming samples along latitude. Ammonites from Montana also exhibit an increase in size and diversity when compared with those from Wyoming. These features are likely tied to gradients such as temperature and salinity in the Sundance Seaway: communities to the north, closer to the mouth of the seaway, would have experienced cooler and more normal salinity water. This pattern suggests that latitudinal gradients play an important role in community structure in shallow, epicontinental seas, and that these relationships can be observed using quantitative paleobiology within a sequence-stratigraphic framework.